A knock sensor is a small vibration sensor bolted to the engine block that listens for detonation (knock or pinging). When it hears knock, the engine computer retards ignition timing to protect the pistons. A failing knock sensor or a damaged harness sets codes such as P0325, and the computer usually pulls timing as a precaution, so the engine feels flat and uses more fuel.
Bad knock sensor symptoms
- Check engine light with a knock sensor code
- Reduced power and worse fuel economy as timing is retarded
- Audible pinging under load if detonation is not being controlled
- Hesitation when accelerating on some vehicles
Knock sensor codes
| Code | Meaning |
|---|---|
| P0325 | Knock sensor 1 circuit, bank 1 or single sensor |
| P0327 | Knock sensor 1 circuit low |
| P0332 | Knock sensor 2 circuit low, bank 2 |
Where is the knock sensor?
Knock sensors bolt to the block where they can hear every cylinder. Inline engines often have one on the side of the block under the intake. V engines usually have two, often in the valley under the intake manifold or on the sides of the block below the exhaust manifolds. On GM LS-family truck V8s they sit in the valley under the intake, and corroded harness connectors there are a well-known cause of P0327 and P0332.
How to test a knock sensor
- Inspect the harness and connector first; wiring damage is more common than a failed sensor.
- Some sensors can be checked for resistance against specification; many are piezoelectric and are more easily checked for an AC signal with a scope while lightly tapping the block near the sensor.
- Watch knock retard on a scan tool: constant retard on a quiet engine suggests a noisy signal or a loose part nearby.
Replacing a knock sensor
- Torque the sensor exactly to specification; over- or under-tightening changes how well it hears knock.
- Clean the mounting surface and do not use thread sealant unless specified.
- On engines where the sensors sit under the intake, replace the harness at the same time if it is brittle or corroded.
Knock sensor vs engine knock
The knock sensor listens for detonation, which is a sharp pinging under load. A deep knocking that follows engine speed is usually mechanical, such as a worn rod bearing, and no sensor will fix it; see engine knocking and spun rod bearings. Use the fuel octane your engine requires, since low-octane fuel in an engine built for premium causes real detonation. If a bearing has failed, compare a replacement on our engines page.
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Trouble codes: P0326 Knock Sensor 1 Range/Performance · P0328 Knock Sensor 1 Circuit High · P0324 Knock Control System Error · P0330 Knock Sensor 2 Circuit Bank 2 · P0333 Knock Sensor 2 High Bank 2 · all OBD2 codes.
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