A knock sensor is a device inside your engine that detects the symptoms of pre-detonation, a condition that occurs when the mixture of air and fuel inside a combustion chamber ignites sooner than it is supposed to. Another name for predetonation is “engine knock”.
Your engine is constantly going through four distinct cycles - intake, where each piston in its turn moves downward, drawing air and fuel into the combustion chamber; compression, where the piston moves back up to compress the fuel vapor; combustion, where the spark plug ignites the compressed air and fuel, causing a rapid expansion of gas that drives the piston downward; and exhaust, where the piston moves up again, expelling the byproducts of combustion out through the exhaust system.
If the timing of the combustion cycle (also known as the “ignition” or “power” cycle) is off, the result can be poor fuel efficiency, a reduction in engine power, and even serious internal engine damage. Fortunately, when pre-detonation or pre-ignition occurs, vibrations associated with the event can be felt by the knock sensor. The sensor then communicates with the engine control module (the computer that manages engine function) that adjusts ignition timing, air/fuel mixture, and other engine factors to correct the problem.
The most common sign of a bad knock sensor is when the check engine light comes on. Not only does the knock sensor continuously monitor the engine for signs of vibration related to pre-detonation, but the engine control module that it communicates with continuously monitors the health of the sensor itself. Other common signs of a bad knock sensor include:
Replacing a knock sensor itself is not a particularly difficult task. It is usually bolted to the engine block and can be unfastened, unplugged, and removed. The new sensor is installed in its place. Access to the component, however, can be a far more involved process on many vehicles, making knock sensor replacement tricky.
For instance, it might be necessary for a technician to remove the entire intake assembly on top of your engine to access the knock sensor. The throttle body, intake plenum, hoses, wires, and more might need to be removed. Other vehicles might need to be raised and supported in the air so that the engine can be accessed from below. The starter assembly or other components might need to be removed to get to the knock sensor.