A knock sensor is a key component of a modern fuel-injected engine’s ignition timing system. It listens for telltale noises that indicate abnormal combustion, which is also known as engine knocking or detonation. When it hears knock, it signals the engine computer to retard ignition timing to suppress it. This protects the engine from damage. But like any engine sensor, knock sensors can fail. Here’s a look at why knock sensors break and how to diagnose knock sensor problems.
What is a Knock Sensor?
A knock sensor is a piezoelectric acoustic detector that clamps onto the engine block. It “listens” for specific frequencies that are the result of detonation. When detonation occurs, it creates pressure pulses in the engine’s structure that vibrate at a frequency of around 6 kHz to 20 kHz or higher depending on the engine design. The knock sensor contains a piezoelectric element that generates a voltage in response to vibration. So when it detects vibrations within the knock frequency range, it generates an AC voltage signal that tells the computer the engine is knocking. The sensor has a couple of wires that connect it to the PCM. The computer then retards ignition timing to prevent the engine from pinging itself to death.
How Does a Knock Sensor Work?
The knock sensor is a low-tech but highly effective engine management component. It has a simple job-to listen for engine noises that signal abnormal combustion. It does this through its built-in piezoelectric element. When the sensor is clamped to the engine block, it reacts to vibrations in the block’s structure. It generates a voltage signal that increases in response to the vibration amplitude. Most sensors will generate 0.5V to 1.5V or higher depending on the intensity of detonation. This voltage signal is sent to the computer. When the PCM receives a voltage signal within the knock range (typically above 0.4V), it interprets it as knock. It will then retard ignition timing to suppress the knock. This reduces the sensor voltage back to zero as knock subsides. The PCM is constantly adjusting timing this way based on the knock sensor input to keep the engine safely out of detonation.
Why Do Engines Knock?
Detonation or knock happens when the air/fuel mixture ignites spontaneously before the spark plug fires. This causes a second flame front within the cylinder in addition to the normal combustion initiated by the plug. The result is multiple flame fronts that collide, creating pressure spikes that resonate through the cylinder. This banging or pinging sound represents pressure pulses hammering on the piston crown, cylinder head and engine block at up to 20 kHz frequency. This can damage pistons and rings over time if allowed to continue. Detonation occurs when the air/fuel mixture gets too hot due to compression, excessive spark advance or a combination thereof. So the knock sensor is vital to prevent this heat-related damage.
What Can Cause a Knock Sensor to Fail?
Like any engine management sensor, knock sensors are subject to failure. Here are some of the most common reasons knock sensors break down or stop functioning properly:
Wear and Tear
Knock sensors have a tough life attached to a vibrating engine block. Over the years, the constant vibration can damage the piezoelectric element inside the sensor. Heat cycles can cause the ceramic element to crack or lose its piezoelectric properties. Vibration can also wear out the electrical connections. So normal aging is one cause of failure, especially on engines over 100,000 miles.
Carbon Deposits
Carbon buildup on the sensor can dampen its vibration response, making it less sensitive. Unburned fuel, oil ash and dirt can all coat the sensor over time, degrading its performance. If your engine burns oil or runs rich, carbon fouling of the sensor is more likely.
Bad Wiring Connections
Like any engine sensor, the knock sensor relies on good, clean electrical connections to work properly. Corrosion, damaged pins, or loose plugs can all cause connection issues. This will create false or erratic voltage signals. Intermittent computer codes are often the result of a bad sensor connection rather than a failed sensor.
Moisture Damage
Since the knock sensor mounts outside on the engine block, it is exposed to water, steam, road salt and dirt. Moisture can seep into the sensor through cracks or its electrical connection. Water mixing with dirt makes an electrically conductive solution that can short out the sensor.
Oil Fouling
Engine oil leaking onto the knock sensor is another potential failure cause. Like dirt, moisture and carbon deposits, oil can dampen the sensor’s vibration response. It also attracts contaminants that further foul the sensor. A cracked housing, bad mounting gasket or leaking valve cover gasket can all cause oil to drip onto the sensor.
Bad Gas
Poor fuel quality that contains impurities can increase engine deposits that foul the knock sensor. It can also raise cylinder temperatures and cause the engine to run hot, putting more stress on the knock sensor. So bad gas is often part of the equation when knock sensors fail prematurely.
Engine Modifications
Modifying the engine to increase performance often requires adjusting the knock sensor parameters so it doesn’t read false knock. But if the tune is too aggressive, it can overwhelm the sensor leading to failure. High compression pistons, high boost turbos, big cams and lean air/fuel ratios all stress engine sensors.
Damaging Detonation
If an engine experiences heavy knock for too long, the extreme cylinder pressures can actually damage the knock sensor. Piezoelectric elements are fragile. Excessive and continuous knock causes extreme sensor vibration that can break down the ceramic element and cause sudden failure.
Diagnosing Knock Sensor Problems
Since the knock sensor is a vital input that determines ignition timing, any problems with it can affect engine performance, fuel economy and emissions. Here are some symptoms of a failing or bad knock sensor:
- Persistent spark knock or pinging
- Rough idle
- Misfires
- Hesitation, stumbling on acceleration
- Poor engine response
- Increased fuel consumption
- Illuminated Check Engine light
- Stored knock sensor code(s)
scans for diagnostic trouble codes (DTCs) that point to knock sensor issues. It can pull codes such as:
- P0325 – Knock Sensor 1 Circuit Malfunction
- P0326 – Knock Sensor 2 Circuit Range/Performance
- P0327 – Knock Sensor 2 Circuit Low Input
- P0328 – Knock Sensor 2 Circuit High Input
These codes indicate voltage problems from the sensor like out-of-range, low or high voltages. This often means a bad sensor, but could also be caused by a wiring issue.
Visual Inspection
A visual inspection of the knock sensor(s) should be part of any diagnostic process. Check for:
- Damage to the sensor casing that allows oil or moisture penetration
- Carbon deposits, dirt or oil fouling on the sensor face
- Loose, corroded or damaged wiring connector
- Detached sensor not fully screwed into block
Any of these findings may point to the problem source. Also note if there is any evidence of oil leaking near the sensor from a bad gasket or seal that can cause oil fouling.
Sensor Testing
You can test sensor operation with a multimeter and a service manual that provides the required voltage values. Tests include:
- Reference voltage at sensor connector
- Ground circuit voltage drop test
- Simulated knock voltage test
These tests check that the sensor circuit is wired properly and is generating the correct output voltage when vibrated.
Monitoring Data
Scanners that link to a laptop allow data monitoring while driving. Graph the knock sensor data over time to look for:
- Erratic voltage readings
- No activity when knock present
- Constant high voltage
- Sharp voltage spikes
This may help confirm if the sensor is malfunctioning.
Replacing a Bad Knock Sensor
Once diagnosed, a bad knock sensor needs to be replaced. On many vehicles, the knock sensor is relatively easy to change. But there are a few things to keep in mind for proper installation:
- Use a quality OEM or reputable aftermarket sensor from Bosch, Denso, Walker, etc.
- Compare old sensor to new one. Make sure replacement matches contact type, wiring and design.
- Clean sensor mounting surface thoroughly to remove dirt, oil residue and old gasket material.
- Apply light coat of anti-seize compound to sensor threads (avoid tip).
- Tighten to factory torque specs. Do not overtighten.
- Reconnect wiring harness to sensor.
- Clear any knock sensor codes.
- Drive and recheck for symptoms. Sensor voltage should show activity.
On some engines, the knock sensor is integrated into the ignition cassette so the entire assembly must be replaced if the sensor is bad.
Preventing Knock Sensor Failure
You can help avoid premature knock sensor failure by:
- Using only good quality gasoline to prevent sensor-fouling deposits
- Fixing any engine oil leaks to prevent oil fouling
- Not extending oil change intervals which allows more combustion deposits to form
- Keeping intake air filters clean for proper airflow
- Tuning engine carefully if increasing performance to avoid sensor-damaging knock
- Checking battery, connections and grounding are all in good shape to provide clean sensor voltage
These measures will keep your knock sensors in good operating condition for maximum engine performance and longevity.
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Conclusion
Knock sensors play an essential role in protecting engines from detonation damage. Like any component, they can fail over time due to factors like vibration, contamination, heat cycling and damage from severe knock events. Symptoms such as rough running, poor performance and check engine lights point to possible sensor issues. Diagnosing knock sensor problems involves code scans, visual checks, voltage tests and data monitoring. Replacing faulty sensors along with better maintenance and tuning can help maximize knock sensor service life.