Prepare for ASE A8 certification test content area “D” (Emissions Control Systems Diagnosis and Repair) and ASE L1 certification test content area “F” (I/M Failure Diagnosis).
Discuss emission standards.
Identify the reasons why excessive amounts of HC, CO, and NO X exhaust emissions are created.
OBJECTIVES: After studying Chapter 61, the reader should be able to: Continued
Describe how to baseline a vehicle after an exhaust emission failure.
List acceptable levels of HC, CO, CO 2 , and O 2 with and without a catalytic converter. • List four possible causes for high readings for HC, CO, and NO X .
OBJECTIVES: After studying Chapter 61, the reader should be able to:
acceleration simulation mode (ASM) • ASM 25/25 test • ASM 50/15 test Federal Test Procedure (FTP) I/M 240 test lean indicator • non-methane hydrocarbon (NMHC) ozone Sealed Housing for Evaporative Determination (SHED) test • smog • State Implementation Plan (SIP)
EMISSION STANDARDS IN THE UNITED STATES
In the US, emissions standards are managed by the Environmental Protection Agency (EPA) as well as some U.S. state governments. Some of the strictest standards in the world are formulated in California by the California Air Resources Board (CARB). Tier 1 and Tier 2 Federal emission standards are set by the clean air act amendments (CAAA) of 1990 grouped by tier. Current Tier 1 standards are different between automobiles and light trucks, but Tier 2 standards will be the same for both types. T here are several ratings given to vehicles, and a certain percentage of a manufacturer’s vehicles must meet different levels in order for the company to sell its products in affected regions.
Beyond Tier 1, and in order by stringency, are the following levels:
TLEV — Transitional Low-Emission Vehicle — More stringent for HC than Tier 1.
LEV — (also known as: LEV I) Low-Emission Vehicle , an intermediate California standard about twice as stringent as Tier 1 for HC and NO X .
ULEV (also known as ULEV I), — Ultra-Low-Emission Vehicle— A stronger California standard emphasizing very low HC emissions.
ULEV II — Ultra-Low-Emission Vehicle — A cleaner-than-average vehicle certified under the Phase II LEV standard. Hydrocarbon and carbon monoxide emissions levels are nearly 50% lower than those of a LEV II-certified vehicle. See Figure 61–1.
Figure 61–1 The underhood decal showing that this Lexus RX-330 meets both national Tier 2; BIN 5) and California LEV-II (ULEV) regulation standards. Continued NOTE: A battery-powered electric vehicle charged from the power grid will still be up to 10 times cleaner than even the cleanest gasoline vehicles over their respective lifetimes.
SULEV — Super-Ultra-Low-Emission Vehicle — A California standard even tighter than ULEV, including much lower HC and NO X emissions; roughly equivalent to Tier 2 Bin 2 vehicles.
ZEV — Zero-Emission Vehicle — A California standard prohibiting any tailpipe emissions. The ZEV category is largely restricted to electric vehicles and hydrogen-fueled vehicles. In these cases, any emissions that are created are produced at another site, such as a power plant or hydrogen reforming center, unless such sites run on renewable energy.
PZEV — Partial Zero-Emission Vehicle — Compliant with the SULEV standard; additionally has near-zero evaporative emissions and a 15-year/150,000-mile warranty on its emission control equipment.
Tier 2 standards are even more stringent. Tier 2 variations are appended with “II,” such as LEV II or SULEV II. Other categories have also been created:
ILEV—Inherently Low-Emission Vehicle
AT-PZEV—Advanced Technology Partial Zero-EmissionVehicle — If a vehicle meets the PZEV standards and is using high-technology features, such as an electric motor or high-pressure gaseous fuel tanks for compressed natural gas, it qualifies as an AT-PZEV. Hybrid electric vehicles such as the Toyota Prius can qualify, as can internal combustion engine vehicles that run on natural gas (CNG), such as the Honda Civic GX.
NLEV—National Low-Emission Vehicle — All vehicles nationwide must meet this standard, which started in 2001.See Tables 61.1, 61.2, and 61.3.
Continued See the table on Page 734 of your textbook.
See the tables on Page 735 of your textbook.
Smog Emission Information New vehicles are equipped with a sticker that shows the relative level of smog-causing emissions created by the vehicle compared to others on the market. Smog-causing emissions include unburned hydrocarbons (HC) and oxides of nitrogen (NO X ).
Figure 61–2 This label on a Toyota Camry hybrid shows the relative smog-producing emissions, but this does not include carbon dioxide (CO2), which may increase global warming. Continued Federal EPA Bin Number The higher the tier number, the newer the regulation; the lower the bin number, the cleaner the vehicle. The 2004 Toyota Prius is a very clean Bin 3, while the Hummer H2 is a dirty Bin 11.
California Standards The pre-2004 California Air Resources Board (CARB) standards as a whole were known as LEV I. Within that, there were four possible ratings: Tier 1, TLEV, LEV, and ULEV. The newest CARB rating system (since January 1, 2004) is known as LEV II. Within that rating system there are three primary ratings: LEV, ULEV, and SULEV. States other than California are given the option to use the federal EPA standards, or they can adopt California’s standards.
Europe has its own set of standards that vehicles must meet, which includes the following tiers:
Euro I (1992–1995)
Euro II (1995–1999)
Euro III (1999–2005)
Euro IV (2005–2008)
Euro V (2008+)
Vehicle emission standards and technological advancements have successfully reduced pollution from cars and trucks by about 90% since the 1970s.
EXHAUST ANALYSIS TESTING
The Clean Air Act Amendments require enhanced I/M programs in areas of the country that have the worst air quality and the Northeast Ozone Transport region. The states must submit to the EPA a State Implementation Plan ( SIP ) for their programs. Each enhanced I/M program is required to include as a minimum the following items:
Computerized emission analyzers
Visual inspection of emission control items
Minimum waiver limit (to be increased based on the inflation index)
Remote on-road testing of one-half of 1% of the vehicle population
Registration denial for vehicles not passing an I/M test
Denial of waiver for vehicles that are under warranty or that have been tampered with
OBD-II systems check for 1996 and newer vehicles
Federal Test Procedure ( FTP) The Federal Test Procedure ( FTP ) is used to certify all new vehicles before they can be sold. Once a vehicle meets these standards, it is certified by the EPA for sale in the United States. The FTP test procedure is a loaded-mode test lasting for a total duration of 505 seconds and is designed to simulate an urban driving trip. A cold start-up representing a morning start and a hot start after a soak period is part of the test.
The federal emission standards for each model year vehicle are the same for that model regardless of what size engine the vehicle is equipped with. This is why larger V-8 engines often are equipped with more emission control devices than smaller four- and six-cylinder engines.
In addition to this drive cycle, a vehicle must undergo evaporative testing. Evaporative emissions are determined using the Sealed Housing for Evaporative Determination ( SHED ) test, which measures the evaporative emissions from the vehicle after a heat-up period representing a vehicle sitting in the sun. In addition, the vehicle is driven and then tested during the hot soak period.
Continued NOTE: A SHED is constructed entirely of stainless steel. The walls, floors, and ceiling, plus the door, are all constructed of stainless steel because it does not absorb hydrocarbons, which could offset test results.
I/M Test Programs There are a variety of I/M testing programs implemented by the various states. These programs may be centralized testing programs or decentralized testing programs.
Figure 61–3 Photo of a sign taken at an emissions test facility. Continued Each state is free to develop a testing program suitable to their needs as long as they achieve the attainment levels set by the EPA. This approach has led to a variety of different testing programs.
Visual Tampering Checks Visual tampering checks may be part of an I/M testing program and usually include checking for the following items:
Fuel tank inlet restrictor
Exhaust gas recirculation (EGR)
Evaporative emission system
Air-injection reaction system (AIR)
Positive crankcase ventilation (PCV)
If any of these systems are missing, not connected, or tampered with, the vehicle will fail the emissions test and will have to be repaired/replaced by the vehicle owner before the vehicle can pass the emission test.
One-Speed and Two-Speed Idle Test The one-speed and two-speed idle test measures the exhaust emissions from the tailpipe of the vehicle at idle and/or at 2500 rpm. This uses stand-alone exhaust gas sampling equipment that measures the emissions in percentages. Each state chooses the standards that the vehicle has to meet in order to pass the test. The advantage to using this type of testing is that the equipment is relatively cheap and allows states to have decentralized testing programs because many facilities can afford the necessary equipment required to perform this test.
Loaded Mode Test Uses a dynamometer that places a “single weight” load on the vehicle. The load applied to the vehicle varies with the speed of the vehicle. Typically, a four-cylinder vehicle speed would be 24 mph, a six-cylinder vehicle speed would be 30 mph, and an eight-cylinder vehicle speed would be 34 mph.
Figure 61–4 A vehicle being tested during an enhanced emission test. Continued Conventional stand-alone sampling equipment is used to measure HC and CO emissions. This type of test is classified as a Basic I/M test by the EPA.
Acceleration Simulation Mode ( ASM ) The ASM - type of test uses a dynamometer that applies a heavy load on the vehicle at a steady-state speed. The load applied to the vehicle is based on the acceleration rate on the second simulated hill of the FTP. There are different ASM tests used by different states. The ASM 50/15 test places a load of 50% on the vehicle at a steady 15 mph. This load represents 50% of the horsepower required to simulate the FTP acceleration rate of 3.3 mph/sec. The ASM 25/25 test places a 25% load on the vehicle while it is driven at a steady 25 mph. This applies a smaller load on the vehicle at a higher speed, it will produce a higher level of HC and CO emissions than the ASM 50/15. NO X emissions will tend to be lower with this type of test.
I/M 240 Test The I/M 240 test is the EPA’s enhanced test. The “240” stands for 240 seconds of drive time on a dynamometer. This is a loaded-mode transient test that uses constant volume sampling equipment to measure the exhaust emissions in mass just as is done during the FTP. The I/M 240 test simulates the first two hills of the FTP drive cycle. Figure 61–5 shows the I/M 240 drive trace.
Figure 61–5 Trace showing the Inspection/Maintenance 240 test. The test duplicates an urban test loop around Los Angeles, California. The first “hump” in the curve represents the vehicle being accelerated to about 20 mph, then driving up a small hill to about 30 mph and coming to a stop. At about 94 seconds, the vehicle stops and again accelerates while climbing a hill and speeding up to about 50 mph during this second phase of the test. Continued
OBD-II Testing In 1999, the EPA requested states adopt OBD-II systems testing for 1996 and newer vehicles. The OBD-II system is designed to illuminate the MIL light and store trouble codes any time a malfunction exists that would cause the vehicle emissions to exceed 1-1⁄2 times the FTP limits. The EPA has determined that the OBD-II system should detect emission failures of a vehicle even before that vehicle would fail an emissions test of the type that most states are employing. The EPA has also determined that, as the population of OBD-II-equipped vehicles increases and the population of older non-OBD-II-equipped vehicles decreases, tailpipe testing will no longer be necessary.
The OBD-II computer is connected to the vehicle’s DLC connector and will scan the vehicle OBD-II system and determine if there are any codes stored that are commanding the MIL light on. It will scan the status of the readiness monitors and determine if they have all run and passed. If the readiness monitors have all run and passed, it indicates that the OBD-II system has tested all the components of the emission control system. An OBD-II vehicle would fail this OBD-II test if:
The MIL light does not come on with the key on, engine off
The MIL is commanded on
A number (varies by state) of the readiness monitors have not been run
Remote Sensing The EPA requires that, in high-enhanced areas, states perform on-the-road testing of vehicle emissions. The state must sample 0.5% of the vehicle population base in high-enhanced areas. This may be accomplished by using a remote sensing device. This type of sensing may be done through equipment that projects an infrared light through the exhaust stream of a passing vehicle. The reflected beam can then be analyzed to determine the pollutant levels coming from the vehicle. If a vehicle fails this type of test, the vehicle owner will receive notification in the mail that he or she must take the vehicle to a test facility to have the emissions tested.
Random Roadside Testing Some states may implement random roadside testing that would usually involve visual checks of the emission control devices to detect tampering. Obviously, this method is not very popular as it can lead to traffic tie-ups and delays on the part of commuters. Exhaust analysis is an excellent tool to use for the diagnosis of engine performance concerns. In areas of the country that require exhaust testing to be able to get license plates, exhaust analysis must be able to:
Establish a baseline for failure diagnosis and service.
Identify areas of engine performance that are and are not functioning correctly.
Determine that the service and repair of the vehicle have been accomplished and are complete.
A popular method of engine analysis, as well as emission testing, involves the use of five-gas exhaust analysis equipment.
The five gases analyzed and their significance follows.
EXHAUST ANALYSIS AND COMBUSTION EFFICIENCY Continued Figure 61–6 A partial stream sampling exhaust probe being used to measure exhaust gases in parts per million (ppm) or percent (%).
Hydrocarbons Hydrocarbons (HC) are unburned gasoline and are measured in parts per million (ppm). A correctly operating engine should burn (oxidize) almost all the gasoline; very little unburned gasoline should be present in the exhaust.
Spark plug wires
Distributor cap and rotor (if the vehicle is so equipped)
Ignition timing (if possible)
Acceptable levels of HC are 50 ppm or less. High levels of HC could be due to excessive oil consumption caused by weak piston rings or worn valve guides. The most common cause of excessive HC emissions is a fault in the ignition system. Items that should be checked include:
NMHC means non-methane hydrocarbon and it is the standard by which exhaust emission testing for hydrocarbons is evaluated. Methane is natural gas and can come from animals, animal waste, and other natural sources. By not measuring methane gas, all background sources are eliminated, giving better results as to the true amount of unburned hydrocarbons that are present in the exhaust stream.
What Does NMHC Mean?
Carbon Monoxide (CO) is unstable and will easily combine with any oxygen to form stable carbon dioxide (CO 2 ). The fact that CO combines with oxygen is the reason that CO is a poisonous gas (in the lungs, it combines with oxygen to form CO 2 and deprives the brain of oxygen).
Clogged air filter
Incorrect idle speed
Too-high fuel-pump pressure
Any other items that can cause a rich condition
CO levels of a properly operating engine should be less than 0.5%. High levels of CO can be caused by clogged or restricted crankcase ventilation devices such as the PCV valve, hose(s), and tubes. Other items that might cause excessive CO include
Carbon Dioxide ( CO 2 ) is the result of oxygen in the engine combining with the carbon of the gasoline. An acceptable level of CO 2 is between 12% and 15%. A high reading indicates an efficiently operating engine. If the CO 2 level is low, the mixture may be either too rich or too lean.
Continued Oxygen The next gas is oxygen (O 2 ). There is about 21% oxygen in the atmosphere, and most of this oxygen should be “used up” during the combustion process to oxidize all the hydrogen and carbon (hydrocarbons) in the gasoline. Levels of O 2 should be very low (about 0.5%). High levels of O 2 , especially at idle, could be due to an exhaust system leak.
Oxides of Nitrogen (NO X ) An oxide of nitrogen (NO) is a colorless, tasteless, and odorless gas when it leaves the engine, but as soon as it reaches the atmosphere and mixes with more oxygen, nitrogen oxides (NO 2 ) are formed. NO 2 is reddish-brown and has an acid and pungent smell. NO and NO 2 are grouped together and referred to as NO X , where x represents any number of oxygen atoms. NO X , the symbol used to represent all oxides of nitrogen, is the fifth gas commonly tested using a five-gas analyzer. The exhaust gas recirculation (EGR) system is the major controlling device limiting the formation of NO X .
Acceptable exhaust emissions include:
See the chart on Page 738 of your textbook. NOTE: Adding 10% alcohol to gasoline provides additional oxygen to the fuel and results in lower CO and higher O 2 levels in the exhaust.
Age and mileage of a vehicle are generally not factors when it comes to passing an exhaust emission test. Regular maintenance is the most important factor for passing an enhanced Inspection and Maintenance (I/M) exhaust analysis test. Failure of the vehicle owner to replace broken accessory drive belts, leaking AIR pump tubes, defective spark plug wires, or a cracked exhaust manifold can lead to failure of other components such as the catalytic converter. Tests have shown that if the vehicle is properly cared for, even an engine that has 300,000 miles (483,000 km) can pass an exhaust emission test.
How Can My Worn Out, Old, High Mileage Vehicle Pass an Exhaust Emissions test?
HC TOO HIGH
High hydrocarbon exhaust emissions are usually caused by an engine misfire. If a spark plug does not ignite, unburned fuel is pushed into the exhaust system. If any of the ignition components or adjustments are not correct, excessive HC emission is likely:
Defective or worn spark plugs or plug wires
Defective distributor cap and/or rotor
Incorrect ignition timing (either too far advanced or too far retarded)
A lean air–fuel mixture can also cause a misfire. This condition is referred to as a lean misfire.
HINT: To make discussion easier in future reference, this list of ignition components and checks will be referred to simply as “spark stuff.”
CO TOO HIGH
Excessive carbon monoxide is an indication of too rich an air– fuel mixture. High concentrations of CO indicate that not enough oxygen was available for the amount of fuel. Common causes of high CO include:
Too-high fuel-pump pressure
Defective fuel-pressure regulator
Clogged air filter or PCV valve
D efective injectors
HINT: One tech remembers “CO” as meaning “clogged oxygen” and always looks for restricted airflow into the engine whenever high CO levels are detected.
If the exhaust is rich, CO emissions will be higher than normal. If the exhaust is lean, O 2 emissions will be higher than normal. Therefore, if the CO reading is the same as the O 2 reading, then the engine is operating correctly. For example, if both CO and O 2 are 0.5% and the engine develops a vacuum leak, the O 2 will rise. If a fuel-pressure regulator were to malfunction, the resulting richer air–fuel mixture would increase CO emissions. Therefore, if both the rich indicator (CO) and the lean indicator (O 2 ) are equal, the engine is operating correctly.
CO Equals O 2
The amount of leftover oxygen coming out of the tailpipe is an indication of leanness. The higher the O 2 level, the leaner the exhaust. Oxygen therefore is the lean indicator . Acceptable levels of O 2 are 0% to 2%.
MEASURING OXYGEN (O 2 ) AND CARBON DIOXIDE (CO 2 ) Continued NOTE: A hole in the exhaust system can draw outside air (oxygen) into the exhaust system. Therefore, to be assured of an accurate reading, carefully check the exhaust system for leaks. Using a smoke machine is an easy method to locate leaks in the exhaust system. Carbon dioxide (CO 2 ) is a measure of efficiency. The higher the level of CO 2 in the exhaust stream, the more efficiently the engine is operating. Levels of 12% to 17% are considered acceptable.
Figure 61–7 Exhaust emissions are very complex. When the air–fuel mixture becomes richer, some exhaust emissions are reduced, while others increase. Continued
A hole in the exhaust system can dilute the exhaust gases with additional oxygen (O 2 ). This additional O 2 in the exhaust can lead the service technician to believe that the air–fuel mixture is too lean.
Find a Leak in the Exhaust System - Part 1 Figure 61–8 A hole in the exhaust system can cause outside air (containing oxygen) to be drawn into the exhaust system. This extra oxygen can be confusing to a service technician because the extra O2 in the exhaust stream could be misinterpreted as a too-lean air–fuel mixture.
To help identify an exhaust leak, perform an exhaust analysis at idle and at 2500 rpm (fast idle) and compare with the following:
Find a Leak in the Exhaust System - Part 2
If the O 2 is high at idle and at 2500 rpm, the mixture is lean at both idle and at 2500 rpm.
If the O 2 is low at idle and high at 2500 rpm, this usually means the vehicle is equipped with a working AIR pump.
If the O 2 is high at idle, but OK at 2500 rpm, indicates a hole in the exhaust or a small vacuum leak that is “covered up” at higher speed .
Using the nose, a technician can often home in on a major problem without having to connect the vehicle to an exhaust analyzer. For example:
Your Nose Knows - Part 1
The strong smell of exhaust is due to excessive unburned hydrocarbon (HC) emissions. Look for an ignition system fault that could prevent the proper burning of the fuel. A vacuum leak could also cause a lean misfire and cause excessive HC exhaust emissions.
Dizzy feeling or headache. This is commonly caused by excessive carbon monoxide (CO) exhaust emissions. Get into fresh air as soon as possible. A probable cause of high levels of CO is an excessively rich air–fuel mixture.
Using the nose, a technician can often home in on a major problem…
Your Nose Knows - Part 2
If your eyes start to burn or water, suspect excessive oxides of nitrogen (NOX) emissions. The oxides of nitrogen combine with the moisture in the eyes to form a mild solution of nitric acid. The acid formation causes the eyes to burn and water. Excessive NOX exhaust emissions can be caused by:
A vacuum leak causing higher-than-normal combustion chamber temperature
Overadvanced ignition timing causing higher-than-normal combustion chamber temperature
Lack of proper amount of exhaust gas recirculation (EGR) (This is usually noticed above idle on most vehicles.)
PHOTOCHEMICAL SMOG INFORMATION
Oxides of nitrogen are formed by high temperature—over 2500°F (1370°C)—and/or pressures inside the combustion chamber. Oxides of nitrogen contribute to the formation of photochemical smog when sunlight reacts chemically with NO X and unburned hydrocarbons (HC). Smog is a term derived by combining the words smoke and fog . Ground-level ozone is PART of smog. Ozone is an enriched oxygen molecule with three atoms of oxygen (O 3 ) instead of the normal two atoms of oxygen (O 2 ). Ozone in the upper atmosphere is beneficial because it blocks out harmful ultraviolet rays that contribute to skin cancer.
TESTING FOR OXIDES OF NITROGEN
Because the formation of NO X occurs mostly under load, the most efficient method to test for NO X is to use a portable exhaust analyzer that can be carried in the vehicle while the vehicle is being driven under a variety of conditions. Specifications for NO X From experience, a maximum reading of 1,000 parts per million (ppm) of NO X under loaded driving conditions will generally mean that the vehicle will pass an enhanced I/M roller test. A reading of over 100 ppm at idle should be considered excessive.
A commonly experienced problem in many parts of the country involves squirrels or other animals placing dog food into the air intake ducts of vehicles. Dog food is often found packed tight in the ducts against the air filter. An air intake restriction occurs and drives the fuel mixture richer than normal and reduces engine power and vehicle performance as well as creating high CO exhaust emissions.
Check For Dog Food?
A Toyota equipped with a double overhead camshaft (DOHC) inline six-cylinder engine failed the state-mandated enhanced exhaust emission test for NOX. The engine ran perfectly without spark knocking (ping), which is usually a major reason for excessive NOX emissions. The technician checked the following:
The Case of the Retarded Exhaust Camshaft - Part 1
The ignition timing, which was found to be set to specifications (if too far advanced, can cause excessive NOX)
The cylinders, which were decarbonized using top engine cleaner
The EGR valve, which was inspected and EGR passages cleaned
After all the items were completed, the vehicle was returned to the inspection station where the vehicle again failed for excessive NOX emissions (better, but still over the maximum allowable limit).
After additional hours of troubleshooting, the technician decided to go back to basics and start over again. A check of the vehicle history with the owner indicated that the only previous work performed on the engine was a replacement timing belt over a year before.
The Case of the Retarded Exhaust Camshaft - Part 2 The technician discovered that the exhaust cam timing was retarded two teeth, resulting in late closing of the exhaust valve. The proper exhaust valve timing resulted in a slight amount of exhaust being retained in the cylinder. This extra exhaust was added to the amount supplied by the EGR valve and helped reduce NOX emissions. After repositioning the timing belt, the vehicle passed the emissions test well within the limits.
See the chart on Page 740 of your textbook.
A tech was attempting to solve a driveability problem. The computer did not indicate any trouble codes (DTCs). A check of oxygen sensor voltage indicated higher-than-normal reading almost all the time. The pulse width to the port injectors was lower than normal.
O 2 Shows Rich, But Pulse Width is Low What could cause a rich mixture if the injectors were being commanded to deliver a lean mixture? Finally the technician shut off the engine and took a careful look at the entire fuel-injection system. Although the vacuum hose was removed from the fuel-pressure regulator, fuel was found dripping from the vacuum hose. The lower-than-normal pulse width indicates the computer is attempting to reduce fuel flow to the engine by decreasing the on-time for all injectors. The problem was a defective fuel-pressure regulator that allowed an uncontrolled amount of fuel to be drawn by the intake manifold vacuum into the cylinders. While the computer tried to reduce fuel by reducing the pulse width signal to the injectors, the extra fuel being drawn directly from the fuel rail caused the engine to operate with too rich an air–fuel mixture.
Excessive hydrocarbon (HC) exhaust emissions are created by a lack of proper combustion such as a fault in the ignition system, too lean an air–fuel mixture, or too cold engine operation.
Excessive carbon monoxide (CO) exhaust emissions are usually created by a rich air–fuel mixture.
Excessive oxides of nitrogen (NO X ) exhaust emissions are usually created by excessive heat or pressure in the combustion chamber or a lack of the proper amount of exhaust gas recirculation (EGR).
Carbon dioxide (CO 2 ) levels indicate efficiency. The higher the CO 2 , the more efficient the engine operation.
Oxygen (O 2 ) indicates leanness. The higher the O 2 , the leaner the air–fuel mixture.
A vehicle should be driven about 20 miles, especially during cold weather, to allow the engine to be fully warm before an enhanced emissions test.