Parked truck for a week and now I have P0420. Reset it and it stayed off about 5 days, now it's back on. Any ideas for troubleshooting to narrow it down? It's a long list of possibilities and I don't want to throw parts at it.

 

Almost always a failed catalytic converter, usually as a result of some other unresolved fault that's dumped too much fuel into the exhaust over an extended period of time. Usual causes are misfires, "lean" codes due to vacuum leaks, upstream O2 sensors, skewed fuel trims or a too-cold thermostat.

Converter performance or lack thereof can be confirmed by graphing the upstream vs downstream 02 sensor voltages. On a failed converter, the two waveforms will be almost identical indicating the cat has gone chemically inert.

If you're subject to OBDII-based emissions compliance testing, you'll have to fix it in order to pass.

1. Try one of the cans of magic juice to see if it can "clean" the converter. Works sometimes, at least enough to pass e-testing.
2. Install anti-foulers on the downstream sensor to back it out of the exhaust flow. This causes the downstream sensor's output to be dampened mimicking a functional converter. May require two of them stacked to achieve desired results.
3. Install "MIL eliminators". Google it.
4. Replace the converter AFTER resolving any condition that caused the old ones to fail or the replacement converter will meet the same fate.

 

Thank you SHO89, this is what I was looking for. I just did the Marvel Mystery oil cleaning procedure yesterday, need to drive it a little more to use up the treated gas, then will reset the code. I had some plugs go bad about 6 months ago, that's probably what caused it, or the Cats may just be old at 280K miles.

 

Not only will MMO not clean cats, it can also possibly hurt them. Get one of these:

https://www.autoguide.com/best-catal...erter-cleaners

 

So far the MMO worked, light is still out after 200 miles. GLC, I was wondering if the MMO would be an issue since it is an oil, but it did the trick so far. Another owner tried MMO on 3 different vehicles and it worked for him, so I gave it a try. I did see another method where Sea Foam was poured into the cat through the O2 sensor hole. Let it soak 30 minutes, then start. Smokes a lot but owners say it worked for them. This may be a better product than MMO since it is not an oil. Wither way, thanks everyone for your input.
One more general comment: cleaning a CAT only works if you have no other issues like misfires, vacuum leaks, or burning a lot of oil. If these issues are present the CAT will self destruct unless they are corrected first.

 

Opinion is you have to many ODDS against you for any long term solution.
Misfires in the past.
Age in miles.
Oil contamination etc
The Cat health is monitored by the rear Ox Sensors for Oxygen storage.
Ox sensitivity is what they do so the content left in the rear CAT is sensed, then the variation or level detected is compared to the front sensor normal variations they do to feedback fuel control signal.
When the variation ratio goes out of limits, the code is set. The rear Ox sensor output should move very little.
Since the Cats depend on running at high temps for them to work, any overheating, contamination or physical break-up of the 'substrate' will be detected.
This can be seen on a Scanner looking at live data action of the rear sensors.
Good luck.