Knock Sensor
What it is
A knock sensor is a microphone-like sensor bolted to the engine block that listens for the vibration of detonation (pinging). Most engines have one or two.
What it does
It lets the computer hear knock and pull back ignition timing to protect the engine. A failed knock sensor sets P0325/P0327 and usually makes the computer retard timing as a precaution — costing power and economy.
Symptoms of failure
- Codes P0325 or P0327 (knock sensor circuit)
- Reduced power and acceleration
- Worse fuel economy
- Pinging or knocking under load (if real knock is present)
- Check engine light with no obvious drivability change
Common fault codes
Which vehicles need it
Most modern petrol engines. On many V6/V8s the sensor sits under the intake manifold, raising labour cost.
Replacement cost
| DIY (part only) | $25–$120 |
|---|---|
| At a shop (parts + labor) | $200–$600 |
| Replacement interval | No set interval — replace on failure. |
| DIY difficulty | Moderate–Hard (DIY) — the bolt is easy, but access is often under the intake manifold |
| Recommended brands | Bosch, Denso, Standard Motor Products, NTK |
Where to buy the part
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Frequently asked questions
Is it safe to drive with a knock sensor code?
Usually short-term, but the computer often retards timing as a precaution, so you lose power and economy. If the engine is actually knocking under load, that can cause damage — do not ignore it for long.
Why is a knock sensor expensive to replace at a shop?
The part is cheap, but on many engines the sensor sits under the intake manifold, so the labour to remove and refit the manifold drives the cost up.