February 11, 2008
^^^ Only if (BOTH) the O2's somehow failed at the same time, causing the codes to show up at the same time. That's a hard pill to swallow (I think it's very unlikely).
However, if the O2's were subjected to any lead, and possibly other chemicals, they may be toast. Did you try any race fuel? Any octane booster that might have had a real lead additive? Anything weird like that go in the tank? If so --) new O2 sensor time.
If you hook up a typical $100 Actron OBD-II sensor to the car and put it in the Diagnostics mode, you can see the output of the O2 sensors, and you will see if they have stopped switching. They should switch back and forth from high to low voltage a couple times a second when warmed up.
I still think that there is something wrong that would affect both cylinder banks equally. Since the code is a 'Rich' code, you have a problem where the computer is seeing the exhaust output too lean even after it has adjusted the mixture to the max richness that the programming allows (that's a lot, actually).
So, either the O2's are lying to the computer (Henry's idea), which would require them BOTH to be lying the same way...
OR, you have problem that is keeping the car from responding to the added fuel that the computer is trying to add. Perhaps the fuel line is clogged somewhere? Or, maybe the fuel line got pinched when somebody jacked the car up? (It's next to the passenger frame rail!). Or, maybe your fuel pump is on the way out and is not delivering enough fuel?
OR, your engine is getting a different amount of air than the MAF is telling it about (bad MAF would cause this, or perhaps a MAF that has been moved to a non-stock housing). Can you temporarily swap on a MAF from another Mustang like yours? How about removing the CAI for a while?
Your blown spark plug sure sounds like a lean backfire to me...
If you reset the codes (disconnect the battery like Henry said before) how long does it take to register the 172/175 codes?