Help: Flashing CEL after replacing O2 sensor

Idratherbsidewayz · 09-02-2009, 05:33 PM

Hey guys,

First off, I drive a Cobb Stage 2 2002 WRX. A little while ago, I got a P0031 code which reads: Front oxygen (NF) sensor heater circuit low input. I checked on some forums and it was mentioned that its a common problem with this gen and that replacing the front O2 sensor fixes everything. I went ahead and ordered a Denso unit and replaced the O2 sensor. Now, after about 15 seconds of idling the CEL comes on flashing and doesn't come off indicating a misfire which is an ignition problem.

A couple of days ago I did a compression test (~140 +-4 all round) for kicks and in the process mixed up the ignition coils when reinstalling. Because the car wouldn't start I tested all the coils for spark. They all worked and I put them back in the correct order. This leads me to believe that the coils are fine.

When I check the codes now with my Cobb unit, the code comes out as 6 question marks. Anyone ever experience the mystery code with their Cobb?

When the P0031 code was still going and my old O2 sensor was in, the sparks came out looking like this: (I scuffed that little black part on the electrode, it was all white before)

My current theory is that it was running lean because of the broken O2 sensor. Why would the new O2 sensor cause the CEL to flash? The only logical thing I can think of is that the new sensor is sending the ECM a signal that it needs to richen the mixture hardcore and it is then too rich to ignite. That, or the old O2 sensor making it lean caused a weak spark to ignite the more explosive mixture.

Anyone have any ideas? I'll be cleaning the sparks with a torch and trying again to see if that changes anything.

Thanks in advance,

Greg T

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Haelan · 09-02-2009, 08:30 PM

If Your Cobb AP isn't on the latest firmware then it has an incomplete OBDII database. That is why when you try to pull some codes they show up as question marks or garbage. You need to get the code(s) pulled from a dedicated OBDII scanner and post them here.

Something else could be messed up causing the misfires. You might also want to try an ECU reset to let it re-learn all fuel trim values.

09-02-2009, 05:33 PM	#1
Idratherbsidewayz Scooby Newbie Member#: 178396 Join Date: Apr 2008	Help: Flashing CEL after replacing O2 sensor Hey guys, First off, I drive a Cobb Stage 2 2002 WRX. A little while ago, I got a P0031 code which reads: Front oxygen (NF) sensor heater circuit low input. I checked on some forums and it was mentioned that its a common problem with this gen and that replacing the front O2 sensor fixes everything. I went ahead and ordered a Denso unit and replaced the O2 sensor. Now, after about 15 seconds of idling the CEL comes on flashing and doesn't come off indicating a misfire which is an ignition problem. A couple of days ago I did a compression test (~140 +-4 all round) for kicks and in the process mixed up the ignition coils when reinstalling. Because the car wouldn't start I tested all the coils for spark. They all worked and I put them back in the correct order. This leads me to believe that the coils are fine. When I check the codes now with my Cobb unit, the code comes out as 6 question marks. Anyone ever experience the mystery code with their Cobb? When the P0031 code was still going and my old O2 sensor was in, the sparks came out looking like this: (I scuffed that little black part on the electrode, it was all white before) My current theory is that it was running lean because of the broken O2 sensor. Why would the new O2 sensor cause the CEL to flash? The only logical thing I can think of is that the new sensor is sending the ECM a signal that it needs to richen the mixture hardcore and it is then too rich to ignite. That, or the old O2 sensor making it lean caused a weak spark to ignite the more explosive mixture. Anyone have any ideas? I'll be cleaning the sparks with a torch and trying again to see if that changes anything. Thanks in advance, Greg T * Registered users of the site do not see these ads.

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09-02-2009, 08:30 PM	#2
Haelan Scooby Specialist Member#: 151045 Join Date: Jun 2007 Location: NW Indiana Vehicle: 2004 JBP WRX Stage II - Monster TD04	If Your Cobb AP isn't on the latest firmware then it has an incomplete OBDII database. That is why when you try to pull some codes they show up as question marks or garbage. You need to get the code(s) pulled from a dedicated OBDII scanner and post them here. Something else could be messed up causing the misfires. You might also want to try an ECU reset to let it re-learn all fuel trim values.