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2002 Toyota Corolla - 2 codes

I get both an OBD Scan P0420 and P0420 pending. What does this mean if both of them are displayed?

“Pending” means the ECM detected a fault but it’s not severe yet. In your case that would be an old code since you now have a full blown P0420, which means your catalytic converter is bad. Have a good muffler shop look at it.

Okay thank you, I will take it to a muffler shop.

Hold the fort here. The P0420 is a two trip DTC (diagnostic trouble code). That means the first time the fault is detected, it is stored as a pending code. If the fault does not reoccur in the next three drive cycles, then it is deleted. The CEL (Check engine light) is not illuminated.

If the fault reoccurs within the next three drive cycles, it is stored as a regular code and the CEL is illuminated. If the fault is NOT detected after the second detection in the next ten drive cycles, the CEL will go out, but the code will be stored for about 30 drive cycles. If the fault reoccurs withing that time frame, the CEL will come back on. If it doesn’t, then the DTC is deleted from computer memory.

Now a P0420 does not mean that your catalytic converter is bad. It only means that the rear O2 sensor signal is cycling too fast. That will happen if the cat isn’t working, but it can also happen if air is getting into the exhaust from a bad exhaust manifold gasket or cracked exhaust manifold or cracked warm up cat. It can also simply be a bad rear O2 sensor or even a bad front O2 sensor. The rear sensor is actually the most common cause in early 2000 Corollas, especially 2003-2008, but common in 98-2002 as well.

Also bad gas, misfire and other things can also cause this code. A mechanic with a dual trace oscilloscope on the outputs of both O2 sensors is one of the best ways to determine the exact cause, but you can expect to pay a diagnostic fee for this, but it would be worth it.


Quit the spreading of false information. My 02 Camry had the P0420 code. Replaced both O2 sensors and 100k miles later the codes have not returned.


There is no code that means Catalytic Converter is bad, replace now…

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Good call by @keith , @PvtPublic and @pyrolord314 ! They did you a rather big favor and may have saved you a good deal of money in pointing out whats what with the 420.

I would also have to agree with the guys here and hold the phone on the “Bad Cat Diagnosis”… that is not what a 420 is telling you.

It may turn out to be a bad cat…but those instances are rare in my experience. The two most common causes of a 420 are failing or failed O2 sensor…up or downstream, leaks in the exhaust system. Please do not just jump to assuming that the cat is bad at this time… You aren’t there yet…


Dave , stop guessing . The site already has a designated guesser .


[quote="VOLVO_V70, . The site already has a designated guesser .
He wouldent happen to be in California would he.

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Nothing false about it. Given the specific code a bad catalytic converter is a strong possibility. I’m glad you didn’t need one. If the OP takes his Corolla to a shop they can easily confirm or reject it as the cause.

it is a piece of black Russian humor on that

a nurse is rolling the patient in the chair, he asks “where are you taking me?”, and she answers “to the morgue”, then he says “but I’m not yet dead?!?!”, where she replies “but we did not get there yet either”

@alan.matsudo_163744, what is the mileage on the car and how much oil it consumes per 1000 miles?

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Yes how much oil does this engine use? These engines are known for blowing oil past the coked up rings. If it is using oil you are wasting money on a new catalytic converter. It will just ruin the new one in short order. If the car runs ok just drive it with that code as it will only return once the new cat is ruined from the oil contaminating it.

About 1/4 of a quart every 1,000 miles. I have 337,000 miles on this car.

at that mileage, catalyst may indeed be worn, but it is still worth checking if sensors may be the issue and/or leaks may be present

if it is only 1/4 of quart of oil per 1000 miles used, this car is in great shape, considering mileage!

Concur the first step is to have your shop compare the signals from the pre-cat O2 sensor and the post-cat. That’s the best way to test if the cat is good or bad.

Have you ever tried the catatlytic converter cleaner they advertise? Would you recommend that I try that also?

most likely it is a snake oil

it will not make things worse, but it will most likely be effective only in lightening your wallet

in your situation, as DIY-capable person, I would:

  • raise the vehicle and made a visual inspection of the exhaust system for rust, specifically trying to determine if you have exhaust leaks due to rust-through perforation[s]
  • if yes => go to muffler shop, end of story
  • if not => replace downstream sensor, liekly it will be under $25 for non-original, but decent sensor
  • clean the codes, drive on
  • if you are lucky and codes do not pop back up after 150-250 miles, consider matter settled
  • if not, your path lies to the muffler shop

Your car is burning oil like a lot of the 8th generation Corolla do.Mine was a 1999 and sold it because of that. It will get worst unless the piston rings are replaced with a revised version.There are a lot of tutorials on Youtube for the piston rings replacement.

his oil burn is well within the limits, especially considering mileage

tell about it to any Subaru owner, they will laugh and say it is “no burn”

I would say “hold on to this car as long as you can”

OKay thank you. I will look into it.