02 wrx CEL help

[USMC]

Hy everyone, I'm new with subbies so please help me. I have a n 02 wrx with 142k on it. It is a manual an all stock besides an aem CAI. I pulled the codes with my code reader and it came up with P0171 and code P0420. I'm assuming some of you know what these codes mean( lean mixture on bank 1, and catalytic converter below threshold.) do these codes go together? Do you think both the 02 sensors are bad? Where should I start? All the plumbing on the CIA is tight. Please help, thanks!

[Calvinball]

Are you the original owner, or did you just buy it?

If just bought, are you sure the downpipe is stock?

[Mikie13]

Does sound like bad o2 sensors to me.

Agreed with the downpipe question Will above has posed...

[Calvinball]

Does sound like bad o2 sensors to me.

Agreed with the downpipe question Will above has posed...

And I agree with 02 sensor, but I figured it's easier to check the downpipe first, assuming the car was purchased used

[USMC]

Are you the original owner, or did you just buy it?

If just bought, are you sure the downpipe is stock?

Just bought it ( got a good deal)
Im pretty sure the down pipe is stock. Looks like a cat right after the turbo and it has metal heat shields covering it.
Also the thing runs awesome if that helps.
So you think both the 02 sensors are shot?

[Calvinball]

Just bought it ( got a good deal)
Im pretty sure the down pipe is stock. Looks like a cat right after the turbo and it has metal heat shields covering it.
Also the thing runs awesome if that helps.
So you think both the 02 sensors are shot?

If the downpipe is stock, there's some possibilities. One or both sensors could be bad, any shop can test those for you pretty easily. It could also be the catalytic converter itself.

[USMC]

If the downpipe is stock, there's some possibilities. One or both sensors could be bad, any shop can test those for you pretty easily. It could also be the catalytic converter itself.

Do you test the voltage coming out of the sensors? I can test them myself with a voltmeter. Do you know what the voltage that they are suppossed to put off when theyre good or bad?

[Calvinball]

Do you test the voltage coming out of the sensors? I can test them myself with a voltmeter. Do you know what the voltage that they are suppossed to put off when theyre good or bad?

Yes, you can test it yourself then. I believe the important thing is not so much the voltage, but that they are fluctuating. They should not read steady. Google time...

Bam. Here ya go: How to Test an O2 Sensor | eHow.com

[SD_GR]

PO171 indicates a bad O2 sensor but that is last on the list of things I'd suspect causing the CEL. I'd troubleshoot PO171 fully first, leaving the O2 sensor for last. You already know the intake is incorrect. Did you clean the MAF? Please search for troubleshooting PO171, there is a whole list of things to do posted.

PO420 indicates a bad O2 sensor also but I'd suspect wiring first, or an aftermarket or absent cat etc.

[Donkey]

The P0171 can also be caused by an aftermarket air intake that is skewing the MAF reading. The P0420,even with an aftermarket highflow catted downpipe,can still be triggered.