I have a 2003 Montana with 230k miles. When I accelerate really hard to pass (particularly uphill) the SES light comes on and the PO131 code is thrown. I can clear it and drive without issue until the next hard acceleration. There is no trouble idling or running - just the light and the code. Since the 02 sensor before the cat is hard to get to, I would like to try and confirm that its bad before I attempt replacing it. My question is this: is there a way to further test or confirm a bad O2 sensor without pulling it? I have a Tech II but I'm a complete novice at using it. Wondering if freeze frame info would help me? Never used the freeze frame data before...

The code indicates low voltage from the O2 sensor, which can be anything from a faulty O2 sensor to an intake gasket issue.

This site has great step by step troubleshooting info.
http://troubleshootmyvehicle.com/gm/3.8 ... n-sensor-3

I'd heed the TechInfo advice to only buy Denso or ACDelco when shopping for a replacement sensor, if that is the cause.

Thank you for the help Mike!

You're welcome.
Hope it's something simple.
Do let us know what it was caused by.

If you rule out O2 sensors and are sure that there are no post-MAF intake leakes, check/clean your MAF.

If I recall, a Montana uses a barrell style MAF sensor, and the Thermistors are prone to getting dirty and/or sooted up should you be using aftermarket oilable filters.

FYI, P0131, is a universal standard code for all manufacturer's, and for GM is the same design theory on all GM vehicles. Doesn't matter if it's a Montana or my V. What may be different is the actual troubleshooting steps, but the theory remains the same. The point being, you could do a search on P0131 here, and even though it's a Bonneville, the overall concept is the same.

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