What are the symptoms of a failed knock sensor?

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... show more 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.)
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