The eccentric shaft sensor is a key component of BMW's VALVETRONIC system, which controls intake valve timing and duration. It monitors the rotation and position of the eccentric shaft. Failure of the eccentric shaft sensor can result in deteriorated engine performance, excess fuel consumption, excess emissions, and the engine warning light turning on. Signs of failure include sluggish engine performance, rough engine operation, and a sudden increase in fuel consumption. The eccentric shaft sensor is important for proper functioning of the VALVETRONIC system, so its upkeep is important.