I have a '98 TA, very mild build (small call, milled 243s) with full exhaust. Car has been just fine for past 5-6 years, but just started running rough out of the blue. Part throttle or full throttle, feels/sounds like it's not firing on all cylinders. Ran a scan and no DTCs are showing, but it does indicate Oxygen Sensor and Oxygen Sensor Heater are incomplete. (I have not seen those on previous scans) Car was tuned at a shop many moons ago.

Where do I start looking for possible codes that may have been turned off?

I can post a log file of a cold start up, or the tune here if it helps.

Thanks in advance to any who pitch in.

 

I would first check all wiring, connections, and grounds. Sounds like a compromised circuit or two.

 

Thanks, I'll get on that in the morning and will report back. Any suggestions on where to start, or just go through everything?

 

I would check all that would apply..

 

#2 plug wire is arcing to the header, time for new wires.

 

boom......