QUESTION: I have a 2001 Subaru Forester S, purchased new, which now has slightly less than 60,000 miles on the meter. This has been a great car; I have had little to no real trouble with it. Lately, however, the "check engine" lamp turned on and, after using my scan tool, I discovered that "Code P0328: knock sensor 1, circuit high input" was the problem. After reading about the knock sensor, I cleared the code and continued to drive the car for quite some time before the "check engine" light again turned on. But as time progressed, the light would turn off, then on, then off, etc. Finally I decided just to order a new knock sensor to replace the questionable original sensor.

Have you already covered knock sensors in a previous column? If not, would you consider doing so in the near future?

—Victor D. Bennight

ANSWER: You’re correct it’s been a long time since this topic was discussed. Knock sensors, also called detonation sensors, can be found on many but not all cars and light trucks. An engine may have either one or two of these small, microphone-like sensors installed where vibrations from abnormal combustion can best be sensed.

Normal combustion starts with the firing of the spark plugs. The ignited air-fuel mixture builds pressure gradually yet quickly across the top of each piston, pushing them firmly downward. Detonation, which is an uncontrolled explosion of a portion of the air-fuel charge, can result from excessive heat and pressure, quickly damaging pistons, head gaskets and rod bearings. High-performance engines, with a greater-than-typical compression ratio and perhaps a supercharger or turbocharger, are almost certain to employ a knock sensor.

Knock sensors contain a piezoelectric crystal, which produces a tiny AC electrical signal when pressed by the vibration of a small plate or pendulum. This signal is delivered to the engine-control computer, and if the frequency falls within the range of combustion chamber detonation, ignition timing is retarded incrementally until the knocking subsides. Some engine systems aggressively seek the edge of detonation, advancing timing to maximize engine performance, then backing it off slightly to be safe. Using higher-octane fuel in these vehicles can improve performance.

It’s possible for the sensor to be confused by other engine noises, such as connecting rod knock, wrist pin rattle, piston slap or a few others, which can result in unwarranted timing changes. Knock sensors can also degrade in performance or develop a faulty circuit connection, causing an illuminated "check engine" light and various diagnostic trouble codes being set. Many systems "ping" the sensor with a DC voltage to ensure the circuit to the sensor is intact and look for any sensor signals to ride atop the resulting pulled-down DC voltage. Yours may have failed this test.

Testing a knock sensor can be done in several ways. Using a pro-grade scan tool, one can observe sensor activity and timing response under high-temperature throttle snaps and/or actual road conditions. Sensor output can also be checked by observing voltage output or a scan tool value while carefully striking the engine with a metal object, near the sensor. This test is better at condemning a bad sensor than verifying a good one, as the vibrations created aren’t especially realistic. The best tests, of course, follow published procedures for the specific vehicle.

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