randian

The check engine light is on. I had the spark plugs and plug seals replaced recently to fix an oil leak that was fouling plugs, so hopefully it's not misfiring (engine runs smoothly). Doesn't seem likely that both O2 sensors would have failed or been improperly installed. No abnormal sounds. Does a bad cat make the car run rich? Or is the richness coming from somewhere else, fouling the cats? What I noticed after the O2 sensors were replaced is gas mileage was up and power was up, but once the check engine light triggered gas mileage has tanked by 10-20%.

Codes came back, MIL on. Took it to auto zone and I'm showing P0420 and P0430 codes. Since I've replaced the O2 sensors that means both cats are bad? I also found out I had a filthy air filter and replaced it, but I assume that won't fix anything. Gas mileage has dropped notably since the O2 sensors were replaced so it looks like I'm running rich. I hate chasing this problem, replacing stuff to no effect is getting expensive.

I wasn't asking about whether the smog people would know if a code has been cleared. I was asking whether the PCM shutting off the MIL implies that the PCM cleared the code. If it does, no problem at the smog check. If it doesn't, or if it only cleared a temporary code but not a permanent one, then I need to get them cleared before I do the smog check. Unfortunately I don't remember the exact codes, only that they were some sort of "reduced efficiency" codes, which the dealer said meant the oxygen sensors were bad.

That was the dealer opinion. Three sensors "really off" and one "somewhat off", they said. They wanted nearly two grand to fix it, so I took the repair elsewhere.

I had been getting check engine indicators that would appear for a few days, then disappear for a few days, then reappear ad infinitum. I took it to a shop and they said I had "reduced emissions efficiency" codes on all my oxygen sensors and that the sensors were reading way off. I had all four replaced. It coded 3 or 4 days after that repair. Argh! Did they not fix it? I had it scheduled to be looked at again when the check engine light turned off. Now the shop can't check it! Then I had the good fortune (so it appears) to break my (original 14 year old) gas cap, because after I replaced it the check engine light hasn't triggered for ~3 weeks since. 1) Can I be confident the cap really permanently fixed anything? 2) Was it all the gas cap in the first place, so I wasted money on the oxygen sensors? 3) Will I need to go back and have codes cleared to pass smog, even though the PCM has cleared the Malfunction Indicator Light?