3.0 Check Engine Code P0420

[red_ranger97]

Hey:

I've got a '97 with a 3.0 V6, and very high miles (just went to 278,500). The truck runs great, uses really no oil (maybe 1/2 quart every 3,000) and looks good.

Over the last year or so, I've had an intermittent problem showing up with the check engine light. It pulls a code P0420. To try and fix it (and per the code), I replaced the catalytic converter (now has 10,000 miles on it since the new replacement). That didn't seem to do the trick -- The CEL stayed off about 2 weeks, and the shop that did the work had told me in advance they weren't 100% sure it was the problem so no options there. After being frustrated about spending the $$ to no avail, I replaced the rear oxygen sensor (after the cat) last fall, and both front oxygen sensors last month.

As the truck runs fine and gets its usual mileage and decent power, normally I wouldn't particularly care about it at this point. However, I'm moving to a state where it will have to pass an inspection and doubt that it will with the CEL illuminated.

Any advice on other things to try? Thanks!

 

[red_ranger97]

Bump.

Any other recommendations out there? Given that the truck runs good, any ways to deal with this check engine light?

Thanks!

 

[Sasquatch_Ryda]

It can also be the upstream o2 sensor. The upstream sensors will switch back and forth between rich and lean in closed loop mode and the down stream (after cat) sensor will stay steady. If the up stream sensors a staying steady it can cause this code to be set as well.

Does the truck run ok, no miss fires? If it is miss firing it will be dumping unburnt fuel into the cat killing it in short order also setting this code.

If the down stream sensor is mirroring the upstream sensors, the cat is no good.

 

[bg@f.ord]

My problems

If anybody noticed my check engine light problems.I have an update.I had to smog for registration.I wish my results were better,but that's not always the case.I had very low compression on one cylinder.I had a shop (friends of the family)handle it.After a V6 valve job,I feel like I'm driving a race car.They're the bomb.Tune Craft La Mesa,CA.I used to do my own work.Didn't trust anyone.Good price and my clothes aren't ruined.Now I feel like I can upgrade my truck & submit a picture in the future.I'm glad I had faith.------------------------------BRIAN

 

[red_ranger97]

It can also be the upstream o2 sensor. The upstream sensors will switch back and forth between rich and lean in closed loop mode and the down stream (after cat) sensor will stay steady. If the up stream sensors a staying steady it can cause this code to be set as well.

Does the truck run ok, no miss fires? If it is miss firing it will be dumping unburnt fuel into the cat killing it in short order also setting this code.

If the down stream sensor is mirroring the upstream sensors, the cat is no good.

Thanks for the suggestions, I appreciate any advice on this. I replaced both upstream o2 sensors just over a month ago, and the downstream sensor last fall. The truck runs great, with no apparent misfires. I get decent power and fine mileage with it (I've had it since new, and now has 280,000 on it.)

I will say that I re-set the CEL a week ago, directly before going on a long drive on the interstate. The truck ran fine, and the light stayed off for the entire first half of the trip. I stopped for lunch for about an hour, and got gas. When the engine got warmed up again the light came on. I don't know for sure, but it seemed like my mileage went down early on the next hour of my trip -- does it go into a default rich mode first?

Also, I'd hate to think it would be a bad cat after getting it replaced so recently.

Is it possible that somehow my truck isn't correctly switching between lean and rich to show the variation across the cat? Could this be a problem with a different system?

Thanks again!