Bad Knock Sensor: Common Symptoms and Testing Procedure

Your car contains what seems like an endless array of sensors, one of which is the knock sensor. Although the knock sensor doesn’t get as much publicity as some of its counterparts, it does play a crucial role in keeping your car’s engine running right.

When the knock sensor fails, you’ll likely notice one or more symptoms that you’ll want to address right away. If you’re wondering, “What does a knock sensor do?” or “What does a knock sensor do for a car? ” read on to find out.

What is a Knock Sensor

The knock sensor uses an internal piezoelectric element to detect abnormal combustion, known as spark knock, inside of the engine. The location of the knock sensor is usually on the left side of the engine block, just below the intake manifold.

Spark knock (detonation) is an undesirable phenomenon that creates an abnormal increase in cylinder pressure, often resulting in a metallic pinging noise from the engine.

Common causes of spark knock include low octane fuel, overly advanced ignition timing, abnormally high engine operating temperatures, and carbon buildup inside the engine’s combustion chambers.

If left unchecked, spark knock can cause costly internal engine damage. That’s why all modern vehicles use at least one knock sensor to monitor the engine for spark knock.

The knock sensor creates an alternating current (AC) signal that gets sent to the engine’s computer, which is often referred to as the powertrain control module (PCM). When the PCM detects spark knock, the module will retard ignition timing until the knock is gone.

ELI5

A knock sensor is like the engine’s ear that listens for bad sounds. Sometimes, the fuel in the engine burns the wrong way and makes a pinging noise, like metal tapping. This can hurt the engine if it keeps happening. The sensor detects the noise and tells the car’s computer. Then the computer makes adjustments to stop the noise and keep the engine safe. This helps the car run smoothly.

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Not everyone is an auto repair expert. ELI5 (“Explain Like I’m Five”) provides layperson-friendly explanations of complex automotive concepts, systems, & parts, so you can fully understand their importance. Great for casual car owners and DIY beginners! Powered by AI.

A knock sensor detects abnormal combustion (spark knock) in an engine using a piezoelectric element. Spark knock causes a metallic pinging noise and can lead to increased cylinder pressure. Common causes of spark knock include low-octane fuel, advanced ignition timing, high engine temperatures, and carbon buildup. If not addressed, spark knock can damage the engine. Knock sensors send signals to the engine’s computer, which adjusts the ignition timing to prevent damage. Modern vehicles use knock sensors to monitor and protect the engine from spark knock.

For additional technical information on the operation of knock sensors, read our advanced discussion about what knock sensors do.

Signs of a Bad Knock Sensor

When the knock sensor fails, it will usually cause one or more noticeable symptoms. The most common knock sensor problems include:

Illuminated Check Engine Light

The most common sign of a bad knock sensor is an illuminated check engine light. When the PCM detects a problem with the knock sensor or its circuit, the module will turn on the check engine light and store a corresponding diagnostic trouble code (DTC) in its memory.

Pinging Noise From the Engine

If the knock sensor fails, the PCM may not recognize or correct spark knock. As a result, you may hear a metallic pinging noise from the engine. The noise is often most noticeable when the engine is under a heavy load.

Poor Engine Performance

A faulty knock sensor can also cause the PCM to misadjust the ignition timing, resulting in poor engine performance. If you’re wondering, “Will a knock sensor cause a no-start?” it’s rare for this to happen, although a bad knock sensor can make starting your vehicle more difficult.

How to Test a Knock Sensor

It’s a good idea to test a suspect knock sensor before you rush out and buy a new one. Before getting started, however, you’ll want to have the vehicle’s repair information handy. Manuals, such as those from Chilton, are good, but a subscription to a repair database (e.g., ALLDATA or Mitchel 1 DIY) is even better. You can find more information on accessing quality repair information in our article on repair manuals.

Note: The following are general guidelines for educational and entertainment purposes only. Consult your vehicle’s factory information for specific repair instructions and recommended safety procedures.

Step 1. Check For Diagnostic Trouble Codes (DTCs)

The first thing you’ll want to do is check for DTCs that could narrow down the troubleshooting process. Cars built after 1996 with on-board diagnostics (OBD) II will usually set a code when there’s an issue with the knock sensor or its circuit. Some older cars with OBD I will also store a code when there’s a problem with the knock sensor. Faulty parts like bad spark plugs can cause a knock sensor code, so you’ll need to inspect your vehicle thoroughly if it sets a DTC.

You can check for diagnostic trouble codes using a scan tool or code reader. These days, you can even get an inexpensive OBD II code reader for your smartphone.

It’s important to point out, however, that DTCs do not tell you the exact problem with the car. Codes merely serve as a starting point for further diagnostics.

Step 2. Perform a Visual Inspection

Next, you can move on to performing a visual inspection. You’ll want to look for issues, such as damaged wires and poor connections. Make sure that the knock sensor’s electrical connector is clean and tight.

Repair any issues found during the visual inspection, clear the DTCs, and see whether the problem returns.

Step 3. Test the Knock Sensor Directly

Testing a knock sensor can be tricky. There are two primary types of knock sensors: wideband piezoelectric and resonance piezoelectric.

Wideband piezoelectric sensors pick up vibrations within an entire range of frequencies. On the other hand, resonance piezoelectric sensors only respond to vibrations that are within the same frequency range as spark knock. For this reason, resonance sensors are sometimes referred to as “tuned” knock sensors because they’re tuned into the same frequency range as spark knock (typically, somewhere between 5,000 and 9,000 Hz).

In the past, when wideband piezoelectric sensors were more common, a popular test method was to tap on the engine near the knock sensor. If the sensor responded to the vibrations, you knew it was working to some degree.

But that test does not work on the newer, resonance-style sensors. Most professionals test these sensors using an alternate method, which involves forcing the engine to ping (exhibit spark knock) while monitoring the sensor’s output signal.

You can learn more about testing both types of sensors in the video below:

Knock Sensor Location

The knock sensor is typically screwed into the engine block or manifold. But the exact location will vary by vehicle. To determine the exact location of the sensor(s) for your application, you’ll want to consult a repair manual or repair database.

Where to Get a New Knock Sensor for Your Vehicle

Once you’ve confirmed that there’s something wrong with your knock sensor, don’t put off replacing it. Otherwise, it can cause issues for your ride and even affect other parts, resulting in expensive repairs down the line. If you’re in the market for a top-notch knock sensor replacement that won’t break the bank, consider shopping online at CarParts.com.

We source our knock sensors from top manufacturers in the industry, so you’re sure to get one that’s built tough. They come at competitive prices because we want you to get the best value for your money.

After entering your car’s specifications into our vehicle selector, use the search filters to find the knock sensors that match your preferred brand, price, and specs. If you have any questions, our round-the-clock customer representatives are always happy to help.

Shop now and take advantage of our best deals for knock sensors.