5 Symptoms of a Bad Knock Sensor (and Replacement Cost)

Picture this: you’re sitting at a stop light on a hot day, getting ready to merge onto the highway. As you pull out and step on the gas, you hear an awful noise that sounds like shaking a bunch of marbles in a can.

The car hesitates, stumbles, then proceeds to accelerate slower than normal. What just happened? You’ve probably just experienced engine knock.

What is Engine Knock?

four stroke gasoline engine

Engine knock, pre-ignition, and pre-detonation are all names for explosions in the combustion chamber that happen when they’re not supposed to.

A combustion engine requires a carefully balanced mix of air and fuel. This air fuel mixture must be ignited at precisely the right moment. That moment is usually just before the cylinder reaches top dead center (TDC) on the compression stroke.

Virtually every engine knocks to some extent. There are countless variables in the environment that can’t always be controlled for, such as carbon buildup, fuel quality, and quick changes in the position of the throttle plate such as stabbing the gas.

Engine knock is usually only harmful when it happens at high loads, such as when you’re at full throttle. Cruising knock happens while driving at a constant speed. Cruising knock is very common and much less concerning.

An engine knock sensor is designed to detect these unplanned explosions and send a voltage signal to the ECU or powertrain control module. From there, the module will delay the ignition’s timing and prevent engine detonation.

Retarding (delaying) the ignition timing means the spark plugs fire a bit later than they used to. The piston will be a little higher in the cylinder and the combustion event will not make as much power on the combustion stroke.

Lowering the power output by retarding the ignition timing will decrease cylinder pressures, making the combustion event safer for the engine. The main downside is that your car will be slower until the ignition timing returns to normal. 

This will help prevent damage to the engine as it goes through the pre-ignition process. If your knock sensor were to malfunction or go bad in your vehicle, there would be some noticeable symptoms that would present themselves.

Low Speed Pre-Ignition

Subaru WRX

On newer vehicles that have both direct injection and a turbocharger, there is an increased probability to experience pre-ignition when the driver requests a lot of torque at a low low engine speed.

Automotive engineers aren’t quite sure why this phenomenon occurs, but it is thought to be caused by the higher cylinder pressures typically seen on direct injected turbocharged motors with a relatively high compression ratio.

Supposedly there are engine oils that can mitigate this phenomenon, but nothing can prevent it entirely. If you drive a manual transmission vehicle that is both direct injected and turbocharged, consider downshifting before you floor it. Automatic transmissions will likely handle this for you.

What Causes Engine Knock?

Engine knock can be caused by a number of factors, especially when unfavorable conditions are combined.

1) High Intake Temperatures

high intake temperature

Ever notice how your car is slower when it’s hot outside? Not only does heat make the intake air less dense, it will also increase the probability of knock.

On hot days, the temperature of the combustion gases will be higher. This increases the risk of combustion before the cylinder is ready to ignite the mixture.

To combat this, most engines will retard ignition timing in hot temperatures to make combustion safer.

2) Poor Quality Fuel

signs of bad gasoline

Ever put a lower octane fuel in your gas tank than what the manufacturer called for? An octane rating is the fuel’s resistance to detonation via compression. A higher octane rating has a higher resistance to compression ignition.

Using the wrong fuel will likely cause engine knock. Note that some vehicles are designed to run multiple octanes. In these situations, the engine will adjust accordingly. 

3) Carbon Buildup 

carbon deposit

As a vehicle ages, carbon deposits build up on the valves, cylinder walls, and pistons. These carbon deposits can create hot spots that ignite fuel unevenly.

There are ways to mitigate or remove carbon buildup. Some examples are walnut blasting the valves, Seafoam treatment, or running a catch can. A catch can and an air oil separator (AOS) are two devices that clean the oily air mixture that is ventilated from the positive crankcase ventilation (PCV) system.

See Also: Best Fuel Injector Cleaners

4) Bad ECU Calibration

bad ecm

Sometimes an ECU is programmed to run the engine too lean or with very advanced ignition timing. A lean air fuel ratio maximizes fuel economy.

Advanced timing burns the mixture earlier in the combustion cycle, which maximizes power. Both of these situations can cause engine knock when taken to an extreme.

ECUs often run on the leaner side from the factory for emissions purposes. This is usually fine, but occasionally a manufacturer will take it too far.

When this happens, the manufacturer will issue a recall or technical service bulletin (TSB) to flash the ECU. The updated engine management software is designed to give the engine a safer calibration.

Top 5 Bad Knock Sensor Symptoms

Here are some of the most common signs of a faulty knock sensor that you should look out for.

1) Engine Warning Light

oil pressure check engine light

One of the first symptoms you might notice is the engine warning light illuminating on your dashboard. You should take this early warning sign seriously and have your vehicle inspected before the problem gets worse.

Of course, there are many reasons why an engine warning light could turn on and a bad knock sensor is one of them. Regardless of the reason, you shouldn’t take the chance of ignoring it for too long or it could have devastating effects on your engine.

2) Loud Sounds

noise while driving

When the knock sensor starts to malfunction, you will hear loud noises coming from the engine that almost resembles thumping sounds. The longer you go without fixing this problem, the louder those sounds are going to get.

The reason this noise occurs is due to the ignition of the air and fuel mixture inside of the cylinder. Normally, the mixture would reach the combustion point instead. Therefore, sounds like this should motivate you to take your vehicle to the mechanic promptly.

3) Bad Fuel Mileage

high fuel consumption

If you notice that you’re getting fewer miles per gallon than you normally do, then a bad knock sensor could be contributing to that. Again, there are many reasons for why you might get bad fuel mileage.

But if you notice any of these other symptoms in conjunction with bad fuel mileage, then that’s even more reason to believe it is because of a bad knock sensor.

4) Poor Acceleration

car won't accelerate

When you step on the gas pedal to accelerate your vehicle and it doesn’t accelerate fast, then a bad knock sensor is probably preventing the acceleration from being effective. You can be sure of this if you already have the previous three symptoms occurring.

5) Poor Engine Performance

car won't accelerate

The worst symptoms from a bad knock sensor will occur when internal engine components become damaged. If you’ve let this problem escalate without replacing a faulty knock sensor, then your vehicle will begin to increasingly drag and jerk around.

There may even be a burning smell coming from the engine and getting into the cabin of your vehicle. Any further use of the vehicle in this condition could result in the entire engine being destroyed.

Then, you’re looking at many thousands of dollars to replace your entire engine. Don’t wait for that to happen. It is much cheaper to just replace the knock sensor.

Knock Sensor Replacement Cost

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knock sensor replacement cost

The good news is that it won’t cost you an arm and a leg to replace a bad knock sensor. If you have a standard economy vehicle, you can expect to pay anywhere from $120 to $500 for replacing your knock sensor.

The parts cost will be anywhere from $65 to $200, while the labor costs will be anywhere from $50 to $350.

In most cases, the parts costs and labor costs will be about the same. If you can find a good deal on the labor from an independent mechanic, then maybe you can lower your overall costs considerably.

10 thoughts on “5 Symptoms of a Bad Knock Sensor (and Replacement Cost)”

  1. Hello, my 95 corolla has 297967 miles and it has been pinging and knocking when I accelerate. The pinging and knocking is not to loud but it is noticable in hot or cold weather and this happens intermittently. No check light on but I have noticed my fuel economy is not good my car still has good power. Could it still be a bad knock sencor

    Reply
    • It’s possible. Check to see if you have any check engine lights (they may be stored even if the light is not illuminated). If you do have a bad knock sensor, you’ll want to get that replaced soon because engine knock could shorten the life of the engine considerably.

      Reply
  2. Hello there!
    My 2015 GMC Canyon sle has 52000 mi. I am using regular unleaded gasoline. When truck is in lower speed, it makes knocking noise. As I drive above 40 mi per hour, the noise stops. On the freeway I can here that noise too, but not as loud. Do you think changing the knock sensor can help eliminate that nagging noise?
    Thank you
    Mars

    Reply
    • Does the knocking noise change with RPM? I’m not sure what is causing the noise so I can’t say if changing the knock sensor will help. I don’t think it will help to change the knock sensor unless your truck is throwing a code for it, though.

      Reply
  3. Can a bad knock sensor be the culprit in causing code p0300&p0332 bank 2 sensor / also everything is good in vac leaks spark plugs coils injectors , And also can it cause transmission problems as delay gears or run sluggish

    Reply
  4. I had my 2008 Kia Sportage Ex brought to 2 separate mechanics,.
    1 said my engine assembly needing replacing due to an unhealthy engine,
    (codes 0P303 and 0P0305 as well as 0P0300,.(random multi misfiring),.
    we changed the coil pack. Still no change,thats when we were told bad engine due to low
    compressions especially on 3 and 5 cylinders.
    We were going to replace the engine when the other mechanic decided to
    check and change the knock sensors. He said since the Kia was running fairly well,
    he didn’t believe it needed another engine.
    He said it was ideal after he replaced the knock sensors,changed the plugs and wires.
    1 hr after I paid him $360.00,.the check engine came on as I was trying to be sure
    in fact the engine was not at best when in park running nor at stop signs,etc.
    Tomorrow we will return to the mechanic but meanwhile did he do anything useful?
    He did not provide the updated compressions. Or any proof,info.in paper form,
    (will retrieve that tomorrow as well due to mus-communication,saying he gave receipt to
    my son in law prior to my picking the car up. (seemed odd).
    So using our own OBD2 reader,it displays the same.,#3 and 5 and multi censors,(active).
    And check engine light on.,along with more shaky idling,and brief misfires while driving.
    It does drive great,however on the flip side,its still not running right,and the cel is on,.
    what could it be,do I need an engine,did this guy pointlessly do work vs.changing engine,
    there was no knocking sound.,ect,as I read what bad knock senor symptoms are,
    1 does look flailed,melted and spent,*.,(2 of them he replaced,giving us the old).
    I think its air / vac.leak somewhere. Yet the OBD reader says #3 and 5,multi and recently a torque converter clutch,P0741.
    He( the mechanic) seemed oblivious to that when I showed him the photo I took of that read out on my cell from the obd reader.
    Any advice?

    Reply

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