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YES!!, you had it tuned, it runs good, but gas mileage dropped, and check engine light is on,it sounds like the o2 sensors are slow or even dead, i would replace them for sure and that just might solve all your problems,good luck, hope this helps.
Change your oxygen sensors - they're responsible for feedback that the computer uses to determine the air/fuel mixture. They usually are good for about 60k miles and then start to go downhill. When they get to a point that the computer won't trust them, it ignores them and runs on preprogrammed fuel maps that run excessively rich (too much fuel) to safeguard the engine. You've probably seen a decrease in fuel mileage as well. And the O2 sensors have to be up around 900 degrees before they'll work - the car ignores them until then. That's why you run fine after a cold start but run rough once the car is warmed up.
The O2 sensors you want to change are in the exhaust system between the engine and catalytic converters (the "pre-cat" sensors). There are O2 sensors after the cats as well, but those are used comparatively to determine how well the cats are functioning. The primaries are the ones used for fuel mixture. If it's been this way for a few weeks or so, I'd put new spark plugs in as well - rich fuel conditions can foul them and contribute to the rough running.
1)new spark plugs properly gapped.
2)new air filter (check every 35000 miles)
3)new fuel filter (every 35000 miles)
4)1 bottle of STP fuel treatment (red bottle) 4x a year.
5) oil and filter change (castrol or another good quality oil every 3-4000 miles.
6)proper air pressure in tires (check every 2 months
7) service transmission every 40000 miles or less.
New oil and filter
8) new plug wires every 80000 miles.
After market spark plugs could be the problem.
Reinstall the old plugs and see if that fixes it.
This vehicle has an ion sensing ignition system and is real sensitive to the proper spark plugs. I would get the proper plugs from the dealer if the old ones work.
The codes P1311 and P1312 are ion sensing codes.
dunno bout rotating the engine to put those plugs in, but the O2 sensor is right in the way as your arm is stuffed up behind the engine (plugs have to be done from below at the back) so I wouldnt be shocked if you found a disconnected O2 sensor that was never reconnected, or a damaged wiring loom from rough contact with the mechanic's arm. Those plugs are very hard to change so go easy on the guy!