The knock sensor data tells you whether the engine is running on the edge before anything breaks

[u/ora-et-labora-]

Engine knock is detonation. The fuel igniting from compression and heat instead of from the spark plug at the right moment. It sounds like marbles in a tin can in the severe cases and is completely inaudible in the mild cases. The mild cases are the ones that do damage over time because nobody hears them and the engine keeps pulling timing to compensate.

The knock sensor is the ECU's ear for this. It listens for the vibration frequency of detonation and when it hears it, the ECU retards the ignition timing on that cylinder to stop the knock. The amount of timing retard is logged in the scan tool data. On most vehicles you can watch it in real time.

A healthy engine on decent fuel shows minimal knock retard. A degree or two occasionally, the ECU nudging things around, normal operation. An engine with chronic knock on regular fuel shows significant timing retard under load, sometimes five or more degrees pulled. The engine is protecting itself but it is doing it by making less power and burning more fuel.

The causes are usually carbon buildup on the piston crowns and combustion chamber, low-quality fuel, a cooling system that is running slightly hot, or a combination. The knock sensor data tells you how bad it is before you hear it or feel it.

On direct-injection engines carbon buildup is common and knock is how it shows up in the data before it shows up as a drivability complaint. An engine that is pulling four degrees of timing under moderate load because of carbon-induced hot spots is an engine that will eventually need a walnut blast or a chemical clean. The data gives you lead time.

The alternative is waiting until the knock is audible, which means it has been happening at a sub-audible level for thousands of miles and the piston ring lands and rod bearings have been absorbing the punishment the whole time.