Trouble Code P0420

Doh... found an archive post that suggests it's probably oxygen sensor. I forgot to mention I have 96 Explorer 4.0 4WD AT. I see in manual that there are 4 sensors. Should I replace all 4 if one is bad? TIAA

Double Doh. Found another post that suggested having the PCM re calibrated or reset first. Something about the trouble code being stored in memory. It cost $90 but the trouble code hasnt come back after 200 miles. Seems like an inherent flaw like that should have been paid for by the factory. Oh well, Kragen said they would take the 02 sensors back.

How many miles did you have on when you did this?

This code came up on mine also. I did do the cat replacement, as my mileage was high, and I haven't had the code come on again. I haven't had my PCM recalibrated. Anyone know what that's all about? Is this a dealer only type thing requiring special equipment?

Paul

I had 67K+ on it when the trouble code came up. As far as I know only the dealer can flash/recalibrate/reset the PCM but I'm not certain. I found the post about the PCM at

Haven't seen the post on your link. If it is referring to power programmers and chips, all these do is modify the spark and fuel curves as well as the rpm limits. They do nothing to the operating strategy that decides when to set the MIL and send a DTC to memory. In the past, Ford (along with most other marques) have approached the EPA for permission to alter the fault determination strategies to reduce the incidence of "nuisance" MILs.

Dealers are the only ones with this sort of programming data. The original poster would have to go to the dealership and have his car connected to the WDS to determine if a later flash is available for the car.