Code P0031 - HO2S Heater Control Circuit Low...What is this? - Subaru Forester Owners Forum
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post #1 of 10 (permalink) Old 02-24-2007, 03:43 PM Thread Starter
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Code P0031 - HO2S Heater Control Circuit Low...What is this?

This one popped up in the car today. Never even heard of this...any insight?

2005 FXT - FP 18G, STI I/C, Mad Dad Header.
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post #2 of 10 (permalink) Old 02-24-2007, 03:46 PM
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The code list I have says

P0031 Front oxygen (A/F) sensor heater circuit low input ????

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post #3 of 10 (permalink) Old 02-24-2007, 03:46 PM Thread Starter
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Looks like it's a front O2 sensor heater wire and is somewhat common (from some NASIOC searching). Does that jive with anyone? will this mess up A/F in the meantime?

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The code list I have says

P0031 Front oxygen (A/F) sensor heater circuit low input ????
I'm going off what my FSM said, but your makes more sense and jives more with what I found on NASIOC.

I'm assuming this is a heated sensor and it is just the heating that is a problem and not the sensor itself.

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post #4 of 10 (permalink) Old 02-24-2007, 04:05 PM
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Yes, it is the heater which allows the oxygen sensor to warmup and go to closed loop faster.

Unfortunately this will set a CEL and on the turbo also your cruise will probably be disabled also. It should be repaired ( replace the front oxygen sensor ) to get the CEL out and back to normal. But the Air/Fuel ratio should be OK ... After warmup. The heater IS part of the Oxygen sensor.

This is a common failure on both the Front and Rear oxygen sensors.

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post #5 of 10 (permalink) Old 02-25-2007, 01:20 AM Thread Starter
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The CEL went off on it's own. Very odd since I've heard that once the O2 goes, even an ECU reset and it will come right back on.

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post #6 of 10 (permalink) Old 02-25-2007, 04:11 AM
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Since the failure is Heater circuit 'low', that indicates the heating element is open, not shorted.
It MAY still be an intermittent heating element in the oxygen sensor, but this can also be a poor connection. I would check the connector at the front oxygen sensor to ensure it's plugged in and seated. Also ensure none of the wires around it are damaged.

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post #7 of 10 (permalink) Old 02-25-2007, 10:59 AM Thread Starter
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I did damage the wires when I swapped in my header. I re-spliced them, heat shurnk them, taped them and then put the stock "shield" back over it.

It's been fine for 4 months. Maybe I'm just on borrowed time. Thanks for the input guys.

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post #8 of 10 (permalink) Old 02-13-2008, 06:39 PM
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I just pulled this code today. Do I have to get a new sensor to get rid of the code? Clearing it with the scanner does nothing. Is there a "better" sensor I can get?

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post #9 of 10 (permalink) Old 02-14-2008, 04:25 AM
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Todd, on MY02, for the few yrs I had it, this was one of the codes that required the sensor to be replaced. In my case it was the rear sensor.

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post #10 of 10 (permalink) Old 02-14-2008, 06:58 AM
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Ok, thanks. I messed up the wires pretty good when I replaced the headers so I hope that that's all it is.

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