Originally posted by racerm515:
i had P0420 and the cel went off like 2 days later. should i worry about this or since the cel went off am i alright?



Probably.

Many times when this code sets, it is a false positive. Ford eventually partually addressed this issue with a reflash on some of the cars effected. The reflash widened the parameters for setting the code. My car was not one of them with a new PCM code.

I would speculate that ocassionally when the cat monitor in the PCM runs that the pre cat may not be hot enough.

I'll give you an idea about how the cat ineffeciency codes are set. The cat effeciency monitor only runs ocassionally, not all the time. It is the last of the monitors to run, and even then only after certain driving conditions have been met. When the cat monitor runs, it compairs the readings of the upstream oxygen sensors to the downstream oxygen sensors. The upstream oxygen sensor is before any cat. It switches rapidly from lean to rich with the pulses set up from the opening of the exhaust valves. A proper performing cat will even out the rich/lean switching. The downstream oxygen sensor is after the cat. The PCM compares the upstream readings with the downstream readings, and if the downstream readings nearly mirror the upstream readings, the code is set and the CEL is illuminated.

What can you do to minimize the likelyhood of the cat monitor codes? Proper maintenance helps. Clean injectors, good plugs and wires, clean valves, and a clean combustion chamber may help a lot. A slightly lazy upstream oxygen sensor may also contribute. So it helps to ocassionally run a good injector cleaner through the tank, do the vacuum suck intake cleaning, make sure the ignition system is in good shape, confirm that the air filter is in good shape, drive in a spirited manner, and if there is any question about the condition of the upstream oxygen sensor, change it.

That may not keep the cat effeciency codes from ocassionally setting, but if it is only ocassionally there should be no trouble.

Even though my car has ocassionally set those codes, it still has the original pre-cats, and on an emissions test the readings are nearly zero, much lower than would be possible if only the main cat (which is downstream of the downstream oxygen sensors) were all that was working.

I do believe in agressive maintenance which includes frequent injector cleaning as well as new plugs every 60,000 miles. Beyond my agressive maintenance schedule I did replace both upstream oxygen sensors a few months back when the #1 bank set and kept repeating a code. The cat effeciency codes have not come on since, but I don't know if it is related since it is an infrequent event anyway.

I hope this helps.

Jim Johnson 98 SVT 03 Escape Limited