Most modern vehicles have a knock sensor that’s nested in the series of sensors that help the ECU maintain a close relationship with engine performance. The knock sensor itself is more of a safety device that essentially listens to the engine’s timing and tells the ECU if there is a problem. The ECU then uses this information to determine if the engine’s timing needs to be altered.
If you have a bad knock sensor, the ECU is sort of flying blind or getting intermittent messages on engine performance. This can cause a pinging noise from the engine, lead to poor engine performance and a check engine light.
Though this is just the tip of the iceberg regarding the potential signs and risks a bad knock sensor poses to you and your car. Not the least of which is the very real chance that you might end up stranded somewhere with badly damaged pistons or cylinders.
In this article, we will discuss the knock sensor and its purpose, as well as the most common symptoms indicating knock sensor failure, the procedure for testing it, and the steps involved in its replacement.
What is a Knock Sensor and What Does it Do?
The knock sensor found in most modern cars effectively listens to the engine and monitors its vibrations via an internal piezoelectric element. Its primary purpose is to detect abnormal combustion and send that information instantaneously to the car’s ECU.
The ECU then uses that information to adjust the timing or other conditions of the engine’s performance to maintain optimal internal combustion throughout the cycle of the engine. As the ECU makes the necessary adjustments, it continually listens to the information being sent by the knock sensor to make sure the correction is working.
5 Signs Of A Bad Knock Sensor That Need To Be Addressed
The signs of a bad knock sensor typically come on strong, such as a strange pinging noise and the engine feeling down on power. Though a check engine light might precede these symptoms, as many automakers set a low threshold for knock issues.
Some of the most common symptoms of knock sensor problems include:
1. The Engine Feels Down on Power
A bad knock sensor can result in the engine feeling sluggish or lacking in power, as the engine control unit (ECU) may overcompensate by retarding the ignition timing or altering other engine parameters.
Due to poor or inaccurate responsiveness, the ECU essentially “flies blind,” unable to meet the immediate demands of the accelerator pedal and throttle. This situation causes the engine to operate in a “limp” mode instead of running smoothly and efficiently.
2. Poor MPG
Poor fuel economy, or miles per gallon (MPG), is one of the more common signs of a bad knock sensor, as the engine control unit (ECU) cannot adjust the ignition timing for optimized internal combustion. The ECU relies on accurate input from the knock sensor to make real-time adjustments to the engine’s operating conditions.
When the knock sensor is not functioning correctly, it hampers the ECU’s ability to maintain optimal combustion, leading to inefficient fuel consumption. Unburned fuel can even end up causing misfires if you drive with a bad knock sensor long enough.
3. Poor Performance Under Load
A bad knock sensor can indeed lead to noticeably poor performance when the engine is under load, such as when towing something or accelerating at high speed to pass another vehicle. Under these conditions, the engine demands optimal ignition timing and precise adjustments to maintain efficient combustion and deliver the necessary power.
With a faulty knock sensor, the engine control unit (ECU) struggles to receive accurate input and cannot make the necessary real-time adjustments to the engine’s operating conditions. As a result, the ECU may overcompensate by retarding the ignition timing or altering other engine parameters, leading to reduced power output and inefficient fuel consumption.
4. Metallic Pinging Noise From the Engine
The metallic pinging noise, commonly referred to as spark knock or detonation, is a prominent indicator of a bad or malfunctioning knock sensor. This audible manifestation of erratic combustion is primarily due to the engine control unit’s (ECU) inability to accurately regulate the engine’s ignition timing in response to varying operating conditions. The knock sensor’s primary function is to detect such abnormal combustion events and relay the information to the ECU for necessary real-time adjustments.
When the knock sensor is defective or providing inaccurate data, the ECU struggles to compensate for suboptimal combustion conditions, leading to persistent knocking. If left unchecked, persistent detonation can result in severe damage to critical engine components, such as pistons, cylinders, and even the cylinder head.
5. Check Engine Light
When the engine control unit (ECU) detects a bad knock sensor and/or other engine performance issues, it will activate the check engine light to alert the driver of potential problems. At this point, the ECU’s confusion may result in multiple diagnostic trouble codes being stored in its memory.
The codes most indicative of a bad knock sensor are:
How to Confirm If Your Knock Sensor is Faulty
If you’ve been seeing signs of a bad knock sensor and your ECU is throwing codes P0325 or P0332, you can test the knock sensor with a multimeter to confirm it has indeed failed. This calls for the following steps.
If you get an open reading, or no reading the knock sensor is bad and needs to be replaced.
If you aren’t getting a clear or consistent reading from the resistance test, you can use the multimeter to test the suspected bad knock sensor voltage via the following steps.
If it gives you a reading underneath 1 volt, then you likely have a bad knock sensor.
Common Reasons Why Knock Sensors Go Bad
A faulty knock sensor isn’t all that common, as they are meant to last for a long time. Though a few things can cause a knock sensor to go bad before its time. This includes things like:
Is It Safe To Drive With A Bad Knock Sensor?
A bad knock sensor typically causes bad engine performance and can easily lead to a spark knock. Either one of which can make it very unsafe to drive your car. While you might be able to get from point A to point B while suffering through poor engine performance the risk of causing serious, expensive damage to your engine simply isn’t worth trying to drive with a bad knock sensor.
Where Your Car’s Knock Sensor Is and How to Access It
The knock sensor housing is usually screwed directly into the engine block or integrated into the intake manifold or perhaps mounted on the cylinder head. If you can’t find it with a quick visual inspection, you should be able to locate it in your owner’s manual or the repair guide for your make and model of car.
If you’re even the slightest bit unsure, you should check the manuals. With some makes and models it’s easy to accidentally mistake the oil pressure sensor for the knock sensor.
How Much Does It Cost to Fix a Bad Knock Sensor?
Fixing a bad knock sensor is as simple as replacing it, which typically costs between $75 to $350. Though the part cost for a new knock sensor is only around $50 to $150. Replacing a bad knock sensor yourself is typically something the average DIY mechanic can handle. Though this is assuming you diagnosed the problem correctly.
A Step-by-Step Guide to Replacing Your Knock Sensor
If you feel up for it, and you can source the new knock sensor at your local auto parts store you can replace your bad knock sensor with the following steps.
Frequently Asked Questions
What Does A Bad Knock Sensor Sound Like?
A bad knock sensor often sounds like a metallic pinging noise. This is actually a form of spark knock caused by erratic combustion in the cylinders due to improper timing, as the ECU essentially is running blind.
Can Bad Fuel Cause Knock Sensor Damage?
Low-octane fuel or running your engine on degraded gasoline can cause spark knock. This might not technically damage the knock sensor itself but often causes the check engine light to come on. Then the ECU will throw a knock sensor code.
This is more likely to happen if you stored your car for a while without adding a fuel stabilizer to the gas tank. Though some rural gas stations might also have degraded low-octane gas as well.
Most of the fuels you find in suburban gas stations are fresh and not degraded. So, you shouldn’t worry about 87 octane gas causing spark knock or damaging your knock sensor.
Can A Bad Knock Sensor Cause Transmission Problems?
Left unchecked the timing and performance issues of a bad knock sensor can cause transmission problems. Without the correct information transmitted by the knock sensor, the ECU can’t get the timing of the engine correct, which affects power and can also cause the transmission to run in the wrong gear.
A bad knock sensor confuses the ECU by not giving it the information it needs to get the timing right. Common signs of a failing or bad knock sensor usually start with the metallic pinging noise of spark knock and the engine feeling down on power or struggling to accelerate smoothly.
A bad knock sensor often causes the check engine light to come on, and the ECU throws codes like P0325 or code P0332. This might also come with codes for the cylinders that experience combustion issues in the incorrect timing cycle.
If you’ve noticed symptoms of a bad knock sensor, the car is probably unsafe to drive. Especially if the metallic pinging of spark knock is getting louder and louder, replacing a bad knock sensor is cheap, and you can usually do it yourself for under $150. However, the cost of repairing damaged cylinders and pistons caused by driving with a bad knock sensor is much worse!
Jason Farrell is a certified master technician, the editor of Mechanic’s Diary in Pittsburgh, Pennsylvania. He is ASE (Automotive Service Excellence) certified and earned a Bachelor’s Degree in Automotive Technology from Pittsburg State University. With nearly 18 prior years of experience in the automotive field, he has extensive knowledge about Domestic, European, and other foreign makes and models of cars and light trucks. Jason’s experience working as a technician and service manager at dealerships, gave him the experience and know-how of most aspects of inspection, diagnosis, and repair from engine and drivability to electrical, HVAC, brakes, steering and suspension and everything in between.