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Last night I changed the front O2 sensor on the 99 outback. I had a little trouble getting it loose so I ran the car to warm up the pipe without bothering to reconnect the wires. I expected a MIL and got one. I left the battery disconnected overnight but still had a MIL this morning. Do I need to clear the code with a OBDII scanner, or do I have a problem with the new sensor?

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When I ran the car this morning the light was still on. I then shut the car off after running it for about 15 minutes and left for work. I just called my wife at work and she said the light never came on during her 45 minute drive. I am confused. Do they always act this way? Is something actually wrong?

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With OBD II, simply disconnecting the battery isn't enough. The computer stores the code in memory even if the power is interrupted. The computer actually has to see that the problem is no longer there during several running cycles before it turns the light off.

Of course, you could always clear the code instantly with a scan tool.

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OBD2 sometimes will act that way. The codes won't clear with a simple battery disconnect. They need to be cleared with a scan tool.

 

In your case the computer probably needed some time to see that the signal is good again, and it turned the light out itself. turning the car on/off again did that. From some reason O2 sensor codes are weird......I can run my car without an O2 sensor for a while, get no code, then all of a sudden it pops one up.

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