Originally posted by gary ng:
ah, that clear things up. thanks again.

Again, out of curiosity, anyone knows what is the point of the pre-cat testing while there is another main cat after that(other than extracting more money from us) ?

BTW, I saw some local mechanics offer fuel system flush service, would that be equivalent to the B12 treatment ?

Oh, I forgot to mention, the O2 sensors have been changed may be 2 times(the last change is about a year ago or so, forgot which one at the moment though) and I don't get other DTC code so I would assume they are still good. But may be it is good idea if I have to change the pre-cat anyway(and assume they don't charge me extra labour for the O2 as they have to go to the same area).



I have no idea what was in the mind of the engineers when they put the downstream oxygen sensors (cat monitor sensors) in front of the main cat. It is a mystery to me.

The way the cat monitor works is that after certain driving conditions have been met, the cat monitor in the PCM runs. It checks the readings of the upstream oxygen sensors to the downstream oxygen sensors. What is actually being checked is the switching frequency of the oxygen sensors. A normally functing upstream oxygen sensor is constantly switch from .1 volts (lean) to .9 volts (rich) due to the constant pulsing of the exhaust stream as the different exhaust valves open and close pulsing fresh exhaust into the exhaust system. After exhaust passes through a properly working cat, the pulses are diminished, nearly gone. f when the cat monitor runs, it sees that the downstream sensor is switching at about the same speed, it sets a code and turns on the CEL.

What can cause a false code? Precats not designed big enough. Engine running rich so that the precats are loaded up. Upstream sensor weak so that the difference between the upstream and downstream sensors is not different enough. Dirty cats from a poorly maintained engine. At least these things are what comes to mind.

I have not seen a CEL for cat codes on my car for nearly two years. Prior to that it would come on about once or twice a year. I would clear the code and keep driving it. The first time I saw it, about 50,000 miles or so, I installed the Ford scanner and drove the car and found the upstream and downstream sensors working as designed. About a year ago I had the bank one upstream sensor fail and replace both upstream sensors. Maybe that has made a difference. I don't know. It doesn't run any different.

Every time I have had a smog test (Southern California requires a dyno loaded gas reading) it has had exceptionally low readings. The reading are nearly zero, so I know that all of the cats are working well. From what others have posted, removing the precats and getting tested with the main cat only results in reading that are just below failing.

Jim Johnson 98 SVT 03 Escape Limited