Our '96 Explorer had the CEL come back on a week or so ago. I took it to AutoZone a couple days ago and they said it was showing code P0420, message of "Catalyst efficiency below threshold (bank 1)"...or something like that. The guy that pulled the code said that if he had to guess he would guess it was a vacuum leak somewhere. After doing some preliminary research online I've found that he doesn't know what he's talking about.

Based on searching these forums, and other regions of the internet, I've found that the cause of this code is most likely one of the following:
-PCM needs to be reprogrammed/updated
-Bad oxygen sensor
-Bad catalytic converter

Then, just a few minutes ago I found a post on some other forum where somebody posted "possible causes for DTC P0420" from the "'96 Ford factory OBD-II manual", and they listed the following:
-Use of leaded fuel
-Oil contamination
-Cylinder misfiring
-Damaged HO2S sensor
-Damaged ECT sensor
-Downstream HO2S wires improperly connected
-Fuel pressure too high
-Damaged exhaust system pipe
-Damaged exhaust manifold
-Damaged muffler/tailpipe assembly
-Damaged/Bad Catalytic Converter

Does all of that sound correct to anyone who knows about this? I'm overdue for an oil change and I just changed the spark plugs about a month ago. Could waiting too long for an oil change cause "oil contamination"? If a cylinder was misfiring I'm assuming it would be immediately obvious - it would run extremely rough or something - is that correct?

Also, if anyone has dealt with this code in the past on a similar vehicle I'd appreciate hearing what you did to fix it.

 

Since the ONLY code that you are getting is the P0420, it's most likely a bad cat convertor.

It's a shame it did'nt happen sooner as the cat has an 8 year/80,000 mile federal emisions warranty on it.

Edit.
If it was ANY sort of an engine problem, you'd be getting an upstream -before the cat- O-2 code.

 

It was way past the 80,000 mile point when we purchased it. It had a little over 100,000 when we bought it, and we've since taken it up to 150,000.

Do cat convertors just go bad like that? I assumed they did, but I was reading some other site that said they almost always go bad because of a problem elsewhere on the vehicle, and if that other problem isn't fixed the replacement cat converter will eventually go bad too. Is there an element of truth to that?

 

Hard to say how long the cats are good for. I bought my 87 Ranger new & it has 226K miles on it, still has the original cats & still passes California emmisions with room to spare.

Not sure what would kill a cat early, a rich condition would probably overheat it.

I know that antifreeze in the exhaust is death on O-2 sensors. Dont know if it would harm a cat.

BTW, if you have a GOOD independant shop near you, thats where i would have it looked at.

Dealers are for warranty work only.

Good luck with it.