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Hello, this particular car was burning too much oil and i rebuilt the engine with new pistons. No more oil burning now. The issue is, i have a P0420 code which refers to the cat running below threshold efficiency. No other DTCs are obtained. I checked the inlet and outlet temperature of the cat and seems like 200 and 300 deg F respectively during idle (engine was warmed up by 15 minutes of running prior to testing). Checked the front O2 sensor voltage and seems it is changing from 0.1 to 0.9 volts. Checked the MAF sensor voltage and it increases with higher throttle. Checked the TPS, IAC resistances and they are within spec. Sprayed carb cleaner over the engine while it was running and i could not find any leaks. Short term fuel trim (bank1) is -6% and long term fuel trim (bank1) is +6%. My OBDII software (PCMScan) is also giving me some STFT and LTFT on bank 3, which i do not understand as the engine is 4 cyl in line. So far the only items that i could not check are the rear O2 sensor voltage and fuel pressure. The engine seems to idle just fine around 700 rpm, once in a while it drops down to 550 though (which leads me to point to MAF), but again runs just fine. I would say, little hard to accelerate though. I do not know what other items to looks for. Any suggestions? Appreciate your comments.
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A P0420 is set by the downstream O2 sensor. If it moves similar to the upstream O2 sensor, then the computer throws a P0420 code. The Gen 8 Corolla was plagued by this issue. The software is overly reactive and tailpipe tests usually show that it is operating within standards. The Gen 9 got a reflash, the Gen 8 did not. So this is a VERY common problem on this car. (I have answered the P0420 in a Corolla question more times than I can count)
Something you can do is replace the downstream O2 sensor with a Denso and hope the cat isn’t too far gone, but in my experience, it is seldom the O2 sensor. However, if it is not a Denso brand, things like this can occur. Something to remember is that you should only use Denso O2 sensors in Toyotas.
I have had success replacing the CAT with kits sold on EBAY. So I say this… more than likely, a new CAT is needed.
Also, for Idle, A Gen 8 should Idle between 650 to 750, so if it is truly going down to 550 then something is amiss.
As far as your fuel trims, they are within spec + or – 20%.
The MAF should be measured on the Grams per second or else you wont be able to read much. May need a cleaning. The wire for the MAF is down inside. Not the red bulb that is obvious (That is the IAT sensor, and it seems to be working properly, but can be cleaned as well with MAF Cleaner)
TPS’s on Toyotas practically never go bad… Wouldn’t spend any time there.
FWIW I have rebuilt 7 1ZZ-FE engines and have owned 6 Corollas. (I flip them) :whistle:
Good luck.
-Karl
Thank Karl, I appreciate your comments.
About the cat, since the downstream temperature is higher than upstream, i would like to believe that cat is doing what it is supposed to do. As you mentioned the rear O2 sensor can also throw in the P0420 code, I do not know how to differentiate between a bad O2 sensor and a bad cat by measuring voltage on the sensor. In both cases, I suppose the voltage readings will not be steady, is this right?
Interestingly, I used to have the MIL on after driving 80 miles. After few months, now it shows up on dash after 10 miles of driving. Something degrading?
I actually cleaned the MAF sensor with carb cleaner few days ago, before posting on this forum. I have little concern about the IAC valve though, it has a high pitch sound at idle, but goes away as soon as the throttle is increased. Somebody posted a video of this on youtube (https://www.youtube.com/watch?v=yldykQZd2S8). However, i do not see how the IAC would cause the P0420 issue.
Thanks,
TanvirYou also need to consider the engine was burning oil before, and could have damaged the cat. Perhaps not enough to clog it, but enough to coat the sub-straight enough to cause the code. Just for the heck of it, you might consider cleaning it to see if you get a result.
Please keep us updated.
OK,
A few things:
1. The P0420 is only set by the downstream O2 sensor. NOTHING else does it. The IAC has nothing to do with a P0420.
2. As I stated earlier, you can have a good cat, and the code will still be thrown. It was an issue with the software.
3. Reading the O2 sensor by backprobing it is a technique, but will not really help. No matter how you read it. I read mine with a scan tool… The code is thrown when the rear O2 sensor is sending pretty much the same voltage as the front (upstream) (which will be jumping from .1 to .9)… A good signal from the downstream should hold around .7 v and above at idle when at operating temperature. A scanner can allow you to look at both… but all it will tell you is what I already suspect. The downstream is jumping around like the upstream (stochastic as it is supposed to be)… So it does you no good to read the downstream…
4. The only way to get rid of this code is 98% of the time replacing the CAT. It CAN be the downstream O2 sensor, but very rare… The downstream is not fun to change. It goes through the floorboard and you have to raise the carpet and move a plastic air channel on the floor… I digress…
5. BTW… You never want to clean a MAF with Carb cleaner… It should be done with MAF cleaner… That said, Even Toyotas says they are not supposed to be cleaned with MAF cleaner, but I have not seen issues with using it.
Eric suggested cleaning the cat… I still have not formed an opinion on this… Have tried all techniques…Some success, some failure, but since if you remove the Cat, it is easy and relatively affordable to replace.
Simply go to Ebay and type “2000 toyota corolla catalytic converter”
For $134 you get a new resonator and pipe as well as catalytic converter…
I wish you luck, but there is no magic fix for this…
-Karl
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