What P0331 means
The knock sensor is a piezoelectric structure-borne sound sensor bolted to the engine block; the module evaluates its signal per cylinder and retards ignition when it detects knock.
How the engine computer detects it
Sensor B is the second sensor – bank 2 on V engines, but on some four-cylinders also bank 1. With a fault the module retards ignition across the board as protection – noticeable loss of power and higher consumption. P0331 means the circuit is electrically sound but the signal is implausible. GM sets it when the filtered background noise stays below a threshold – typically a loose sensor or one not bolted down properly – in the first trip; Saab's GM-based engines also when excessive knock persists on several cylinders or a whole bank for 8–10 s. Ford does not use this code.
Written from public technical sources and not yet checked by a mechanic. How we write these pages
Symptoms
- Check engine light on
- Loss of power, sluggish response from ignition retard
- Higher fuel consumption
Common causes
- Sensor loose, wrongly torqued or mounted with sealant or on a dirty surface Most likely
- Engine software or calibration (bulletins for some models) Possible
- Genuine knock on a whole bank or mechanical noise Possible
- Sensor aged or damaged (low sensitivity) Less likely
How to diagnose P0331
- Read every stored code with an OBD-II scanner. If misfire or fuel trim codes are there too, fix those first: they often cause this one.
- Inspect the parts involved: knock sensor b, knock sensor wiring, engine control module.
- Work through the causes above from most to least likely, cheapest checks first.
- Clear the code and drive a full drive cycle to confirm the repair held.
Reading the code
- P
- Powertrain
- 0
- Generic (SAE) code
- 3
- Ignition system or misfire
- 31
- Specific fault
Questions drivers ask
Can I drive with code P0331?
Usually yes for a short time, if the car drives normally, the temperature gauge reads normal and the check engine light is steady. If the light flashes, the engine loses power or overheats, or other warning lights come on, pull over and have it checked.
Will my car pass an emissions inspection with P0331?
Probably not in states that test vehicles with an OBD-II scan: an emissions-related code that keeps the check engine light on is usually an automatic fail. Rules differ by state and county, so check yours.
Can I fix P0331 myself?
Often yes, with an OBD-II scanner and basic tools. Difficulty: medium. Start with the cheapest checks before replacing expensive parts.
This page is general information, not a substitute for a professional diagnosis. Repair costs are rough estimates for parts and labor and vary by vehicle and region.