I am beyond frustrated and hope someone can shed some light on this ongoing issue. I have a 2004, Toyota Matrix, 170k miles. In the last 4 years, I have replaced the Catalytic Converter 3 times. From my understanding there are 2 in the Matrix. I replaced one it in Oregon about 3 1/2 years, and they apparently did something different where they welded out the old cat and welded in a new one. Which isn’t great from what I hear after the fact. Fast forward a year and a half the other one is apparently bad, I have them do a factory replacement in California, super expensive. I am now in Texas and about a year ago they tell me my cat is bad AGAIN. I again replace it. I believe this was the one from Oregon, 3 months later I get the 420 code again. OF course the place I just replaced it with is out of business. So, now I am coming up on inspections and have to start all over with a $100 fee just to check it out. I have been doing some research and apparently there was a recall on Toyota Matrix/Corrolla 2005 - 2009, one year after mine for the ECM Engine Control Module. Apparently this had a problem and caused the cars to spit out the 420 code when the cats weren’t bad and also caused the cars just to break down for no reason. Which has also been happening to me, mine happens when it rains, (it’s not the spark plugs or the distributor cap, no one can figure out why it will just break down) 8 mechanics have tried in 3 different states to no avail. So I am wondering if there is a chance it is my ECM and should’ve been part of this recall. I have also heard there are some people with similar issues that are trying to file a lawsuit against Toyota for having to replace Cats when it wasn’t necessary. I myself am about 2k in for cats in the last 3 1/2 years, with needing to replace it again apparently. Anyone have any thoughts on this or where I can start?
After the first catalytic converter was installed, was the ECM reprogrammed?
Tester
I have no idea. They didn’t tell me 
You will probably have to visit a Toyota dealer to have the software update performed, few independent shops are capable.
Toyota technical service bulletin EG026-04;
Some 2003 – 2004 model year Matrix vehicles equipped with a 1ZZ–FE engine may
exhibit a M.I.L. “ON” condition with Diagnostic Trouble Code P0420 (Catalyst System
Efficiency Below Threshold) stored in the Engine Control Module (ECM) (SAE term:
Powertrain Control Module/PCM). Follow the repair procedure to reflash the ECM (PCM)
and replace the catalytic converter assembly.
Hmm. I am wondering if this may be it then, because the car is running fine right now. Thank you. I will make an appointment with my Toyota Dealership.
Good ideas above. Suggest to start there. If your cats are actually repeatedly failing however, one cause can be an exhaust system leak. Or a cracked exhaust manifold. Cracked exhaust manifolds used to be a common Corolla problem according to Tom and Ray, but not sure if that applies to a 2004. But any leak in the exhaust system can confuse the ECM and cause it to inject more gasoline that it should, which will damage an otherwise healthy cat. Something like that would show up during an emissions test usually, as higher than normal HC levels. It may or may not affect performance/drivability.
SUPER common on 7A-FE engines
So common, that at my local junkyard you won’t find a single exhaust manifold on those engines, because the scavengers remove them, as soon as they’re dropped in the yard
Guess it’s not the timing belt off a notch because you’d get multiple misfires. Somehow you are getting too rich of gasoline, probably not too lean. If the ECM causing it to fire too late, maybe it needs reprogramming. They reprogrammed my 2009 and it got 3 MPG more than before. The Oxygen Sensor is probably sending too much gasoline in or not enough
The spark plug wires were replaced too? Also, are the shields under the engine still there and secure? If they are missing or drooping while driving, they will scoop up water and short things out.
Have you checked the sensors? My sons Camry (2004) with 197,000 miles threw a P0420 code. I ran a diagnostic with live data while the engine was running. The output from the bank 1 sensor was not low, it was ZERO! That told me the sensor was bad. I bought a new one and popped it in. Problem solved, no cat replacement required. A year later everything is still good.