How to diagnose a faulty knock sensor

What is a knock sensor

Knock sensors detect engine vibration or detonation. Learn in detail what they are and how they work here, but in a nutshell, they send a signal to the engine control module (ECM) to prevent engine knock. 

A faulty knock sensor can seriously affect an engine’s condition. Here we’ll explain how to diagnose the symptoms and advise on the best fix.

What does the knock sensor do?

The knock sensor is designed to detect signs of engine ‘knock’ – pre-ignition and other explosions outside of the main firing process. 

The knock sensors work by containing a stack of piezoelectric crystals that create signals in voltage when compressed by engine knock. These signals are sent to the ECU, which then optimises the air/fuel mix to prevent further occurrences. This achieves maximum vehicle performance and fuel economy, as well as reducing emissions.

Is the knock sensor important?

Yes, a knock sensor is essential for preventing engine knock. These detonations can severely damage an engine, causing it to run inefficiently, with less power and significantly reduce its lifespan.

What are the most common symptoms of a faulty knock sensor?

There are some signs to look out for that a knock sensor could be faulty. These telltale signs tend to affect engine performance and overall condition. Any of the symptoms below could signal that the knock sensor needs a look. 

‘Check engine’ light is on 

Any problem picked up by a sensor – or any problem with the sensor itself – is relayed to the ECU. The ECU then signals the driver’s attention to a problem by illuminating the ‘check engine’ light. 

Engine knocking

The knock sensor is equipped to deal with engine knock by having the ECU adjust the air/fuel mix in the cylinder, thus reducing the amount of misfiring and the amount of knock. Increased engine knock – an excessive knocking or vibrating – may mean the sensor isn’t functioning correctly to send that signal.

Decreased acceleration

If the knock sensor isn’t working, the ECU can’t tell if it’s got the right mix of air and fuel in the cylinder. To protect the engine, the ECU will hamper the ignition timing, protecting the engine from unnecessary damage but removing the oomph from the engine.

Reduced engine power

The loss of power isn’t limited to trouble accelerating. The vehicle’s onboard electronics are acting to preserve the engine and prevent further damage, which means reduced torque throughout the journey.

Lower fuel economy

With its acceleration reined in, the vehicle’s fuel economy will drop too, as the engine isn’t burning the fuel as efficiently as it could be if the knock sensor were operational.

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Issues could be caused by incorrect installation or use of a faulty component, such as a knock sensor with bad wiring. 

Excessive vibration could have shaken loose a connection, caused damage to the sensor or could be preventing the sensor from doing its job.

How to diagnose a knock sensor

If any of the above common symptoms are occurring, it’s recommended to test the knock sensor to see whether or not there is a fault. There are several ways to do this.

Test with a multimeter

Use this to check the wiring of the sensor to the control unit. Connect to the wiring harness and then measure the resistance values. A normal resistance reading should be around the 0.1 ohms mark. Any wide deviation would point to a wiring harness fault.

Test with an oscilloscope

With the engine running and idle, connect the test probes of the oscilloscope to the knock sensor and then record the signal waveform it produces. If it fails to display a knock signal, it suggests a fault. As a further test, while still recording, tap gently against the engine block near the sensor. If that knocking isn’t detected either, you have a defective sensor or circuit on your hands.

Test with a scan tool

This is a common and straightforward way to diagnose a faulty knock sensor. With a diagnostic scanner, you can quickly check for error codes stored in the ECM. A code related to a knock sensor would indicate a fault. You may wish to then test the sensor with the methods outlined above.

Error code is stored

• P0325: Knock Sensor Malfunction (Sensor 1, Bank 1), indicating a problem with the electrical circuit of the knock sensor on bank 1.
• P0326: Knock Sensor Range/Performance (Sensor 1, Bank 1), indicating that the signal from the knock sensor on bank 1 is outside the normal range.
• P0327: Knock Sensor Low Input (Sensor 1, Bank 1), indicating that the signal from the knock sensor on bank 1 is weak.
• P0328: Knock Sensor High Input (Sensor 1, Bank 1) indicating that the signal from the knock sensor on bank 1 is too high.

Knock sensor replacement