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.
What is a Knock Sensor and What Does it Do?
The knock sensor uses an internal piezoelectric element to detect abnormal combustion, known as spark knock, inside of the engine. 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.
The Top 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.
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.
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: Where to Find It
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.