The engine was running well, even though it was throwing a code for a bad O2 sensor, bank 1, sensor 2. In that time, I replaced the fan clutch, removing the air filter housing and the ram pipe as well. I put everything back together, tightened a squealing belt, and test drove it. Everything seemed to work fine, apart from the fan seeming to run rather constantly even when cold. The test drive went well.

The next day, I set about replacing the O2 sensor, and I drove to the store to buy some anti-seize. As soon as I fired it up, it was loping like crazy, and almost stalling itself. Revving the engine seems to smooth it out, but when driving, coming off the line, it just bogs right down. When I get up to speed a bit, it's better, but it's still seriously down on power.

Now, I replaced the O2 sensor, and the symptoms remain. I let it run for about 5 minutes, to warm up and get used to the new sensor, but there was no change.

Now, this is an OBDII car. The ONLY code it is throwing is the O2 sensor code. No other codes are being thrown. I can't for the life of me figure out what it is.

What could have changed between me replacing the fan clutch, the good test drive, and the bad test drive the next day?

I checked everything, and it all seems fine.

 

My guess is something didn't seat with the air filter housing or pipes. Sounds like that kind of problem, you know you had them off, so start there. Maybe something just shook loose.