Knock sensors are crucial tools that allow your engine to run its life. Don’t know what a knock sensor does? No need to stress; we’ve got you covered. This guide entails all the information you need about knock sensors, from the symptoms of a bad knock sensor to its replacement cost. We’ve also listed reasons for engine knocking and simple tips to try and prevent engine knocking.  

What is a Knock Sensor?

In simple terms, a knock sensor listens to and records the engine sounds. Your engine may make strange noises or knocks, also known as pinging, spark knock, or detonation. A knock sensor is placed outside the engine to detect the vibrations that cause engine knock.

The sensor comprises two parts- a piezoelectric resistor and a crystal. The piezoelectric parts create counterweight voltage when put under pressure or in contact with vibration. This phenomenon generates an electric charge like the sensor’s output signal to the ECU (Engine Control Unit).

Generally, the Engine Control Unit will ignite the air-fuel mixture from the intake manifold by triggering a spark in the cylinder. While the engine prepares for the air-fuel mixture to burn, ignition timing is adjusted by the Engine Control Unit for maximum power efficiency. This process is called spark advance and retard.

A delay would result in decreased power, whereas if the process is hurried, it will lead to pre-ignition in the cylinder, which could cause permanent damage if ignored.

The engine knock sensor could help you detect and prevent dangerous explosions and notify the Engine Control Unit by sending voltage signals. The Engine Control Unit will delay the ignition timing to prevent engine detonation.

Uses of the Knock Sensor

A knock sensor essentially retards the ignition timing. In other words, it delays the firing of spark plugs. It will place the piston in the cylinder at a slightly higher position and lower the power generated while combustion.

All this will ultimately reduce the cylinder pressure and make the combustion process much safer for your engine. The only catch here is that it will reduce your car speed until the ignition timing gets back to usual. But it’s better to move slowly than not move at all. 

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, some noticeable symptoms would present themselves.

In a nutshell, a knock sensor will notify you in case of a misfire or strange combustion sounds. Your car’s check light will most likely turn on if that happens.

What Causes Engine Knock?

Several factors can combine to cause engine knocking. Here are the most common reasons:

1. Increased Intake Temperatures

Your can has a higher probability of suffering an engine knock on a hot day. Heat causes the temperature of combustion gases to rise. This increases the risk of untimely firing up before the block is prepared for ignition.

As a result, your engine may alter the ignition timing during high temperatures to reduce the risk of early combustion.

1. Low-Quality Gas

A fuel’s octane rating indicates its capacity to rest detonation due to compression. Gas with a higher octane rating is more resistant to compression ignition, increasing the ignition timing. Whereas using a lower octane fuel can cause an engine knock.

1. Carbon deposits

With time, carbon deposits may build on the pistons, valves, and cylinder walls. Such carbon build-ups generate hot spots, leading to uneven fuel ignition and knocking.

You can remove or mitigate carbon deposits to maintain the ignition timing and prevent engine knocks. You can try Seafoam treatment, walnut blast the valves, or use an air oil separator (AOS), and catch can clean the carbon debris.

1. Poor ECU Calibration

ECUs may be programmed to run on the leaner front to maximize fuel economy. But this could eventually lead to an engine knock if taken too far.

As a result, you might need to get your ECU flashed for updated software to ensure a safer calibration.

Can You Prevent Engine Knocking?

Yes, you can prevent your engine from knocking. Like elsewhere, in this case, too, it is better to be safe than sorry. Now that you know what causes engine knock, you can try and prevent it. Here are some tips that you can follow to avoid an engine knock.

• Change your spark plugs regularly.
• Get gas-filled from a reputable gas station.
• Use good quality fuel additives to prevent carbon build-ups.
• Use good quality fuel with the recommended octane ratings for your vehicle. 
• Reduce the throttle and get your engine checked as soon as you hear any pinging or knocking.

Top 4 Bad Knock Sensor Symptoms

Mishandling the knock sensor can tamper with its functioning. You could have accidentally hit it with a wrench or reconnected the battery and damaged it.

We’ve listed the top signs of a bad knock sensor that you must keep in mind.

1. Slow Acceleration

If your car stutters while accelerating, it might be due to a bad knock sensor. While driving your vehicle at high speed or with a heavy load, the engine wouldn’t feel right. If you notice something off with the engine, get it checked, even if the check engine light doesn’t turn on.

A bad knock sensor will decrease the torque and lead to slower acceleration in your vehicle. On the other hand, a well-functioning knock sensor ensures smooth and quick acceleration.

1. Decreased Fuel Mileage

Another common symptom of a bad knock sensor is poor fuel mileage. Your fuel-efficient car will lose some efficiency as its system senses a faulty knock sensor. However, several other factors can also cause low mileage. But if other symptoms of a bad knock sensor corroborate with this one, chances are you have a bad knock sensor.

1. Check Engine Light (CEL) Warning

This is the most obvious of all symptoms. You will see a warning signal if your knock sensor is malfunctioning. You will see the Check Engine Light on your dashboard. Nonetheless, several things can cause your check light to turn on, be sure to check your car manual to decipher what it could mean.

Whatever the reason, a check light serves as a warning signal. Hence it would be best if you visited your local mechanic soon.

1. Loud Engine Noise

Pre-ignition knocks can cause loud thumping sounds, like the noise made by rocking marbles. If overlooked, the noise will increase progressively as you drive. This happens because of the untimely combustion of the air-fuel mixture in the engine cylinder head before reaching the combustion point.

Any strange noise from the engine should be enough to raise the alarm. Timely checks can save you precious time and money.

What Will Happen If You Drive With A Bad Knock Sensor?

After going through the symptoms, if you’re still thinking of driving your vehicle with a bad knock sensor, keep reading.

Technically, you would still be able to drive your car with a bad knock sensor. But it will burn a lot of extra fuel and eventually damage the engine.

An unattended faulty knock sensor might lead your car to jerk and drag. Your vehicle might shake violently, vibrate and misfire when you start the engine. It could also fill your car with a strong exhaust or burning smell. These are signs of permanent damage to the car’s internal engine parts.

In the worst case, driving your vehicle with a bad knock sensor will lead to a total engine failure and cause you thousands of dollars to replace the engine itself.

Knock Sensor Replacement

A much cheaper and safer option is to get the knock sensor repaired or replaced as soon as you see any of these symptoms.

If you know your way around engines and cars, you can replace or repair them within an hour or two. You can look up on YouTube for tutorial videos to change your car’s knock sensor.

Step 1 – To begin with, click photos of your engine and the knock sensor for reference. This will help you to put everything back in place.

Step 2 – First, disconnect the battery and free the engine of wires, hoses, and brackets for easy access.

Step 3 – Now, get a new knock sensor for your car, and find the broken one to replace it with.

Step 4 – After replacing the sensor, refer to the photos that you clicked before to reassemble all the parts.

Step 5 – Reconnect the battery, shut the car hood, and start your car to see if the Check Engine Light has gone away.

Knock Sensor Replacement Cost

If you don’t have practice in engine DIYs, it is better to take your car to a mechanic. How much your mechanic takes would largely vary from region to region.

But the rough replacement cost will range between $120 to $500 for replacing the knock sensor of a standard vehicle. It is usually split equally between part cost and labor charges, with $60 to $250 each.

You might even get a better deal if you can negotiate labor costs with the mechanic. In any case, it is much more efficient to repair or replace your bad knock sensor timely than to bear the damages later.

To Sum Up

A knock sensor is vital for your engine’s health despite being a small part of the engine. It keeps your engine in check for weird engine sounds. Even though every engine knocks at some point or the other, you can limit its extent with the help of a knock sensor. A bad knock sensor can invite trouble for the engine and should not be ignored. Poor acceleration, loud engine noise, CEL warning, and poor fuel mileage are the four main symptoms of a faulty knock sensor.

If left unchecked, your car could jerk or produce a burnt oil smell while driving. This will eventually result in full engine damage. Thus, you should look for the above-mentioned bad knock sensor symptoms and replace the malfunctioning knock sensor on time.