EXPERT IN AUTO PARTS
15 August 2025 (updated) Table of Contents
The knock sensor is an important monitoring device in modern internal combustion engines. Mounted directly on the car engine block, it listens for unusual sounds, specifically engine knocking or "pinging", and sends a signal to the engine control unit (ECU) to adjust the ignition timing accordingly.
This helps protect the engine from serious damage and maintains efficient performance. In this article, we’ll explain how knock sensors operate, what happens when they fail, and what to do if your vehicle displays a fault code like P0325.
When combustion in the engine cylinders doesn’t happen as intended, such as when the air-fuel mixture ignites too early due to excessive heat, it creates sharp pressure waves. These generate a distinctive knocking sound. The knock sensor detects this sound and alerts the ECU, which responds by adjusting the ignition timing to prevent damage.
The sensor is usually a piezoelectric device that converts vibrations into electrical signals. It may have one or two terminals. A single-terminal sensor grounds itself through its body, while a dual-terminal design includes both a ground and a signal wire. These signals are monitored by the Active Control Engine Mount (ACM) or ECU, which expects a stable signal under normal conditions. Irregularities indicate either knocking or a sensor malfunction.
A faulty knock sensor often triggers the check engine light on your dashboard. Other symptoms may include:
However, in some cases, the only sign may be a diagnostic trouble code (DTC), such as P0325, without any noticeable changes in driving behaviour. The onboard computer will need to be scanned using an OBD2 scanner to find the source of the error.
The P0325 code indicates a malfunction in the knock sensor circuit, specifically for sensor 1 in bank 1. This can arise from several causes:
In some cases, this issue may be caused by legitimate engine problems, which show symptoms that are similar to a faulty sensor. For example, causes may range from ignition and timing faults to the use of poor-quality fuel.
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-52%
RIDEX Knock Sensor
without cable
Article number: 3921K0003
compared to
RRP
£21.10
£9.99
Price per item
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-57%
RIDEX Knock Sensor
without cable
Article number: 3921K0006
compared to
RRP
£28.50
£11.99
Price per item
price incl. 20% VAT, excl. delivery costs
-58%
RIDEX Knock Sensor
without cable
Article number: 3921K0020
compared to
RRP
£29.20
£11.99
Price per item
price incl. 20% VAT, excl. delivery costs
-59%
RIDEX Knock Sensor
Article number: 3921K0016
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RRP
£32.20
£12.99
Price per item
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BOSCH Knock Sensor
Article number: 0 261 231 188
£17.49price incl. 20% VAT, excl. delivery costs
Before jumping to a replacement, a proper diagnosis is essential to avoid unnecessary expenses. Start with the following steps:
If the sensor proves to be faulty, replacing usually isn’t so complicated. Depending on the vehicle make and model, as well as your experience, you may be able to perform the replacement yourself using basic tools. If you’re unsure, it's best to leave the job to a qualified mechanic.
It’s possible, though unlikely, for the P0325 code to vanish if it was triggered by a temporary glitch. However, ignoring it may lead to undetected knocking and long-term engine damage. Repeated exposure to knocking and irregular combustion can harm pistons, bearings, and other key components, resulting in costly repairs.
While your car might still be drivable, continuing to operate a vehicle with this code is not recommended. Driving with a faulty knock sensor prevents the ECU from adjusting ignition timing to prevent engine damage. If the sensor has failed, replace it as soon as possible.
The cost of replacing a knock sensor depends on your vehicle make and whether you choose to do it yourself or hire a professional. The sensor itself typically starts at around £33. Labour charges will vary depending on your car model and your mechanic’s hourly rate.
A DIY replacement can save you money, especially if the sensor is easy to access. However, some vehicles require partial engine disassembly, which is better handled by an experienced mechanic.
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BOSCH Knock Sensor
Article number: 0 261 231 226
£20.99price incl. 20% VAT, excl. delivery costs
BOSCH Knock Sensor
Article number: 0 261 231 036
£28.99price incl. 20% VAT, excl. delivery costs
BOSCH Knock Sensor
Article number: 0 261 231 007
£34.99price incl. 20% VAT, excl. delivery costs
-51%
Article number: 106812
compared to
RRP
£46.99
£22.99price incl. 20% VAT, excl. delivery costs
Article number: ESS-NS-004
£12.99price incl. 20% VAT, excl. delivery costs
A malfunctioning knock sensor might not be the only issue if your engine is displaying performance problems. It’s useful to understand related parts such as the crankshaft sensor or O2 sensor. Working together, these sensors provide vital data to the ECU, helping ensure your car runs smoothly.
The knock sensor is a vital line of defence for your engine, safeguarding it against abnormal combustion that could lead to serious damage. Understanding how it works, recognising the symptoms of failure, and taking prompt action when issues arise can save you time, money, and headaches in the long run.
Crankshaft Sensor: what you need to know
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