2001 Corolla With P0420 Code - Toyota Corolla, Chevy Prizm (1998-2008)

kulkarni_ash

I have Toyota corolla 2001 with 140,000 miles on it

I get error code P0420 Catalyst System Efficiency Below Threshold (Bank 1).

I worked with a mechanic who wants to change catalyst on it, which i dont want until some more test and

I did the following

1. I had the code reset and drove for about 60 miles that day, the light did not come on

2. I checked it on machine same day, it showed catalyst and EVAP not ready, 02 sensor ready

3. Next day i drove for 10 miles the light came back on.

4, Checked it catalyst is ready but EVAP is not ready, but 02 sensor is ready

Any ideas or suggestions, i had the original 02 Sensor (which i changed last year along with MAF sensor, same mechanic) and was willing to put it on and try it again which he says he wont do.

Ashish

Chances are - it is not the cat. Can't always go by the OBD-II code, as with any diagnostic work, only shows you part of the whole picture.

It was actually quite helpful to have system monitored - as it appears you were querying the I/M readiness of the onboard computer. Since in the end, it showed cat and O2 sensor READY - that means they passed the internal computer test - but EVAP failed means you potentially have a EVAP related problem, most likely a bad hose or bad vacuum switching valve. It takes time for it to test all the components, that's why you got no CEL in the beginning, until it finished its internal test.

I remember that you had some issues in the past and swapped the O2 sensor and MAF sensor - probably both are still functioning OK - just lazy sensors that may have flagged other CELs before.

I would have a complete EVAP system diagnostics - pulling individual hoses, checking valves, checking operation of electronics - it is quite involving (done it myself TWICE - about 4 hours of testing first time through). I have a link to the Toyota FSM in a link in my sig under EVAP. Keep at it - can be pretty tough, I've messed with my system all too often - about the only thing I was unhappy about the car - otherwise it was fine.