The knock sensor is an essential component of the engine management system, there to detect and manage spark knock. Spark knock is what happens when an air/fuel mixture ignites too early, or if the fuel is too low octane (which allows it to burn too quickly), or if the combustion is uneven, leading to colliding combustion wavefronts. Problems in the fuel mixture or the engine timing are common causes, but fuel quality and mechanical or sensor malfunctions can also be causes. The result is a shockwave that reverberates through the engine.
Knock sensors are simple sensors that respond to shock waves. Some engines use one, while larger engines often use two, and they are typically mounted in the engine block close to the crankshaft, so they can be difficult to access. Where access is restricted and there are two knock sensors, most of the time they are both replaced.
Knock sensors can also be delicate; they can be damaged if dropped or over-tightened. Because their position on the engine is often low down in between two cylinder banks, some models are prone to being damaged by water intrusion if the engine is washed.
As the knock sensor detects shock waves in the engine block, anything else that causes shock waves could lead to a false code or unusual running problems. A bad motor mount, for instance, can cause enough vibration in the engine to trigger the knock sensor. Then the engine can try to compensate by retarding the timing, which reduces power and fuel mileage. When this attempt doesn’t work, the PCM may turn on the engine light to check the knock sensor in spite of its not being the cause. Other things that can cause spark knock are excess carbon in the cylinders, timing problems, mechanical problems like a bad rod bearing, and overheating issues.
Costs of Knock Sensor Replacement
On average, for most vehicles, the cost of replacing a knock sensor is about £345. That includes an hour of diagnosis, as the sensor is rarely replaced except as the result of an engine light, and there is generally time involved in narrowing down other possibilities.
For some more specific examples on common vehicles, using £70 to £100 an hour as a labour rate and including a standard diagnostic charge:
- For a 2010 Ford F150 with a 5.4-liter engine, there are two knock sensors underneath the intake manifold, and the labour to replace them is 3 hours. A factory sensor lists for around £40, and a non-OE part costs about £30. This makes the job of replacing the sensors about £250 using OE parts or about £240 using aftermarket parts.
- For a 2003 Toyota Camry with a 2.4-liter engine, the labour to replace the knock sensor is 1.4 hours. A factory sensor lists for £210, and a non-factory part costs about £95. This makes the job about £310 using OE parts, or about £200 using aftermarket parts.
- For a 2011 Grand Caravan with a 3.6-liter engine, there are two knock sensors, and the labour to replace them is 1.6 hours. A factory part is listed for £40, and an aftermarket part costs about £15. This makes the cost of replacing the knock sensors about £150 using factory parts or about £130 using aftermarket parts.
Signs that a Knock Sensor Needs to be Replaced
In almost all cases, the first thing that would be noticeable is an engine light. In most cases where the PCM is unable to rely on the information from the knock sensor, it typically “de-tunes” its engine management, so the engine light would be accompanied by a reduction in power and poor fuel mileage. In other cases, a bad knock sensor could cause an engine knock if the PCM is relying on bad information or the absence of information.
The diagnostic procedure for a knock sensor code is pretty straightforward, mostly involving ruling out a few other simple possible causes and replacing the sensor.
Yes, though engine power and efficiency will be negatively affected until the knock sensor is replaced.
Yes, in almost all cases. The PCM closely monitors the information from the knock sensor and verifies its plausibility against engine timing and other sensor data.
Yes, any mechanical problem that causes the knock sensor to signal a problem, which the PCM is then unable to remedy, may set a knock sensor code. Bad rod bearings or bad engine mounts are examples.