98 GTP - P0420 PCM Questions

[eippermx]

I disconnected the battery to R & R the e-manifold last week as a normal safety precaution. It looks like it took 120 miles of town & highway driving for the PCM to complete its diagnostics. The GTP's computer set twice the P0420 code. See my posts in the thread for PCM thread: http://www.automotiveforums.com/vbul...d.php?t=733619
Questions:
- How often is P0420 checked by the PCM?
- If I do have a bad cat, wouldn't the P0420 code keep setting or does the PCM count twice that it detected a catylist issue and no longer reports it until it is reset?
I am trying to understand if my cat at the time of the code set was just burning out junk that has accumulated in it (an assumption here) from the previous motor that had a "grenaded/fragged" cyl#4 piston and the back 02 sensor picked that condition up. After the burn off the sensor was satisfied and the PCM saw the condition within its data base limits and didn't report it any more. (???)

[BNaylor]

I would take the DTC at face value and replace the CAT convertor. The CAT has deteriorated and operating below the required threshold when the PCM looks at both the front and rear 02 sensors to make a determination. Plus there are alot of variables as to when that specific DTC will set so it may not set every drive cycle or on a specific trip. See some engine parameters below:

- Engine speed is 1,410- 3,100 rpm.
- Calculated TWC temperature in PCM is above 574 °C {1065 °F}.
- Calculated load is 15- 50% (at 2,000 rpm)

On my Actron scanner DTC P0420 shows up as a pending code. From what I understand the SES/CEL MIL illuminates if the PCM detects the above malfunction conditions in two consecutive drive cycles or in one drive cycle while the DTC for the same malfunction has been stored in the PCM. DIAGNOSTIC MONITORING TEST RESULTS and PENDING CODE are stored if PCM detects the above malfunction condition during first drive cycle.

Hope the info helps.

[eippermx]

Bob, thank you for your post. It's a done deal. I'll have a new magnaflow cat installed next week on the vehicle.