Got a new cel over the weekend P0139. Rear o2 sensor slow circut response. The possible causes are numerous according to Mazda's manual that I'm looking at online. Going down the list. Purge solenoid valve malfunction, loose o2 sensor (checked already). exhaust leak (can't hear any), bad o2 sensor, fuel line pressure malfunction, fuel leakage, fuel pump failure, AIR system malfunction, leaking coolant (can't see, smell any, coolant not low) PCM malfunction. Some I can eliminate some not so sure. Any speculations.

A little history on my car, 2004 manual, 43,000 mi. started premixing 3,000 miles ago. only 4 oz for a full tank. have racing beat intake, rp highflow cat, greddy exhaust. Can't think of anything else that would be relevent. Thanks all.

the exhaust and intake will often throw codes. If its running fine, reset code and keep rollin.

^+1

How old is the RP Supercat? It has been know to throw rear O2 CELs before.

Its the high-flow CAT.

The CAT is about 8-9000 miles old.
That's what I'm thinking, but the only reason I'm not 100% is there are not drivabilty issues, so could the CAT be falling apart rather than clogging? Or is it just the quality of the CAT, not very good so doesn't work the way it was intended.?

Over the last couple of days the cel has gone off and on twice, sorry if I'm beating a dead horse here but I just want to be sure, I don't want to just replace parts at random until I get lucky.

Its not an important CEL unless you are noticing a drop in performance.

Yeah no performance drop, not worried either I just hate that light. I'm mainly wondering cause I've had the cat for 6000 miles and no cel, but I guess nothing lasts forever.

its ur cat

Unless you're loosing A LOT of performance, small ones are hard to detect by Butt Dyno.

Have u take the cat out and look at it ?

Yeah prob the cat is probably causing it id just ignore it haha

I haven't pulled it to look at it yet, tonight I will have access to a lift and will pull it.