Bad cat or bad diagnosis?

[DoraDexplora]

I am a new poster to this forum so be gentle.

I have a 96 XLT 4.0 with 110,000 miles and a check engine light on. Two months ago, the light came on with a P0154 but last week when I was going to get the O2 sensors replaced, the code came up P0420, nothing about 0154. Pep Boys and $95 will get you a recommendation to replace the Cat.

Here is a fly in the ointment, I read that a stuck thermometer will kill a Cat and I had one open for two years before it closed and needed replacement.

Replace the cat or is it residual o2 problems? If I disconnect the battery, will that clear the check engine light and the codes so I can get another reading of what is broken?

Any direction would be appreciated, thanks.

 

[00XLS]

Yes, disconnecting the battery will clear the codes, if there is still a problem they'll come back, Heres a tip. Go to autozone and they'll pull 'em for free.

 

[DoraDexplora]

Thanks, will do.

 

[DeRocha]

I am a new poster to this forum so be gentle.
Here is a fly in the ointment, I read that a stuck thermometer will kill a Cat and I had one open for two years before it closed and needed replacement.
Any direction would be appreciated, thanks.

A stuck wide Open thermostat will cause the engine to be continously cooled (the temp gauge will drop all the way to the cold side and your heater won't give off any heat). When cold the engine will use more gas so the extra fuel will in effect shorten the life of the CAT, but won't kill it for a long while (several years). Basically it comes down to how poorly your engine has been running. The better it runs the longer the CAT will Last.

 

[DoraDexplora]

Here is the strangest part, yesterday I took it to the local mechanic to check the engine light out. He didn't have the code for the alarm so he disconnected the battery to stop the horn from honking which cleared the recent code but not the "history" which is stored until erased by an OBDII tool. There were no codes in the history even though two within the last year two were read (P0154 & P0420). Does anyone know about a glitch in the software that would turn the light on for no reason and not post codes to the history?

 

[DeRocha]

I believe the error codes can be cleared if the computer detects the problem has gone away after multiple "testing" cycles.

 

[Runnin'OnEmpty]

DoraDex, the P0154 is "no activitiy detected from sensor circuit bank 2 sensor 1 (this is the first and only one on the drivers side). If the code no longer exists, I'd suspect a break in the wiring, or a bad plug-in contact.

The P0420 could very well be related to the P0154 code. If the codes are cleared, and the MIL is off, I wouldn't replace anything yet. I would inspect the wiring to the driver's side O2 sensor, unplug the sensor and spray WD-40 in the plug assembly, and re-plug........it might have only had a momentary glitch in the O2 wiring circuit.

EDIT: OK, after re-reading your post, it seems the MIL is still on? If so, and if the O2 wiring is OK, then I'd replace the sensor. The cat may or may not be bad. The cat downstream sensor gives info to the PCM in conjunction with the other O2 sensors, so if one O2 was bad for a time, it might have also triggered a false P0420 (bad cat code).