4 Signs of a Bad Knock Sensor (and Replacement Cost)


You’re driving down a long country road, of course, and you can’t tell what’s going on in the engine compartment. It’s because everything is so quiet these days.

But you notice a bad engine that wasn’t there before or you may feel a strange thud when you accelerate. Maybe your mileage dropped quickly or something is wrong. While the problem could be a variety of things, a faulty knock sensor could be the culprit.

What is a knock sensor?

What does a knock sensor do?

A knock sensor is a small circular device located in the block or intake manifold of most internal combustion engines. The bottom of the sensor screws into the actual block or collector, and the top of the sensor is made of a silicon donut, a piezoelectric crystal, and then an electrode.

Your car’s computer constantly listens and recognizes every sound that occurs in the engine compartment. Sounds are important because there will be normal and abnormal sounds. The latter means some problem in the engine compartment, which will then throw up an engine code on your dashboard.

Another type of knock sensor is an ion sensor. It is a new idea used in some BMWs, Ferraris and other high-end vehicles. The method basically projects the current through the spark plugs and detects any differential current, which could indicate out-of-spec detonation. It’s not a widely used method, but it’s still great!

What does a knock sensor do?

So this is a general explanation of how sensors communicate with the driver of a motor vehicle, but what exactly does a knock sensor do?

Basically, this will let you know if there is a misfire or strange combustion that doesn’t match the other engine sounds. You will most likely see a check engine light, but if you don’t and the car still looks strange, take the car to your trusted mechanic.

Signs of a Bad Knock Mary Sensor

noise when driving

One sign that your knock sensor is bad is that your car will feel strange when you accelerate or load it faster than usual.

To test this, bring the car up to operating temperature. Now, when you step on the accelerator again, try to pay attention to how fast the tachometer is moving in the RPM (revolutions per minute) range.

If you feel a stutter when accelerating and it’s not as smooth as before, it could be your knock sensor.

#2 – Bad gas mileage

gasoline in a diesel engine

Another sign that your knock sensor is bad is if you suddenly start getting very low fuel mileage.

Since you’ve obviously kept meticulous mileage data for each tank (I know you have!), you’ll know in a microsecond when your miles per gallon are dropping. Well, not really, but you get the idea.

This is because your car’s computer detects a faulty sensor and will then kill the engine by simply changing the timing.

#3 – Check Engine Light

check engine light

This is probably the easiest to spot: If your knock sensor fails, you’ll just see a check engine light.

For most cars, this will just be a solid light, but if it is flashing, check your manual to find out what this means for your particular vehicle. At this point, take the car to your favorite trusted mechanic. DTC P0325, DTC P0327, and DTC P0332 are common fault codes for knock sensor malfunction.

#4 – Slow time

causes the cylinder to fail

The last symptom of a bad knock sensor is, in my opinion, the best, and I am aware of that. When your knock sensor is faulty, your car will effectively slow down time without further damaging the car, but enough to get you to a mechanic.

Another great thing is that if you have an EcoBoost, a high compression engine like Mazda’s SkyActiv technology, or a flex fuel engine, you will experience a greater impact as the knock sensor wears off.

Knock Sensor Replacement Cost

For an average vehicle, the cost to replace the knock sensor at a shop ranges between $120 and $500. This includes parts that will typically cost between $65 and $200, and labor that will cost between $55 and $300.

Replacement process

If you have some mechanical experience and want to do it yourself, consider an hour or two of your time to replace that pesky sensor. YouTube is usually a big help too.

First, take a bunch of photos with your phone so you have a reference for how things will look when they’re together. Be sure to disconnect the battery and keep pipes, cables, and other supports out of the way. You’ll thank yourself later.

Then make sure you get the replacement sensor first and find where your sensor is broken. It may sound silly, but make sure you have the right part and at least it looks good. I can’t tell you how many times I picked the wrong part and it was a complete roll of the dice from there.

Once you’ve changed the sensor, check the photo you took earlier and put everything back together. Now reconnect the battery and start your car to see if that CEL is gone.

Related: 8 Best Wire Strippers (for Automotive Work)

What causes a knock sensor malfunction?

Generally, the knock sensor will go bad with improper handling. Maybe you’re working on the car and you press a key.

Or, you unplug it and plug it back in with the battery connected and fry it. Take care with these sensors, and they should last the life of the engine.

Can I drive with a faulty knock sensor?

Technically, you can drive with a bad knock sensor, but depending on your engine addiction, you won’t go far and will burn a ton of gas.

When the knock sensor goes bad, it is very important to replace it. This is because it is dangerous to run your engine with a faulty knock sensor, and in some cases, it can prevent you from discovering catastrophic engine failure quickly enough. Then you have a complete car.



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