What are the symptoms of a failed knock sensor?

Question

2001 Subaru Forester, manual transmission, 200K miles.  I've got a chronic evaporative emissions false alarm, so my "check engine" light (CEL) is on more often than off during hot weather.  Every now and then I read the codes to see if there's anything else lurking in the computer.

Twice during August, I read a P0328 code, which is for the knock sensor.  (That's in addition to my false alarm evap codes.)

I've got no obvious driveability problems: the car handles normally, there are no strange noises, my fuel economy remains very high.  (31 MPG in this hot weather.  I drive with a light foot.)

I'm thinking that this is another false alarm: a sporadic code from a failed self-test that would not have lit the CEL by itself.  But it still shows up when I read the evap codes.

If I really did have a bad knock sensor, what symptoms would I see?

I don't want to spend more money chasing a random false alarm.  (I spent too much chasing those evap codes.)

Anonymous · Accepted Answer

The pcm uses the knock sensor to tailor the timing curve for the ignition system, when it registers a fault code due to a malfunctioning knock sensor the pcm then uses a set amount of ignition timing which is usually less advanced and may lead to poor performance and or spark knock(ignition ping) when you use poorer quality gasoline. When the knock sensor is working properly the pcm can adjust the ignition timing to the maximum ignition advance for improved combustion efficiency  and better fuel economy. If you don't notice reduced performance or pinging I would think it may be a false alarm caused by bad wiring or some other intermittent electrical problem.

Anonymous · Answer

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How much is a knock sensor? You are paying an extra $0.20 per gallon for fuel now and can't utilize the extra octane most of the time. If the light stays on, you should replace the sensor even if the car runs good because when something else fails you will not know until you notice other symptoms like mpg dropping off or misfire, etc.

hafter · Answer

Subaru Knock Sensor

Robert M · Answer

REMOVE THE KNOCK SENSOR and clean it out! BAD oil can cause this as well, and also take $20 to the local OIL CHANGE CENTER and have the throttle body and IDLE AIR CONTROL VALVE cleaned up to SHOWROOM clean! The CODE for a BAD KNOCK Sensor can only meant hat your engine is going OFF KILTER with the AIR FUEL mixture! SUBARUS are VULNERABLE to sticking IDLE AIR CONTROL VALVES ( which FINE TUNE the amount of AIR entering the engine) and also FILTH baked into the THROTTLE BODY! A CLEAN IAC is VITAL for AIR/FUEL mixture and can TRIGGER fault codes from many engine points! IF your computer COUNTS a few knocks, it may be due to this CDIRTY IAC vavle thingy! It CANNOT respons to O2 senosr signals if it is CLOGGED UP with SDLUGE or FILTH! GOOD LUCK!!

Lesus · Answer

If the engine doesn't knock, it will set the ignition timing so that it never knocks. I think that would affect  performance, unless the sensor only failed occasionally.

Technobuff · Answer

Knocking. Often called pinging.
Can result in broken pistons, rings etc., and total engine failure.
A metallic sounding knocking noise when running full throttle up a steep hill, steep enough you would need to normally downshift.

Kenny · Answer

Delete the code and see if it comes back.  

Symptoms is you hear knocking or ping when accelerating.




Damaged spark plug electrode will also be a sympton.

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What are the symptoms of a failed knock sensor?

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