What are the symptoms of a faulty knock sensor?
4 Answers
Performance symptoms of a faulty knock sensor: engine knocking, power loss, accompanied by piston slap. A faulty knock sensor can also lead to: inability to detect engine knock signals and failure to automatically retard ignition timing. The causes and consequences of a faulty knock sensor are introduced as follows: 1. During engine operation, factors such as excessive ignition timing advance (ignition advance angle), engine load, temperature, and fuel quality can cause engine knocking. When knocking occurs, because the gas combusts before the piston reaches top dead center, it may produce noise and reduce engine power in mild cases, or damage the engine's mechanical components in severe cases. 2. If the knock sensor is damaged, the engine control unit (ECU) cannot monitor the degree of knocking. When the ECU detects engine knocking via the knock sensor, it reduces the ignition advance angle, leading to an erroneous judgment that no knocking has occurred. In this scenario, the ECU will increase the ignition advance angle, causing the engine to run roughly and shake due to knocking. 3. After the knock sensor fails, it will automatically retard the ignition timing to prevent engine knocking, so the engine will operate in a state of retarded ignition timing. 4. Engine fuel consumption will increase, combustion efficiency will decrease, and engine power will decline.
I just experienced this last month when the knock sensor went bad, and driving was really uncomfortable. The most obvious symptom was sluggish acceleration - you'd step on the gas but the engine would just roar without moving forward, like it was choking. The fuel consumption suddenly became ridiculously high too, with a full tank running 70-80 km less than usual. The lack of power was bad enough, but the check engine light kept coming back on after being cleared. The mechanic said when this sensor fails, the ECU defaults to retarding ignition timing to protect the engine, but you sacrifice both power and fuel efficiency. In severe cases, black smoke comes out of the exhaust with a very pungent smell - definitely wouldn't pass emissions testing like that.
When the knock sensor in my old car failed, the most annoying part was the abnormal engine noises. Cold starts were relatively normal, but after running for more than half an hour, there would be a metallic clattering sound from the engine bay, like someone tapping an iron pipe with a screwdriver. It was especially noticeable when going uphill, making me hesitant to press the accelerator too hard. The repair shop used a diagnostic tool and found a P0327 trouble code, indicating the sensor wasn’t responding. Before replacing it, they specifically checked the wiring harness connector and found severe oxidation. After spraying it with rust remover and reconnecting it, the issue was resolved—so don’t rush to replace parts; checking the wiring first might save you a few hundred bucks.
The most dangerous aspect of a failed engine knock sensor is actually the hidden damage. During one maintenance session, I was alerted that this sensor had malfunctioned, and I discovered the ECU had been operating in a conservative mode. Prolonged ignition delay can cause a drastic increase in carbon buildup in the combustion chamber, potentially leading to piston rings getting stuck due to carbon deposits. Another hidden risk is uncontrolled exhaust temperatures, which can cause the catalytic converter to overheat and glow red—seeing your car's undercarriage emitting a red glow on a summer highway is downright terrifying. If you notice a metallic rattling sound resembling flint striking metal during hard acceleration, get that sensor checked immediately.
