The Subaru SVX World Network

Knock sensors detect detonation, aka 'knock'. High compression, coupled with incorrect ignition timing, too low octane fuel, high combustion temperatures, carbon build-up in the combustion chamber or on the piston and an inoperative EGR system can all cause it to occur.

Knock sensors 'listen' for specific frequencies that indicate the problem. They are piezo devices that convert motion into voltage. As such, they always generate some voltage when the engine is running. What the ECU is looking for is a particular range of frequency and voltage. Once a specific threshold has been crossed the ECU will react by retarding timing in stages and watch for continuing detonation. Over a specified amount of time, if the threshold is no longer crossed the ECU will begin returning the timing to its original program.

A knock sensor code does NOT indicate that the engine is knocking. The ECU expects to see some output from the sensor(s) whenever the engine is running, as the engine will always generate some 'noise'. This 'noise' doesn't necessarily have to be in the detonation region. It is the lack of any signal that will cause the ECU to store a code.

With all that being said... The sensor used on the SVX is very similar to those found on many Nissans. They are notorious for throwing codes, both on the SVX and on Nissans. I've replaced a number of them only to later find that they continue to throw codes. I have watched their signals while back-pinning at the ECU and have watched them set codes while there was absolutely no signal deterioration. I've spoken to many other techs about this and we've all come to the same conclusion: it's a glitch in either the ECU's internals or the ECU's programming.

Additional info: a knock sensor code will never turn on the 'check engine' light. It will remain in memory until the memory is cleared.

What should you do? Hmmm... well the proper thing to do is to monitor the sensors with a graphing meter or o-scope for drop-outs. Orrrr... clear the code, start the engine and let it run for a few minutes and re-check the codes. Chances are if it immediately resets the code you have one or two dead sensors, if not the code was probably a glitch. Best bet? If you can, unplug and re-plug the sensors a few times to clean their connector contacts.

BTW, if you decide to replace a sensor Subaru issued a TSB years ago stating that the sensors should be replaced in pairs. If you need help replacing them use the search button and I am positive you'll find all kinds of help previously written.