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2004 Legacy: codes P0420 and P0139 After Turning On A/C

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Hi everyone,


I have a 2004 Legacy L, 2.5L with 116,000 miles. I bought it a year ago primarily because it had a huge number of things recently-replaced by a local Subi shop with a good reputation. It also had a good, verifiable history of being well-maintained. No problems until now.


It threw codes P0139 (Rear 02) and P0420 (cat efficiency.) I had the codes read at Autozone, had them cleared, and they didn't come back on.


Took it to my local non-Subi mechanic anyway a few weeks later, he wanted to replace the cat. Took it to an independent Subi mechanic, he said the cat was around a 3 year-old replacement and looked fine. No need to replace.


Drove the car for a few weeks, and right after running the A/C the engine warning light came on. Same codes, had Autozone read and clear them. No problem for a couple of weeks until I needed the A/C, then the warning light again and the same codes.


I don't think it's the actual cat, because the independent mechanic could have easily sold me one but swore it looked fine. But the codes definitely seem related to increased engine load by using the A/C.


Any ideas on how to proceed? Thanks in advance!

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Thank you both for your replies. The A/C belt looks fine, with good tension, and I had it inspected less than a month ago by my Subi mechanic.


Unless someone on the forum has a better idea, I'll replace the rear 02 sensor, clear the codes, and see what happens.

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