I've got a check engine light on with a knock sensor code set on my '99 Legacy Outback. The service manual calls for 560 kohm, mine measures 545 kohm. Not much off spec, but I suspect far enough trip the code. This is the third knock sensor I've replaced, the first two were OEM. Any ideas about what's causing the sensor to fail so often?

Phil, a couple things. First, since the return (ground) connection is made by physical contact with the crankcase, you'll want to make sure that the contact patch is clean and free of corrosion, and that the mounting bolt is tight. Second it's possible that one of the several connectors along the electrical path between the sensor going back to the ECU may have an intermittant open contact. Last you may have an abrasion situation where the lead has rubbed through it's insulation, and is intermittantly grounding out at some point.

 

I believe a short would code as "sensor low" and an open would code as "sensor high".

 

The sensors may have all been fine.

Try replacing the sensor with a resistor and see if that turns out the light. If that works then I would guess that the wiring is ok.

 

I don't think an ommeter measurement will tell you much about the sensor being bad unless it is shorted internally.