Code P0420 question
- Thread starter mgcorreia
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TheDurk
Elite Member
1. Exhaust leak anywhere before rear sensor.
2. Rear O2 sensor
3. Front O2 sensor.
4. Bad/damaged cats.
undergroundatx
New member
Before you replace any parts, try cleaning the Mass Airflow Sensor. My '97 SR5 throws that code every few months. I just pop out the sensor, spray it clean, let it dry and reinstall. Clear the codes and you'll be good for a while. Also, check to make sure your fuel cap doesn't have a dry-rotted gasket.
3rzrunner
Member
Yes the 3rz's exhaust manifold is poorly designed and seem to crack. Mine did as well as a few others check your manifold for leaks.
I just got the LCE long tube headers after it cracked.
Nelsonmd
New member
P0420 has nothing to do with the MAF sensor. It takes a while to throw the P0420, it is typically the last test to finish and can take months to finally trigger. Cleaning the MAF has nothing to do with "fixing" your code for a while, it just takes that long to finish the test again.
Search here for P0420, there is tons of info. Durk pretty much summed it up though. You could get an O2 sensor spacer to bring the sensor out of the exhaust flow, but that is illegal and may not fix your problem. You can also clean your CAT's (look on youtube), but since you can replace them for under $350, it wasn't worth it to me to go through the trouble of removing them and putting them back on without knowing it would fix it.
undergroundatx
New member
I only learned this trick after replacing the O2 sensors and having the code come back within two weeks. Cleaning the MAF clears the code for several months. My '97 has 276k miles and gets 300-400 miles per week at a solid 17mpg.
Nelsontacoma
New member
This is solid advice. My 96 threw that code every time the ECM would run the catalyst readiness test. All I had to do was replace the catalytic converter. $200 bucks and the code has not returned. It even cleared the catalyst readiness monitor for the first time since I bought the truck 1 year ago.
My tacoma keeps throwing the code too but haven't bothered to fix it yet as the light clears itself within a day or two anyway without me doing anything to it.
Also, the P0420 code is the "catalyst efficiency" code. It has nothing to do with your mass air flow sensor. The test for the catalyst efficiency monitor only runs when you drive over 45mph for 3 minutes straight without stopping or heavy acceleration. This is why it takes a while for the code to show back up. Basically the computer checks the readings from the upstream and downstream 02 sensors and compares the readings to see how well the catalytic convertor is "cleaning" the exhaust during this test.
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Ha! I knew it couldn't be that easy. Anyways I cleaned the MAF 3 days ago and still haven't seen the code come back up. It looked pretty dirty, but not horrible. My 4runner drives noticeably better now. Before cleaning the MAF, after I reset the CEL I'd have p0420 come up at the latest by the next day. I drive on the freeway to work every day over 45 mph for 20 min each way. I'm taking a trip to the mountains Sunday, its a 3 hr drive each way, mostly freeway, if that doesn't trigger the p0420 I don't know what would.
I'll look into cleaning the cat converter and getting a new set of o2 sensors. I want to replace both of them right? Where's a good place to buy these parts?
I'll look into cleaning the cat converter and getting a new set of o2 sensors. I want to replace both of them right? Where's a good place to buy these parts?
...and I know the p0420 relates to a bad o2 sensor and/or bad cat converter. But if these were bad, wouldn't I be getting horrible mileage? I currently get 18-19mpg average and great performance in my 4cyl manual 4runner, how could they be bad? I don't hear any rattling out of the cat converter either.
Nelsonmd
New member
Don't get me wrong, cleaning the MAF is a good thing and can produce noticeable improvement, but it just isn't related to the P0420 code. The catalyst check sequence is actually a bit more than just driving above 45 for 3 minutes, it requires a number of things that can take a long time to complete. Also, if you are just borderline out of spec on CAT efficiency (meaning you are kind of going back and forth across the spec line), then the ECU will possibly throw a pending code and wait for multiple confirmation measurements before finally throwing the code. This is also why it can take months to finally be below threshold long enough to actually throw the code again.
I commute 80 miles a day both on the highway and in the city and it would sometime take mine months to come back on, sometimes a day or 2, even with the exact same driving pattern. I also go up to the mountains regularly, same deal. It's voodoo.
You only need to replace the rear O2 sensor. The front sensor is an A/F sensor, not an O2 sensor. Also, only buy Denso O2 sensors, get it off amazon, they are the cheapest around. Bad CATs won't really cost you performance or fuel efficiency, just cleanliness of your exhaust.
T4R_Finally
New member
Nothing at all wrong with cleaning the MAF - should probably take the time to do it everytime you change the oil. I don't, but it's not a bad idea. It's fairly easy to do and once you've bought the cleaner, it doesn't cost anything.
However, as has been stated, doing that has not done anything to keep that code from coming back. It can take hundreds of miles for it to come back. 3 days does not put you in the clear. If you have an OBDII scanner, most will show you when all the system tests have been completed and only then can you judge success with the code not coming back. When I first bought my 2000 a couple of years ago, I got the CEL and the P0420 code within 20 miles of driving it. I stopped at the first Auto Zone I found and they cleared it for me. I drove it a couple hundred more miles with no CEL and it came back on again. After researching here, I decided to swap the rear O2 sensor before replacing the CATs. It wasn't until it was driven for a month or so that I felt comfortable that the code was gone for good. That was over 2 years ago, and now it's back. But also do not see a drop in performance/mpg. I suspect an exhaust leak but haven't found one yet. You'd probably be surprised how often the issue is just a faulty sensor.
Just like the potential pink milkshake issues with the radiators, I suspect these vehicles are getting to where the engines are outlasting all these other parts expected lifetime.
TheDurk
Elite Member
Not urgent at all. Just be aware that it can take a while to diagnose and fix and then set ready on all the monitors so you can clear inspection. So don't wait until the week before to deal with it.
On the other hand, if the code only pops up every few months, just ignore it.
undergroundatx
New member
Yeah, even though it's "not related to the P0420", if you leave your gas cap loose, it will be the code you receive.