Motor Vehicle Expert
OBD fault code guide

P0325 Code Meaning UK

P0325 means “Knock Sensor Circuit Malfunction”. In simple terms, the engine computer has detected a problem with the knock sensor signal. The knock sensor helps protect the engine from detonation, pinking or abnormal combustion by allowing the ECU to adjust ignition timing.

📅 Updated June 2026 🇬🇧 UK diagnostic guide 🛠 Knock sensor circuit advice
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✓ Knock sensor fault explained ✓ UK repair cost guide ✓ MOT-aware advice ✓ Heavy internal linking included
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Quick answer

What does P0325 mean?

P0325 means the ECU has detected a problem with the knock sensor circuit. The knock sensor listens for abnormal combustion, often called knock, pinking, pinging or detonation. If the ECU cannot trust the knock sensor signal, it may reduce power, adjust ignition timing or store an engine management fault.

Do not assume P0325 always means the sensor itself has failed. A faulty knock sensor is common, but damaged wiring, corrosion, poor connector contact, incorrect sensor torque, engine vibration, poor fuel quality or ECU issues can also trigger the code.

Most common area

Knock sensor, wiring, connector corrosion, poor earth or incorrect sensor fitting.

Main risk

The ECU may not detect detonation properly, so engine protection can be reduced.

Best first check

Scan all codes, inspect wiring, check connector condition and confirm sensor signal before replacing parts.

Code meaning

P0325 — Knock sensor circuit malfunction

The knock sensor is usually mounted to the engine block. It detects vibration patterns caused by abnormal combustion. When knock is detected, the ECU can adjust ignition timing to protect the engine.

P0325 usually means the ECU has detected an electrical or signal problem in the knock sensor circuit. It does not always mean the engine is actually knocking, but it does mean the knock detection system is not working as expected.

Mechanic view

Why P0325 should not be ignored

A knock sensor fault can make the engine feel flat because the ECU may use a safer ignition strategy. On some vehicles the car may drive almost normally, but if the engine does start knocking, the ECU may not react correctly.

This is why P0325 is more than just a warning light. It is part of the system that helps protect pistons, valves, head gasket, spark plugs and the catalytic converter from combustion-related damage.

Symptoms

Common symptoms of a P0325 code

Symptoms vary depending on the vehicle. Some cars only show an engine warning light, while others feel sluggish or lose power.

Engine management light

The most common symptom is a steady check engine light or engine management light.

Reduced power

The ECU may pull ignition timing or enter a safer running mode, making the car feel sluggish.

Pinging or knocking noise

A metallic pinging noise under load may suggest real knock, poor fuel, overheating or timing issues.

Poor acceleration

The car may feel slow to accelerate or weak uphill if ignition timing is being limited.

Misfire symptoms

Knock, poor combustion or timing issues can sometimes appear alongside misfire-related symptoms.

Higher fuel use

If the ECU changes timing strategy, fuel economy may suffer.

Common causes

What causes P0325?

P0325 can be caused by the knock sensor itself, but the circuit, wiring, connector and engine conditions should be checked too.

Very common

Faulty knock sensor

The internal sensor can fail or stop producing the expected signal.

Electrical

Damaged wiring

Heat, oil leaks, rubbing or previous repair work can damage knock sensor wiring.

Connector

Corrosion or poor contact

A loose, wet or corroded connector can interrupt the knock sensor signal.

Fitting issue

Incorrect sensor torque

Some knock sensors must be tightened correctly. Too loose or too tight can affect signal accuracy.

Fuel quality

Poor fuel or wrong octane

Poor fuel can cause real knock, especially under load or on turbocharged engines.

Engine condition

Overheating or carbon build-up

Excessive heat or deposits can increase knock tendency and confuse diagnosis.

Safe to drive?

Can you drive with P0325?

Short careful driving may be possible if the car runs normally and there are no knocking noises, misfires, overheating or power-loss symptoms. However, P0325 should not be ignored because the knock sensor helps protect the engine from abnormal combustion.

✅ Lower risk: steady engine light, no knocking noise, no overheating and normal running.

⚠️ Medium risk: sluggish acceleration, poor fuel economy or repeated fault code after clearing.

🚫 Higher risk: metallic knocking, flashing engine light, overheating, misfire or major power loss.

Can You Drive With Engine Light? Engine Management Light Guide
Important warning

Knocking noises need quick attention

If you hear a metallic knocking, pinging or rattling noise under acceleration, do not treat it as just a sensor code. Real engine knock can damage pistons, valves, spark plugs, head gasket and catalytic converter if ignored.

A P0325 code with overheating, misfires or loss of power should be diagnosed quickly.

Car Losing Power Engine Misfire Symptoms
UK repair costs

Typical UK repair costs for P0325

Costs vary depending on engine access, diagnostic time, sensor location and whether the fault is wiring or sensor-related.

Diagnostic scan and inspection

Typical range: £40–£120.

Knock sensor replacement

Typical range: £80–£300+ depending on access and part cost.

Wiring or connector repair

Typical range: £60–£250 depending on fault tracing time.

Intake removal labour

Typical range: £150–£500+ if the knock sensor is buried under the inlet manifold.

Engine knock investigation

Typical range: £80–£250+ if real combustion knock or mechanical noise needs checking.

Additional repairs

Costs vary if overheating, misfire, timing or fuel-related faults are also present.

Repair Costs Guide UK
Diagnosis flow

How to diagnose P0325 properly

✅ Scan all stored, pending and permanent codes.

✅ Record freeze-frame data before clearing anything.

✅ Check whether P0325 appears with misfire, lean, timing or catalyst codes.

✅ Inspect knock sensor wiring for heat damage, oil contamination or rubbing.

✅ Check connector condition, corrosion and pin fit.

✅ Confirm power, ground and signal circuit where applicable.

✅ Check sensor mounting and torque if it has recently been replaced.

✅ Listen for real knocking, pinking, overheating or mechanical engine noise.

✅ Confirm fuel quality and related engine running faults before replacing parts blindly.

Real experience

What I would check first

On a real P0325 fault, I would not just fit a sensor straight away. I would first inspect the wiring and connector because knock sensors often sit in hot, awkward areas of the engine. Oil contamination, brittle wiring or a poor connector can cause the same code.

If the car also has lean codes such as P0171 or P0174, misfire codes such as P0300 to P0304, or catalyst code P0420, I would diagnose those together. A poor-running engine can create knock-related symptoms, and a knock sensor fault can make the car feel low on power.

P0171 Lean Code P0300 Misfire Code P0420 Catalyst Code
MOT impact

Will P0325 fail an MOT?

P0325 itself is not normally what directly fails an MOT. The MOT issue is what the fault causes. If the knock sensor fault causes an emissions-related engine warning light, poor running, misfire, visible smoke or emissions problems, it can become an MOT concern.

⚠️ Higher risk: engine warning light, misfire, rough running, smoke or poor emissions.

⚠️ Medium risk: reduced power, repeated fault code or related lean/misfire codes.

✅ Lower risk: repaired fault, no warning light and normal engine running.

Engine Light MOT Guide Common MOT Failures
Used car buying advice

P0325 on a car you want to buy

If a used car has P0325 stored, treat it as a proper diagnostic point, not just a small sensor issue. The repair may be simple, but on some engines the knock sensor is difficult to access and labour can be expensive.

Before buying, check whether the car has loss of power, misfires, overheating, poor fuel economy, previous engine repairs or other stored codes.

Used Car Buying Guides Inspection Checklist
Frequently asked questions

FAQs about P0325

Common questions about P0325 knock sensor circuit malfunction, safe driving, repair costs and MOT impact.

What does code P0325 mean?

P0325 means the engine computer has detected a problem with the knock sensor circuit.

Can I drive with P0325?

Short careful driving may be possible if the car runs normally, but knocking noises, overheating, misfires or major power loss should be treated as urgent.

What is the most common cause of P0325?

A faulty knock sensor is common, but wiring, connector corrosion, poor sensor mounting and engine noise can also cause it.

Can P0325 cause loss of power?

Yes. The ECU may reduce ignition timing or use a safer strategy if it cannot trust the knock sensor signal.

Will P0325 fail an MOT?

It can contribute to an MOT issue if it causes an engine warning light, poor running, misfire, emissions problems or visible smoke.

Should I replace the knock sensor first?

Not before checking the wiring, connector, sensor mounting and related engine faults. Diagnosis should come before parts replacement.

About this guide

Written for practical UK fault finding

Motor Vehicle Expert explains diagnostic trouble codes in clear, mechanic-style language for UK drivers. This P0325 guide is designed to help you understand knock sensor circuit faults, likely causes, safe driving advice, repair costs and MOT implications before replacing parts.

Fault codes should always be treated as a diagnostic starting point. Knock sensor faults can involve sensors, wiring, connector condition, engine vibration, fuel quality and related running faults, so proper testing is better than guessing.