Ever had a P0420 code (cat) appear and go away by itself?

In January 2007 at 53,769 miles, the check engine light in my '05 2.5i wagon (PZEV) came on, and it threw a P0420 code (cat converter efficiency below threshold). Both cats were replaced under warranty at that time.

In November 2009 at 104,882 miles I had the WVH18 ECM reprogram done. (according to the letter from SOA, the ECM needed to be reprogrammed because under certain driving conditions, the cats may develop hi temps that exceed design specs).

The other week at about 145,250 miles the CEL came on while driving on the freeway, and stayed on for the next 1000 miles or so. I hooked up an OBD-II reader which showed the P0420 code. I didn't reset it. Today - for no apparent reason - the CEL stopped illuminating and the cruise control stopped flashing. I hooked up the OBD-II reader again, and it still says P0420.

I have an appointment at the stealership for next Tuesday to check out the CEL issue. But now the CEL is no longer illuminating, yet the P0420 code still shows up in an OBD scan.

The fact that the P0420 code is still present - does that mean there is still something going on with my cat (that's not bad enough to cause the CEL to come on), or does it merely mean that the cat problem has resolved itself and the code just stays there until it's reset?

I kind of want to have it checked out at the stealer, because if something's wrong with my cats, I'm very close to the PZEV emissions warranty running out at 150K miles. However, they charge $100 to diagnose CEL problems. if they're just going to tell me that since the CEL went off by itself, everything is all good and the code just needs to be reset, I can reset the code myself.

I popped this code 2 months 2 weeks ago.
Was a week after my 105k km (~65k miles) oil change. Popped the hood to check o2 sensor, realized was missing my oil fill cap. Bought a new cap, then cleared ecu by popping battery for 20 minutes while I cleaned off the excess oil about my engine compartment.
Since then, haven't had any codes pop with ~3k miles driven.

Now would this example be the same situation for you? No knowledge, just passing along my experience.

Don't know how applicable this is, but this has been an issue on Lexus vehicles for years. My GS400 would intermittently do the same thing your CEL does. Even replaced a cat because of it. Turns out, the root issue was faulty post cat O2 sensors. Replaced them, and no problems since.

Try swapping O2 sensor for known good one, or replace with new. Should be cheaper than a new cat.

FYI, I don't own an lgt, YET, but just thought I'd try and help.

I replaced o2 sensors as they went bad, but replacing them as needed can trick the computer into thinking that there is a problem with the cat. Try replacing up and down stream sensors....

The CEL and cruise lights stayed off for about 200 miles and numerous ignition cycles, but they came back on again while driving on the freeway and now they're staying on.

I hooked up the code reader, and now I have two codes, one P0420 and one P0420 P (I guess the "P" means "pending", but it's weird, the code reader manual says that a pending code will say "PD" for pending. The code reader says I have 2 DTC codes and 0 pending codes.)

Originally Posted by TenzoGT

I replaced o2 sensors as they went bad, but replacing them as needed can trick the computer into thinking that there is a problem with the cat. Try replacing up and down stream sensors....

Good to know, I may end up doing that. However, I think I'll take it to the dealer first to see if the problem might be covered under my emissions warranty. The original set of cats only lasted about 53K miles before they sh#t the bed, and this replacement set has about 92K miles on them...

So I took it to the dealer today, turns out the cats were bad again. They quoted me $4,000 to replace them , but I reminded them that it should be covered since it's still within the 150,000 mile emissions warranty applicable to PZEV vehicles.

They called the warranty clerk to confirm, and they're going to order the cats and replace them under warranty.

Good thing they went bad now (at 147K miles), another 3,000 miles and I would have been SOL!

So you are on your third set of factory cats now?

Originally Posted by slowGT

So you are on your third set of factory cats now?

I will be once they install these new ones.

Have them check your injectors and O2 sensors when you are there since if you have had that problem more than once there has to be a problem. If the injectors are unbalanced so one gives more or less than the others then the cat will get unnecessary strain to take care of the problem.

I agree.

What gas do you run? Any gas additives? Running too rich, or what is your gas mileage like compared to other PZEV's?

Killing two cats within 150,000 miles seems strange to me.

Although I see your ECM was reprogrammed, so hmmm.

Originally Posted by ehsnils

Have them check your injectors and O2 sensors when you are there since if you have had that problem more than once there has to be a problem. If the injectors are unbalanced so one gives more or less than the others then the cat will get unnecessary strain to take care of the problem.

Is there a DIY way to check the injectors and sensors? I never suspected that, I change my spark plugs every 30K miles and the ceramic parts are always the same tan color.

Originally Posted by slowGT

I agree.

What gas do you run? Any gas additives? Running too rich, or what is your gas mileage like compared to other PZEV's?

Killing two cats within 150,000 miles seems strange to me.

Although I see your ECM was reprogrammed, so hmmm.

Arco 89 octane, I never put any extra additives in. My mileage is around 20-25 mpg around town, low to mid 30 mpg highway.

I just started having this same issue on my 2006 2.5i, 138k. I have not had the reprogram under recall done yet, and I'm hesitant to take it to the dealer now since I figure they will want to fix something expensive, like the cats. Unless, they are covered under warranty? How do I know if they are? My car is newer than the OP's car, but it does not have pzero badge on the back of it.

Originally Posted by itzed

I just started having this same issue on my 2006 2.5i, 138k. I have not had the reprogram under recall done yet, and I'm hesitant to take it to the dealer now since I figure they will want to fix something expensive, like the cats. Unless, they are covered under warranty? How do I know if they are? My car is newer than the OP's car, but it does not have pzero badge on the back of it.

Your owner's manual folder should have a warranty booklet that will lay that out. The PZEV warranty runs to 150K, not sure about other models. My '05 doesn't have a PZEV badge, but it's a PZEV car, however, it's from California. It would be worthwhile to bring it in to the dealer, but if your cats need replacement and they're out of warranty, an independent shop can do the work much cheaper.

The cats will never just go bad by themselves. If a cat is bad then there's got to be something else wrong with the car (O2s, fuel system problem, etc.) I have a little flier here at work floating around somewhere that explains it better than me. I'll try to find it.