OBD-II trouble code
P0325: Knock Sensor 1 Circuit (Bank 1 or Single Sensor)
The engine computer detected an electrical fault with the knock sensor. Without knock detection, the ECM pulls back ignition timing as a safety measure, which reduces power and hurts fuel economy. The engine still runs, but not at its best.
Quick facts
- System
- Powertrain
- Category
- Sensors / Ignition
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $150 – $800
- DIY difficulty
- Intermediate DIY
Browse every code in P0300–P0394, or start from the full code library.
What does P0325 mean?
The knock sensor is a piezoelectric vibration sensor mounted on the engine block (or sometimes on the cylinder head) that listens for the high-frequency vibration patterns produced by engine knock — abnormal combustion where the air-fuel mixture detonates rather than burning smoothly. When the engine control module (ECM) detects knock, it pulls back ignition timing on the affected cylinders to stop the detonation.
P0325 is set when the ECM detects a circuit fault with knock sensor 1 — either no signal, an open circuit, a short to ground, or a short to power. Without a working knock sensor, the ECM can't safely advance ignition timing for optimal power and efficiency. Instead, it falls back to a conservative timing strategy that protects the engine but costs noticeable power and fuel economy.
The most common cause is a failed knock sensor — they're exposed to engine heat, vibration, and contamination over the years, and they eventually wear out. After the sensor itself, damaged or chafed wiring is the next most common cause, followed by corroded connectors. On certain engines, the knock sensor sits under the intake manifold, making replacement an expensive job because the manifold must come off.
Common causes
- Corroded or loose sensor connector
- Damaged or chafed sensor wiring
- Damaged sensor harness from rodent activity
- Improperly torqued knock sensor (over- or under-tightened — torque spec matters for these)
- Failed knock sensor, common after 100,000-150,000 miles and the usual answer once the wiring and the mounting are clear
- Coolant or oil contamination on the sensor (especially on engines where it sits below the intake)
- Faulty ECM input circuit (rare)
Symptoms
- Check engine light is on
- Reduced power and slower throttle response
- Lower fuel economy
- Possible engine pinging or knocking under load (ECM can't pull timing on detected knock)
- Slightly rough running in extreme cases
- Failed emissions test in some jurisdictions
Diagnostic steps
- 1.Inspect the knock sensor connector and visible wiring for damage, corrosion, or signs of rodent activity.
- 2.On engines with the knock sensor under the intake manifold, plan for intake removal as part of any deeper diagnosis.
- 3.Test knock sensor resistance with a multimeter against the manufacturer's specification — many piezoelectric sensors read in the hundreds of kilohms to a few megohms, and some carry an internal resistor purely so the module can watch the circuit. Read the result the right way round: a value far outside specification, or an open or a short, condemns the sensor, but a value inside specification does not clear it. A piezo element fails by losing output, not by changing resistance, so an in-spec sensor can still be dead.
- 4.Tap the block near the sensor with a light hammer or a wrench handle, ignition on and engine off, while watching the knock signal or the knock-retard value on the scan tool. A sensor that responds to tapping is still generating output; one that never moves is not, whatever its resistance measured.
- 5.Verify the knock sensor torque against the manufacturer's specification. Over- or under-tightened sensors produce inaccurate signals.
- 6.If the signal has to be watched under real load on the road, have a passenger read the tool rather than reading it yourself while driving. Note also that many tools show knock retard rather than a raw sensor voltage, so know which channel you are looking at before drawing any conclusion from a flat trace.
- 7.If wiring and torque check out, replace the knock sensor with an OEM or known-quality part. Aftermarket sensors of unknown quality are a frequent source of repeat failures.
Repair cost
$150 – $800
Knock sensor replacement is $150 to $400 on engines with easy access. On engines where the sensor sits under the intake manifold (common on V6 and some V8 engines), the same replacement can run $400 to $800 because of the labor required to remove and reinstall the intake. OEM sensors are worth the premium on this one — knock sensor quality matters.
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Related repairs
DIY vs shop
This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.