* Retard Ignition Timing: This is the most common response. The Engine Control Unit (ECU) relies on the knock sensor to detect unusual combustion events. If the sensor is malfunctioning or sending incorrect signals (either indicating knock when there isn't any, or not detecting knock when there is), the ECU will err on the side of caution and significantly retard (delay) the ignition timing. This reduces engine performance, making the engine feel sluggish, weak, and less responsive. Fuel economy may also suffer.
* Reduced Engine Power: The retarded ignition timing directly leads to a reduction in power output. The engine simply won't be able to produce as much horsepower or torque.
* Check Engine Light (CEL): A bad knock sensor will almost always trigger a check engine light, indicating a diagnostic trouble code (DTC) related to the knock sensor circuit.
* Inconsistent Performance: The engine's performance might fluctuate, becoming unpredictable. This is because the ECU is constantly trying to adjust the ignition timing based on flawed input from the faulty sensor.
* (Less common) Engine Damage (in some cases): While usually the ECU's response is to prevent damage, a faulty knock sensor can *indirectly* contribute to engine damage in extreme cases. For example, if the sensor is constantly reporting no knock, the ECU might advance the timing too aggressively, leading to actual detonation and potential damage over time. This is less likely than the other symptoms, but possible.
In short, a bad knock sensor usually results in reduced performance and a check engine light. It's crucial to have it diagnosed and replaced to prevent potential (though less likely) long-term engine damage and restore optimal performance.
