rock99

i have this P0420 check engine light code and i dont really know what to do. i had the same code pop up about 18 months ago when i was getting it my 2000 4runner to pass smog. i replaced an Oxygen sensor and code went away, passed smog no problem. now the code came up again this past weekend. i use the vehicle very rarely mostly for short trips around town.

could the O 2 sensor be bad again?
is it a more serious problem?

seems to run fine but i will have to get it smogged again in 6 months and will need to address it sooner or later.

any suggestions/help?

Riverman

Did you disconnect the battery during that time?

I had P0420 after doing some work on my truck that required the battery to be disconnected. My mechanic explained that the PCM had to relearn the status of the catalytic converter and that the P0420 would go away over time if I just drove the truck for a while. He was right - the light went off after another couple days of driving.

If you haven't disconnected your battery, it could be a sign that your catalytic converter is going bad. That's the job of those two o2 sensors - one is in front of your cat and one behind, and they use the difference in readings to determine whether your cat is cleaning the exhaust well enough. So no matter how many o2 sensors you replace, it will never fix that problem.

Bosser

Hello,

I've been fighting the P0420 code since June (it is now october). CEL came on and brought it in. First they changed the front cat, fine for a 2 days then light came back on. Then they swapped out the rear cat. Same thing, fine for a week or so then the light came back on. They said they changed out the sensors and cleared the code but then it came back again. Finally after talking to the manager, they did a complete change out of everything, both cats and sensors. The sensors are the densos but the cat i believe is still an aftermarket cat from napa. This kept the light off for about 3 weeks, but boom...light came... (btw, the truck is an '04 tacoma with 320K miles...) any suggestions? Do i bite the bullet and get the OEM cat?

vasinvictor

iTrader: (3)

Check for any exhaust leaks. Sounds like you shouldn't be having any real problems just spring for THIS MOD, the MECHANICAL O2 SIMULATOR

Dear reader, please be aware this above link will take you to a Honda Civic forum where you might see things you can't unsee. Potentially NSFW/NSFL. Click at your own risk. The mod, however, does work. Source: my own 1998 Mazda MPV 4x4 minivan.

Bosser

i checked for leaks, the only thing i could find is that my muffler does have a small hole in it. Would this be enough to trip the 02 Sensor?

vasinvictor

iTrader: (3)

No, there's usually a small hole at the bottom rear of a muffler for condensation drain anyway. You'd be looking for a rather large leak. I'm sure they would have discovered and fixed it while replacing your cats, if there was one. If you used an aftermarket cat, this could be the cause, especially if it's a higher flowing cat. Hate to see you buy another cat and still have the problem. An oscilliscope would be able to tell you if the rear o2 was functioning properly.

TheDurk

No, because any leak has to be upstream of the downstream O2 sensor. The muffler is downstream of both.