p0420 engine code after every oil change???

Hey all,

I’ve noticed every time I change my oil I get a P0420 pop up on the first time I drive after an oil change.

If I clear the code, it comes back again for a week or two. Then it stops coming back.

I have a new o2 sensor (the one that’s easier to change) which did clear my code last time I got this code. In the past I cleared it multiple times without replacing any parts.

It’s a 3.4l 5mt 4x4 TRD at 190k miles. I use mobile 1 high mileage 5w30. The truck runs good and always passes smog, although it’d fail right now on a technicality due to the check engine code.

I actually do have to pass smog soon to get the truck registered.

What’s going on? Is this normal?

 

It's a bank 1 catalytic converter, its failed and it takes a few cycles to trigger again.

If you clear it prior to smog they will see the test is incomplete.

 

Not sure How changing the oil is tripping a PO420 code .

I have never managed to cause the problem not sure even how.

Do you change the oil with the Truck on the ground or lift it up in some way??

Maybe if it is lifted up the exhaust flexes enough to cause a minor leak between the 2 cats someplace enough to trip the code then after enough time it seals again code goes off .

This is the only way I can think of .

Unless your some how causing enough oil or raw gas to dump into the exhaust that fouls the sensors but the smoke should be noticed .

It happens by pure chance??

 

I have no idea. I filled the oil maybe a little too high. It’s at the very top of the range on the dipstick (at the top dot).

 

Maybe actually fix what's wrong instead of continuing to clear the code. You're not fixing anything by clearing it.

It wouldn't fail on a technicality, it'd fail because your cat isn't working correctly.

You can try replacing the other O2 censor, but I'm betting it's your cat. Magnaflow makes one that's significantly cheaper than the OEM ones: $200-300 iirc.

 

Lubrication system and exhaust system... seems there should be no connection physically, or electrically.

Just grasping at straws here. You don't by chance disconnect your battery cable when you change oil, do you?

 

I don’t disconnect the battery. My hypothesis is overfilled oil burns off and overwhelms the cat.

I don’t get any sense the truck is actually polluting. It runs good and cleanly as far as I can tell. I don’t want to throw cash at it for a cat. So far I’ve passed every time just by clearing it and driving to actually pass smog. My problem happens once a year when I change oil, not constantly. I just drove 1000 miles with no codes, then I drove the same drive that triggered it the first time and sure enough I got the code. The drive was very hilly that threw the code. I wonder if hills cause oil to burn more.

 

If you're burning ANY oil, that's a problem, especially if going up hills makes you burn more. These aren't 1980's domestic pickup trucks. Toyotas can actually be running like shit (relatively speaking) and still pass the emissions.

I had a 1st gen 4Runner that died while during the sniff test (faulty distributor) and it still passed. The thing had 300k+ on it, an worn out turbo, and the emissions were still nowhere near the threshold of failing.

Look at the snif test results from your previous SMOG tests, that will like tell you if your cat is going bad, or if maybe just your 02 censors are going bad.

 

Do you have a meter that can track live data?

If not the oriellys one can, you usually just have to walk the attendant through it lol

Track what the voltages look like from the front and the back O2 sensors if it looks anything like mine you've got a bad cat... you want the second O2 sensor to read a basically flat line

 

@pontoon sorry forgot to attach the picture so you get a sense of what I was talking about

 

My truck has always passed the sniff test. That’s why I think these engine codes are bogus.

The engine temp never leaves the middle spot on the range.

I was thinking maybe if it’s over full oil can splash into some places it normally wouldn’t and burn. Idk much about the inside of engines.

I’ll replace the downstream sensor since I have a new one laying around. I had read in the past the downstream shouldn’t affect the results??? I did replace the upstream and fixed a code in the past that way.

 

The codes aren't bogus. It's just that Toyota has a higher standard for noticing when something is wrong than what the SMOG regulations require. I've seen multiple cars that are running like shit and still pass smog. If your emissions are barely squeaking by, then there's something wrong with the engine that should be addressed, though it may not be bad enough to fail a smog test.

As long as your 02 censor is an OEM one, that's a great route to take before buying a cat. Non-OEM ones are known to (sometimes) never work correctly. Many people have gotten frustrated by this, only to replace with an OEM one and their code immediately goes away.

The first 02 censor controls engine performance, and the 2nd one just compares the results from the first one, to see if the cat is doing its job. As @Abeyancer posted, if both censors are showing essentially the same thing, your cat is bad.

So to answer your original question, no, this isn't normal.

What's likely happening is that either your cat and/or O2 censor are marginal, trending towards bad, and any number of random little things cause the ECU to trip an error code. Sure, you reset it and it's fine for a while, but it's clear you haven't actually fixed anything. You clear the code and it's fine for a while because whatever random thing hasn't tripped the code again, until it does. I highly doubt it's actually related to oil changes.

 

What if anything did you do just clear the codes and drive till all the tests run??

 

Two years ago I replaced the upstream sensor, reset code, passed smog, no CEL for 2 years.

About a month ago I changed the oil and the CEL came back. I reset the code once without doing anything and I got the code again. When I cleared it I had gotten the code 3 times I think.

I replaced the downstream sensor, reset the codes in the ECU, then drove it until the ECU passed the catalyst.

Now time to quick get it smogged before my registration expires.

 

Just went to the smog station and passed smog.