I replaced the hole exhaust system on a 92 ford 4 cylinder mustang last year and it passed inspection this year it failed on high CO levels every thing else passed.I was told to check the map. Throttle. And egr valve. Which I did back probed all of them and made sure there was no oil or carbon build up on any of them and they looked almost new.
The first time when it was inspected it failed with a 1.19 and needed to be 0.73. I was told if the car wasnt getting warm enough that it would run to rich.I replaced the thermostat and the heater and all started working again and the car ran very good. Then on the return inspection it failed with a 1.38 reading on the high speed test and failed on the low speed test with a 1.37 and it needed to be at 0.76.I have taken off every thing egr and throttle sensor to check for carbon or oil but they look brand new with very little ware and no oil or carbon any where.
The only thing I could find was a broken o2 sensor and some vacuum lines that runs to a canister down on the frame and replaced the hoses on it. And I know the heater core has to replaced so I just pulled the hoses off the core tubes and connected them together so the temperature sensor would still work. Started it back up and drove it down the road and it started popping back through the throttle body but when I floored it it picked back up after about 2 or 3 pops and ran great. Now its a toss up to when it happens again, it doesnt do it every time maybe every 4 or 5 times driving but then it clears right up and goes on. All this started as soon as I changed the thermo in it and I have never had problems like this by just by changing a thermostat.
I thought it may be a head gasket leaking into one of the cylinders but there is no white smoke or steam coming out the exhaust and no water lose now. What am I missing to check or does it need a rag in the tank with a match in my hand ready to strike and light