Toyota Repair: PO420 - 2001 Chevy Prizm - 75000 miles, 2001 chevy prizm, checker auto parts

Question
Hi Ted,

I have a 2001 Chevy Prizm with about 75000 miles. I live in the Phoenix, Arizona area (drive the car about 25 miles each way to work five days a week). The car hasn't had any problems until now. The CEL just came on, and I took it to AutoZone and Checker Auto Parts, and both came up with PO420. --Money is tight right now, and I am wondering what the most likely cause of this is.

Before I got the code, I was guessing it was a dirty MAF. Now, I am not so certain. --From what I am reading online, it could mean I should replace the O2 sensors, the catalytic converter, etc. The AutoZone guy suggested adding some fuel injection cleaner (I always thought of that as a very temporary fix). I have read other car owners say that they cleaned the MAFs, replaced the O2 sensors, and even replaced the catalytic converter ---to no avail --the CEL came back on.
Do you have any suggestions? (I don't know much about cars at all.) Thank you, very much!!!

Answer
I have a lot of experience with this trouble code and the end result is always that the cat converter is the problem and should be replaced, you can save a lot of money by finding a muffler shop that can weld a new converter in. If the cat converter and the o2 sensors are all working correctly the check engine light will not come back on. The O2 sesnor and cat converter efficiency can be checked with a diagnostic scantool that displays the sensor voltage patterns.