Posted 09 February 2006 - 11:00 PM
Edit: My gawd. What happened to my line breaks? They're definitely not working, even after the edit. My apologies. '97 Legacy Outback. Here's the way I look at it... it might sound repetitive, but I gotta list things out like this to help myself think through this. If I try this in my head, I forget what I started with What I know: 1. Once the ECU is cleared of codes, the code gets rethrown approximately ten to fifteen minutes after starting the engine and moving the car. 2. I've tested the resistance across the heater circuit twice, approximately 5-6 ohms when cold, same as the original O2 sensor heater circuit. When I removed the new O2 sensor when it was still warm and tested it, the resistance was higher, 7-10 ohms. 3. I've tested the voltage on the engine side of the connector successfully, 12 volts. This was with the engine off with the ignition in the run position. What this leads me to believe: 1. The problem is not continuous. With the original oxygen sensor installed, the CEL would come on within a minute or two of starting the engine. 2. There is a problem occurring (obviously), otherwise the ECU would not be throwing the code and the CEL would be popping on. 3. Therefore, the problem is intermittent. Somehow, something is changing ten or fifteen minutes after starting that's causing the ECU to think the oxygen sensor heater circuit is failing. What I assume: 1. Because the Haynes manual states that the diagnostic test to verify the operation of the heater circuit relies on measuring the resistance across the circuit, I assume the ECU is using this same method to determine the operation of the heater circuit. 2. If that is true, then something is causing the resistance to fall outside what the ECU considers tolerable limits. 3. The resistance of the circuit depends on the temperature of the heater. I assume this because of the measurements I took as the heater cooled. Possible sources of failure: 1. The heater itself, but only until ten to fifteen minutes into operation. If the heater is heating to a certain temperature, then breaking the circuit, it would fail. Or if something is vibrating loose within the oxygen sensor, breaking the circuit, and just happening to occur ten to fifteen minutes after starting. 2. The wiring is failing intermittently. Since I've routed the wiring away from the half shaft by going around the power steering piping, I feel I can safely assume the halfshaft is not rubbing on the wiring. It could be the same case as number 1, however... the wiring could be vibrating loose, just happenstance that it's occurring that late. Or I cut the wires too long, which could raise the resistance across the circuit, causing it to fall outside the ECU's limits. I doubt an extra inch or so of wire would cause that, though. Could be that the crimped connections are causing too much resistance, though... or a combination of the two. 3. The ECU is mistakenly reporting that the heater circuit is failing. It could be that the oxygen sensor heater is still operating, but the ECU is mistaking the resistance across it as a failure. Or whatever portion of the ECU that tests the resistance on the heater circuit is failing itself. I'm leaning towards the extra length of the wire and/or crimped connections causing the resistance to be outside of allowable limits once the heater reaches a high enough temperature, ten to fifteen minutes after starting the engine. I can see two possible ways of verifiying this: 1. Clear the codes. Run the car until the code is thrown. Pull the oxygen sensor connection and test the resistance across the heater circuit as quickly as I can. 2. Pick up an OBD2 adapter, a inverter or power adapter for my wife's laptop, and find the appropriate software that would allow me to view however the ECU is testing the heater circuit in realtime. Number 2 is probably expensive and troublesome if I can't find the right software (or the ECU won't let me view that info), but it'd be reliable. Number 1 is cheap, but it's reliability probably depends on how fast I can get the test done, before that heater cools off. Can anyone poke holes into my theories?