Bad Knock Sensor Symptoms? Causes and Replacement Costs

Bad knock sensor symptoms are easy to notice when driving your car, but there are some signs that you might not even realize, like fuel damage and catalyst damage. The knock sensor is a highly important part of your engine as it keeps your car running smoothly.

What Can a Bad Knock Sensor Do With Car

If you notice something wrong with your sensor, you should visit a car repair shop immediately, as this can cause further damage or lead to an accident. In this guide, we take a closer look at the signs of a faulty knock sensor and the replacement costs.

What Are the Bad Knock Sensor Symptoms?

The bad knock sensor symptoms are rough idling, engine misfire, pinging noise from the engine, check engine light is on, catalyst damage, bad fuel economy, acceleration, and poor performance. It also leads to higher emissions. If you notice any of these symptoms, take your car to the repair shop.

Rough Idling

One of the main signs of a defective knock device is rough idling. When idling your car, you might notice that the engine is acting like it’s about to seize up or stop driving completely. It can lead to an accident when driving.

Mechanic Set Up The Idle System of An Engine

Service Engine Soon Light or Check Engine Light Is On

If you notice that your check engine light or service engine light is on, this is a sign that there is something wrong with your knock sensor code and there has been an ignition misfire. The light might be steady or flashing, but either one is bad news.

Check Engine Light_s On In Odometer

You should check the trouble code with an OBD scanner. In some cases, you won’t see a code despite the flashing engine warning light. This could be because the bad sensor has not set a misfire code yet. It’s best to take your car to a mechanic for inspection.

Catalyst Damage

You might also notice that your car emissions are too high, which is due to catalyst damage. A knock sensor malfunction can damage the catalyst system, leading to higher hydrocarbons.

Technician Detaching The Catalyst of Exhaust System

The engine would release a lot of emissions when misfiring and running wrongly. After a while, it would collapse the catalytic converter.

Poor Acceleration and Performance

If your sensor is failing, you will deal with decreased acceleration and engine power. Your engine would default to reduced output to avoid further damage, and this means you won’t be able to go as far as before. Your car would also lose its speed and torque.

This is because the engine reverts to default values to stop you from producing too much force with the defective knock engine sensor. The engine’s computer would then reduce the amount of power that your car can produce.

Poor Fuel Efficiency

Another sign that your sensor is defective is reduced fuel mileage. If you notice that you’re spending more on fuel, there are different reasons that could cause this, but the car knock sensor is one of the likely culprits. Unfortunately, it’s not easy to notice that you’re consuming more fuel.

Pinging Noise

If your sensor is damaged, you might be hearing a pinging noise from the engine called the knock sensor sound. This is because the PCM cannot fix or recognize the spark knock frequency. A damaged sensor leads to a metallic pinging sound, especially when the engine is under heavy load like towing load or driving on rough terrain.

Reduced Power

Your car might lose power if you have a knock engine sensor that isn’t working properly, and this can be very dangerous. The amount of power that you lose depends on the limit of the engine and how much it depends on the sensor. With an older car, you would probably not be able to drive your car with a failed sensor.

Cars with flex fuel and high-compression engines usually experience a lot of power loss when their knock sensor is not working. Until you fix the knock engine sensor or replace it, the reduced power slows down the engine and it leads to transmission problems. If your car slows down, it will give you a chance to park and call a mechanic.

Engine Misfire

If you have a defective sensor, your engine might also misfire when you start. This means that the engine would not be running smoothly but would lose power, stall, or hesitate, especially when you accelerate. This can also cause increased emissions, a lack of responsiveness, and a sputtering sound from your engine.

Mechanic Working On Car_s Engine

Engine misfires can damage the catalytic converter and would waste a lot of fuel on combustion, thereby increasing the fuel usage.

Engine Damage

In the worst-case scenario, and especially if you ignore the earlier signs, a bad sensor can cause engine knock.

A Damaged Broken Engine

Since it can cause pre-ignition, which is the fuel-to-air mixture igniting before the spark plug fires, it can create a shock wave. This would lead to damage to the engine components like the piston, cylinder head and spark plug.

What Are the Causes of a Bad Knock Sensor?

The causes of a bad knock sensor include an overly rich fuel-to-air mixture and hot engine temperatures, worn-out components like piston rings, cylinder walls and valves which increase pressure in the cylinders, and misfiring spark plugs that form erratic combustion.

Here are some of the potential reasons for a faulty sensor:

  • Wrong spark ignition timing.
  • Deposit buildup in the cylinder like dirt and grime.
  • Using the wrong kind of spark plugs.
  • Deposit buildup on the spark plug.
  • Faulty spark plugs.
  • Incorrect fuel and air mixture.
  • Low octane fuel.
  • Mechanical damage.
  • Carbon buildup.
  • Wrong installation of the sensor.
  • Abnormally high temperatures in the engine.
  • Corrosion in the sensor and engine.
  • Internal short circuits.
  • Breaks in wiring.
  • A car accident or impact.

Regardless of the reason, the best solution is to replace the sensor.

What Are The Effects of a Faulty Knock Sensor?

The effects of a faulty knock sensor include failed emissions testing, elevated hydrocarbon and carbon monoxide levels, and rough driving. If you don’t repair it, it can lead to complete engine failure, which is the main effect. Failed emissions testing could be because the fuel and air mixture is incorrect.

Even if there is no internal problem in your car, if your fuel mixture is too rich, your knock engine sensor will fail, and you will fail any emissions test. Your car would also give off higher CO levels since the air and fuel mix aren’t burning completely.

A bad sensor can also cause you to drive roughly. You might notice that your engine is acting rough when you’re driving, and you won’t have a smooth ride. In general, it would adversely affect your riding experience. It’s important to replace your sensor immediately instead of leaving it to become worse.

How Can You Replace Your Faulty Knock Sensor?

You can replace your faulty knock sensor by removing the retaining bracket around it or bending it out of the way. Then, remove the faulty sensor and use anti-seize on the threads of the new one. Screw the new sensor into place with a wrench, and put the bracket back.

If your car has a distribution ignition, you will find the sensor wire going to the dashboard from the engine, so you can check how madly your car is misfiring. You would have to unplug the wire before you replace the sensor and then plug it back in once it is installed. Check your user manual for the knock sensor location.

Some cars come with two wires going to two knock engine sensors. One wire is for the signal, and the other one is for the voltage of the heater element of the sensor. They are plugged into the engine control unit. You should unplug the wire during the installation of knock sensors and then plug it back in once you’re done.

For some models, where the knock sensor located is right beside another one, so you would have to remove both wires from under the dashboard before replacing the sensor. Be careful when replacing a sensor, and avoid leaving any connections in place to prevent damage or an accident.

How Much Does It Cost To Replace a Damaged Knock Sensor?

It costs between $250 and $350 to replace a damaged knock sensor depending on the car repair shop you visit, the make and model of your vehicle, and the extent of the damage. It’s best to visit a professional mechanic that can repair your sensor at a good price.

This price is for a knock sensor replacement. If it is neglected and leads to an engine knock, you would spend up to $3,000. Before you replace your sensor, ensure that you have the correct diagnosis so that you don’t replace the wrong parts. If you do so, you will spend extra money.

Also, if you’re unsure about replacing your sensor yourself, you should work with a car repair shop. But you can save money on the sensor replacement cost by doing the repairs yourself. You can purchase a knock sensor from any reliable shop for between $20 and $200, depending on the make and model of your car.

If your car is still under warranty, some manufacturers require that you use OEM parts. But these would cost more, and it would take longer to get. If you’re hiring a mechanic to take care of the replacement, their labor costs would be between $150 and $250.

Frequently Asked Questions

Can I Drive My Car Without A Knock Sensor?

Yes, you can drive your car without a knock sensor or with a faulty sensor, but you shouldn’t. Driving without a sensor would cause your engine computer to go into limp mode. This would greatly reduce your engine output and increase your fuel usage. Your car would barely move.

How Can I Test if My Knock Sensor is Bad?

You can test if your knock sensor is bad by using a multimeter. Use the multimeter to test the device, and if you see around 10 ohms, then the sensor is fine. You can also use the OBD scanner to check if there are codes like P0325.

What Does the Knock Sensor Do to The Engine?

The knock sensor picks up sound and vibration from the engine and translates it to an electronic signal which is sent to the engine control unit or the engine computer. This would allow the ECU to choose the ignition timing.

What Are the Common Signs and Symptoms of a Bad Knock Sensor?

The common signs and symptoms of a bad knock sensor include engine misfires, rough idling, decreased fuel efficiency, and illuminated check engine light. If left unaddressed, it can lead to costly repairs like rear main seal replacement cost in 2023. Timely detection and proper maintenance can prevent further damage and expenses.


Whenever you notice any of the bad knock senor symptoms, it’s important to inspect your car and replace it as soon as possible. Check out a summary of this article below:

  • The bad knock sensor symptoms include engine misfiring, rough idling, check engine light is on, catalyst damage, poor engine performance and acceleration.
  • The causes of a faulty sensor are the wrong fuel-to-air mixture, worn-out components, hot engine temperatures, and wrong ignition timing.
  • A defective sensor can cause failed emissions testing, elevated hydrocarbon and carbon dioxide levels and rough driving.
  • It is possible to replace the sensor but ensure that it is the culprit and hire a mechanic if you’re unsure.
  • It costs up to $350 to replace your knock sensor at a car repair shop.

With this guide, you know how to repair or replace your knock sensor if you notice any of the symptoms mentioned above. Ensure you work with a mechanic if you are not good with tools.


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