A knock sensor is an engine component that many people may have heard about, but one that most normal people may not fully understand. It is vital in the proper combustion timing of the engine.
The knock sensor is designed to up both normal and abnormal noises from the combustion chamber. When a knock sensor fails, there may be an abnormal tapping, pinging, or knocking noise from the engine, you may experience decreased fuel economy, and your vehicle’s engine will suffer in the performance department.
What Is a Knock Sensor and What are its Functions?
A knock sensor is a device fitted to the engine block or intake manifold that picks up vibrations and abnormal sounds from the combustion chamber. As the name implies, it is designed to pick up both normal and abnormal noises from the combustion chamber. Engine knock is inappropriately timed, unplanned, or secondary combustion within the combustion chamber.
Normal combustion timing will not produce abnormal noises or vibrations, but when the sensor detects abnormalities, it sends the information it receives to the vehicle’s ECU, which, in turn, tries to compensate and delays combustion timing to combat the irregular combustions occurring.
Common Signs and Symptoms of a Bad Knock Sensor
Because the knock sensor is so central to the inner workings of an engine, the signs and symptoms exhibited by a malfunctioning knock sensor are usually limited to the performance and running of the engine itself.
Abnormal Engine Noises
Engine knock is pretty self-explanatory, despite the lack of general knowledge surrounding what it is. Its name comes from the recognizable knocking or pinging noise produced by the phenomenon. It can be difficult to describe the noise if you have never heard it before, but it is apparent when it happens.
It is usually most recognizable under load or during non-idling RPMs. If you suspect that your vehicle is experiencing engine knock because of a bad sensor, it is much easier to diagnose by propping open your vehicle’s hood and stepping on the accelerator while in neutral, allowing the engine to rev slightly. The noise usually becomes more apparent, the higher your engine revs.
Poor Performance an Acceleration
An engine and all of its moving components are designed to work in time and balance. Anytime this timing or balance is off, the engine has the potential to run poorly. If the knock sensor is bad, it will not relay that knock is occurring and will not, therefore, alter ignition timing.
It can be difficult to hear engine knock at speed since road and wind noise is also a factor, so acceleration and excessive engine vibration issues should be examined as soon as they present themselves. The aforementioned sound test can easily be performed when safely in a parking lot off the road.
Poor Fuel Economy
Fuel economy can also suffer from incorrect combustion timing or additional combustion within the engine. Poor fuel economy is rarely the only sign of a bad engine knock sensor because it can be related to so many issues from so many different vehicle systems. With this in mind, fuel economy is more of a supplementary symptom of engine knock than a primary symptom.
Check Engine Light Illumination
The easiest way to specifically identify a bad engine knock sensor is the illumination of the check engine light. Because it is a sensor, it is easy for the vehicle’s ECU to communicate an issue when the information normally received from the sensor starts to slow down or fails to reach the computer at all.
Beyond actually hearing the infamous engine knock associated with the sensor, an OBD scanner is the easiest way to diagnose a faulty sensor because it shows the exact code associated with this specific issue.
Engine and Piston Failure
Engine and piston failure usually only occur if the owner of the vehicle has neglected to have the issue fixed. There are multiple other ways to tell if the engine knock sensor has failed before extensive engine damage occurs. Therefore, it is important to have these issues investigated as soon as problems arise to mitigate costlier future damage to major components.
What is the Expected Lifetime of a Knock Sensor?
Some amount of engine knock occurs almost all the time in each engine due to continually changing environmental factors. A functioning knock sensor can mitigate this and helps an engine run as well as possible under a variety of conditions. Unless there is a serious engineering flaw, a fairly rare occurrence, a knock sensor will usually last the lifetime of the vehicle.
The most common reason for premature knock sensor failure is continued exposure to harsh conditions or physical damage. These are often damaged from other mechanical work done on the vehicle, whether it be from careless movement within the engine bay or overtightening from being reapplied after a different job.
There are, however, some other factors that can cause excessive engine knock – when the ECU cannot compensate for the combustion anomalies within the combustion chamber – or that can cause the sensor to fail completely.
Heat
Although engine knock happens quite often, an excessively hot day can cause an excessive amount of knock. Even the hottest days cannot usually create enough abnormal knocking to make the ECU unable to adjust timing and compensate. But, an already bad knock sensor can work with the heat to exacerbate the knocking issue.
Bad Fuel
Bad fuel is usually not a big concern for most drivers, but it happens enough that it can cause excessive knocking. Like excessive heat, it is not usually the cause of knock sensor failure, but it could cause enough abnormal knocking for the ECU to fail to delay timing enough and mitigate it adequately.
Faulty Spark Plugs
Faulty and clogged spark plugs are the causes of many different issues, including abnormal engine knock, and if persistent enough, knock sensor failure.
Carbon Buildup
Any kind of accretion or buildup is the bane of any sensor since it relies on the information it uses from the environment in which it resides. Carbon buildup is a common cause of sensor failure, especially if it is located in the engine block. Some knock sensors are located on the intake manifold which lessens the chances that it suffers from carbon buildup, but a bad or poorly maintained air filter can let in dust particles that can essentially do the same thing.
How Much Does a Knock Sensor Cost to Replace?
Knock sensors are fairly inexpensive to replace. Like most other engine components, parts and labor costs will depend on the make and model of the vehicle, as well as the placement of the sensor and how much has to be removed to access its location.
The cheapest knock sensor replacement jobs can cost as little as $150 for both parts and labor. The most expensive can soar to $500. Despite the high high-end cost for such a small part, this is the exception rather than the rule, and comparatively, a knock sensor is quite inexpensive for how much important it is.
Final Thoughts
Your engine knock sensor manages combustion timing in your engine, sending information to your vehicle’s ECU so that it can make minuscule ignition adjustments to keep your engine running correctly. A bad knock sensor will exhibit an abnormal knocking or pinging noise, prevent normal acceleration, reduce fuel economy, and cause the check engine light to illuminate. If you experience any of these issues, you should visit a local mechanic to have them diagnosed.
