Knock sensors monitor the combustion chamber for engine-damaging knock and are piezoelectric microphones tuned to the frequencies of engine knock or detonation. When engine knock is detected, the ignition spark is retarded by the ECM. In a 6-cylinder engine, knock sensors are bolted to the left side of the cylinder block under the intake manifold, while in a V8 M62 TU engine, there are four sensors, with two located per bank of cylinders, positioned under the intake manifold.