P0420 efficiency is related to the temperature within the cat converter. You can test the efficiency through the downstream o2 sensor.

To test, remove the downstream o2 sensor. You will need a multimeter and a torch for heat. There’s a total of 4 wires for the o2 sensor. The thicker 2 of 4 wires are the heater circuits. Apply power and ground and use the resistance setting on the multimeter.

Apply heat to the sensor tip and watch for the voltage rise. When it’s slow to rise, the sensor is failing. Compare to how it reacts when heat is removed. When it’s slow to fall, sensor is failing.

Shine a light from the tail pipe end to view the substrate and see if it’s blocked. If it is, cat is failed. Check the soot to see if there’s oil in it. That indicates blowby through the exhaust valves.

 

My wife just totaled her exact same year and model. Airbags deployed so it is junk. I pulled the catalytic converter off it along with the sensor. I'll swap it into the P0420 failure on my vehicle. Coming out was easier than I thought it would be. 4 bolts and unhooking the sensor. Should I have any problems with the exhaust leaking after I do it?

Smog comes up in September on my P0420. It's an oil burner. Wish I could swap out the entire engine in my wife's totaled car, but that's to much work for a 69 year old. My wife's car did not burn oil and the motor survived the head on.

My 2013 Corolla 120K miles is only worth around $7K-$8K, ...if that. I'm not going to spend a bunch to keep it on the road.

Just spent $25.5K "Out The Door" for a new 2025 Corolla LE for my wife after she crashed hers. If mine gets to be too much, I'll do it again for myself.

If I can keep mine rolling and smogged a couple more years, I'll be happy.

I'll post back with the outcome of swapping out the Cats.

 

Inspect your doughnut gasket before you reassemble the exhaust. Do the paper test afterwards. When paper moves, there’s a leak.

It could take as long as 30 days or more for the catalyst monitor to be ready for smog check. Especially if the car isn’t driven very much. Though, you could try cataclean, but that’s not a guarantee to work.

 

Does smog check get out of wack after a P0420 code? I cleared the code with my reader. And now shows no stored codes or pending codes, but has a permanent P0420 code?

I'm swapping the whole Cat assembly from wife's car that just passed smog. We drive it a lot. My wife is afraid to drive the new Corolla I bought her.

 

There’s 2 things that California wants to see.

Catalyst is a readiness monitor that gets the information it needs from the overall performance from the o2 sensor and cat data over time. This monitor will not be ready until it gathers enough data to say it is good to go. The smog station will have you come back.

P0420 DTC will not set until the ecu is satisfied with the data points it needs to turn off the DTC.

A few years ago, I had to service my o2 sensors in my Mazda. I drove around 30 days before the monitor was ready. The codes went away in no time. I drove both city and highway for the drive cycles.

 

An exhaust leak upstream of the sensor will set the code. That's the most diy thing to check.

Beyond that you gotta look at live data or consider a bad cat.

Nobody mentioned exhaust leaks so I had to chime in.

Exhaust stuff is usually rusty. I use oxy acetylene before I touch ANYTHING on the exhaust. Map gas is only for DIY and painfully slow and ineffective.
What I'm saying is: don't snap a stud off in the head, or strip out your O2 bung by not heating and brushing everything off.

If you have pinhole leaks upstream of the second Oxygen sensor, just gloop some red rtv or muffler mud on it 🤞

Burning a quart in 1,000mi isn't great for cats, but I've seen worse oil burners with good cats somehow.

 

An exhaust leak upstream of the sensor will set the code. That's the most diy thing to check.

Beyond that you gotta look at live data or consider a bad cat.

Nobody mentioned exhaust leaks so I had to chime in.

Exhaust stuff is usually rusty. I use oxy acetylene before I touch ANYTHING on the exhaust. Map gas is only for DIY and painfully slow and ineffective.
What I'm saying is: don't snap a stud off in the head, or strip out your O2 bung by not heating and brushing everything off.

If you have pinhole leaks upstream of the second Oxygen sensor, just gloop some red rtv or muffler mud on it 🤞

Burning a quart in 1,000mi isn't great for cats, but I've seen worse oil burners with good cats somehow.

It’s an efficiency code. The cat is not reaching the temp threshold. A leak would not cause a cooling effect.

edit : the cat needs to maintain above 600F to keep working to convert the nox and hc into co2. Oil blowby causes the substrate to get blocked and impact heating.

 

I've swapped out the Cat from the totaled vehicle that is same make model. Bolts were not rusted or too bad to get out on either car. I initially tried an impact wrench, but they would not budge so just a breaker bar and a slight amount of torque popped them loose. Thankfully I had I had a long extension for the front 2.

When I pulled the cat out, it didn't look all gummed up or black on the screen I could see. Maybe it was deeper inside.

Everything seemed to go OK except when I put the carpet back after hooking up the O2 sensor, I pulled the dummy move of putting it over the accelerator peddle causing the car to scream way up in RPMs. Hope this didn't break anything revving the engine that high.

I've drove it around a bit and everything seems OK. Between me and my wife we'll drive the crap out of it the next few weeks. I need to register it by the 24th of September,

I have no check engine light coming on, but imagine from the discussion here that my reader will show the "Permanent P0 420" code until I drive it enough to get the computer to reset it. That code was not reset when I reset the codes using my reader.