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