PO420 Code - What would you do first?

[burnt03]

Code keeps coming back, read some of the posts on the forum...

What action would you take first? Oxygen sensor?

[dorman68]

Code P0420 means you have a catalyst efficiency failure.

• Check for exhaust leaks by the manifold, pipes, converter.
• Check for vacuum leaks at the intake manifold, its very common for the intake gaskets to go bad.
• Check your spark plugs
• Use a scan tool to check the oxygen sensor
• Check downstream oxygen sensor, replace if necessary
• Replace the catalytic converter

The catalytic converter has an 02 sensor front of and behind it. The computer reads the difference between the 2 and determines the efficiency of the converter. Most people who have experience this problem ended up replacing the catalytic converter.

[JasonA]

Originally Posted by dorman68

Most people who have experience this problem ended up replacing the catalytic converter.

I agree; from what I've read, catalytic converter replacement seems inevitable. You may still be covered by the 8 year/80,000 mile Federal emission warranty. I think it's longer if you're in California. That would be a free replacement.

Edit: I see you're in Canada. Not sure about the emission warranty requirements up there.

[ShawnM]

Its extremely rare for it to be anything other than a bad converter.

[fishexpo101]

Miles on car? Anything significant worth noting - oil consumption, blows smoke, anything different in drivability, fuel economy, etc.?

I wouldn't condemn the catalytic converter just yet, until you check some other stuff out first. Poster dorman pretty much summed it up - tThe P0420 code is set when the ECM sees that the waveform of the upstream O2 sensor (pre-cat) and the waveform of the downstream O2 sensor (post-cat) look too much alike. This can be caused my an exhaust leak, bad O2 sensors, or bad catalytic converter.

Should check for exhaust leaks first (especially close to the O2 sensors, I've seen leaks right at the O2 sensor bung), check the O2 sensors (best to backprobe them, if not -data logging with an OBD-II scanner), if those check out OK - then it is likely a bad catalytic converter.

[tigerclaws1318]

also check your mass airflow sensor.

[holler1]

Another failure mode is a faulty ECU. Based on my experience over the last six months with my daughter's 03 Corolla, after checking the O2 sensors and for manifold leaks as indicated above, my next step would be to check to see if the ECU is in the range for the Tech Service Bulletin on the P0420 code and if it has been "flashed" with new software. If it hasn't, I would get it flashed before spending $$$ on a new cat converter. It cost me $94 at the dealer, and the code has not come back after about a month of driving. I will attach the TSB if I can get it uploaded. There is a pretty long thread on this that I just updated last week, so it should be easy to find. If the ECU is a Denso, it can be flashed. If Delco, it can't.

Sorry, don't see a way to upload a file. Look for TSB number EG028-04, Title:
M.I.L. ON DTC P0420
Models:
03 - 04 Corolla
If your VIN is lower than these, it is in the range:
Shokki JTDBR3#E#52048919
NUMMI 1NXBR3#E#4Z339353
TMMC 2T1BR3#E#4C312392

[burnt03]

Thanks for the replies!

On autozone it notes a specific resistance between the +B and HT terminals, that's the way to test it I guess? I have an OBDII reader but doesn't have the real-time reading option.

The car has 180,000k (about 110,000mi) and runs great. Doesn't burn or leak any oil, good gas mileage still, etc.

Is there some way to test the cat? I seem to remember a post that you can use a infrared thermometer on the upstream and downstream sides of the cat and there should be a certain temperature differential?

[holler1]

I tried to test the cat by measuring temp before/after it with an infrared thermometer (now $29 at Harbor Freight). I unhooked a plug wire to cause a rich condition, and after a few minutes the temp after the can started going up. These temps are very approximate and depend a lot on how you aim the meter. Make sure you get it on bare metal and not on the heat shield. I found "normal" temps at idle to be 450-499 F before the cat and 350 after. After unhooking the plug for a minute, and reconnecting it, and waiting about 5 minutes, the temp before was 450 and after rose to about 580 F. This seemed to verify that the cat was doing something. The odd thing was that the temp after the cat didn't go up much while the plug was disconnected, but did later. I interpret this to mean the fuel was so rich it was cooling the whole exhaust stream- not sure of this. Also the exhaust didn't smell- the TSB mentions a rotten egg smell indicative of sulfur for a bad cat.

Again, my argument was that, after verifying O2 sensor operation and checking for gross leaks, I thought the next step was to "flash" a possibly bad ECU rather than spend ~ $1000 on a cat replacement. If the P0420 comes back again on my daughters car, I will suggest to her to replace the cat with a middle cost cat that hopefully will satisfy emissions testing in Chicago. The alternative was to replace the cat and still possibly have it recur due to a bad ECU. Basically, make the cheap fix first and see if it works before making the costly fix.

[zeekh]

Originally Posted by holler1

I tried to test the cat by measuring temp before/after it with an infrared thermometer (now $29 at Harbor Freight). I unhooked a plug wire to cause a rich condition, and after a few minutes the temp after the can started going up. These temps are very approximate and depend a lot on how you aim the meter. Make sure you get it on bare metal and not on the heat shield. I found "normal" temps at idle to be 450-499 F before the cat and 350 after. After unhooking the plug for a minute, and reconnecting it, and waiting about 5 minutes, the temp before was 450 and after rose to about 580 F. This seemed to verify that the cat was doing something. The odd thing was that the temp after the cat didn't go up much while the plug was disconnected, but did later. I interpret this to mean the fuel was so rich it was cooling the whole exhaust stream- not sure of this. Also the exhaust didn't smell- the TSB mentions a rotten egg smell indicative of sulfur for a bad cat.

Again, my argument was that, after verifying O2 sensor operation and checking for gross leaks, I thought the next step was to "flash" a possibly bad ECU rather than spend ~ $1000 on a cat replacement. If the P0420 comes back again on my daughters car, I will suggest to her to replace the cat with a middle cost cat that hopefully will satisfy emissions testing in Chicago. The alternative was to replace the cat and still possibly have it recur due to a bad ECU. Basically, make the cheap fix first and see if it works before making the costly fix.

I also have a P0420 code om my 1999 Chevy Prism ( Toyota Corolla ) CAr deosn't burn any oil.

What does Flash the ECU? What is the ECU, Electronic Control Unit?

[holler1]

ECU or ECM is engine control unit or engine control module- in other words the computer that controls the engine fuel injection, emissions etc. "Flash" means to upload new software to the ECU, something like upgrading your operating system on a computer.

[holler1]

A 1999 car is a different generation of Corolla/Prism, so this probably doesn't apply to your car. There is another forum for the earlier generation.

[zeekh]

Originally Posted by holler1

A 1999 car is a different generation of Corolla/Prism, so this probably doesn't apply to your car. There is another forum for the earlier generation.

Ooops I searched the for the P0420 code and ended up here. Anyway, I crawled under my car. Found that the clamp that holds the Cat to the front pipe was broken. I'm hoping that screwed up the O2 sensor's output enough to give the code. We'll see.

[zeekh]

Originally Posted by holler1

ECU or ECM is engine control unit or engine control module- in other words the computer that controls the engine fuel injection, emissions etc. "Flash" means to upload new software to the ECU, something like upgrading your operating system on a computer.

Thanks for the info. How do you flash the ECU?

[mikered30]

Originally Posted by zeekh

Thanks for the info. How do you flash the ECU?

Only the dealer can do it.