The knock sensor is an essential component of the engine management system, its purpose being to detect and manage spark knock. Spark knock is what happens when an air/fuel mixture ignites too early, if the fuel is too low octane (which allows it to burn too quickly), or if the combustion is uneven, leading to colliding combustion wave-fronts. 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 or “knock”, which 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 on the engine block close to the crankshaft. Where access is restricted and there are two knock sensors, most of the time they are replaced in pairs.
Knock sensors can also be delicate; they can be damaged if dropped or if over-tightened. On a V-6 or V-8 engine, they are usually down in between the two cylinder banks, and can be prone to damage by water intrusion if an engine is washed. Lacking avoidable damage, however, knock sensors are generally not prone to wear or failure. The wiring pigtail, or connection to the vehicle harness, is a common weak point.
What else can seem like a bad knock sensor?
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 it 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. Poor quality fuel or fuel that has the wrong octane rating for the vehicle can also cause engine knock.
Costs of Knock Sensor Replacement
On average, for most vehicles, the cost to replace a knock sensor is about $400. That is including 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 $150 an hour as a labor 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 labor to replace them is 3 hours. A factory sensor lists for $50, and a standard part costs about $35. This makes the job of replacing the sensors about $650 using OE parts, or about $635 using aftermarket parts.
For a 2003 Toyota Camry with a 2.4 liter engine, the labor to replace the knock sensor is 1.4 hours. A factory sensor lists for $246, or a WVE part costs about $70. This makes the job about $606 using OE parts, or about $430 using aftermarket parts.
For a 2011 Grand Caravan with a 3.6 liter engine, there are two knock sensors, and the labor to replace them is 1.6 hours. A factory part lists for $65, or an NTK part costs about $40. This makes the job to replace the knock sensors about $520 using OE parts, or about $470 using aftermarket parts.
Signs that a Knock Sensor Needs to be Replaced
In almost all cases, the first thing that is noticed is an engine light. Common knock sensor trouble codes are P0324, P0325, and P0328. 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, such as wiring issues, engine timing and fuel quality problems, then 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 it’s 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.
It will be one of the engine blocks. On a four cylinder engine, it’s usually bolted to the block underneath either the intake manifold or the exhaust manifold. On V-6 and V-8 engines, it may be on the lower block, but sometimes it is in the valley between the cylinder banks, underneath the intake manifold.
On a V-6 or V-8 engine, yes, moderately hard, if the sensor is under the intake manifold. On most other engines, the sensor can be accessed and replaced fairly easily. The knock sensor will be held on by one bolt, and the main critical thing is to torque the new sensor correctly.