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Working on diagnosing an evap/emission/fuel system problem on my Toyota 2003 Corolla. More on that in this question.

On 3/2, I saw a mechanic use his handheld OBD to read a code P0420. On 3/12, without any code resets, I got my Ozzy OBD reader and read code P0440 using Torque.

I understand that sometimes later codes change because of faults in earlier codes, but I don't believe I've ever seen the primary and only fault change. Is this normal variance that I (because of my inexperience) just haven't seen, or is it indicative of something deeper?

Updates

  • 3/15: After a reset drive cycle, the new code read is P0446.
  • 3/16: After a reset drive cycle, the new code read is P0420. Have confirmed cat failure, so this code seems correct.
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The P0420 and P0440 are completely unrelated (other than both being related to emissions). The P0420 is having to do with catalyst function. The P0440 having to do with EVAP system. There is no way for one code to have morphed into the other code.

P0420 can appear and disappear over time due to the nature of what is being seen by the computer. If the after cat O2 is out of whack a little bit for a period of time, the code will trip. Once cleared, it might not come back for a long time, or it might come right back.

P0440 could be as simple as the gas cap not sealing correctly. You car is new enough, I doubt this is an issue, but it's always good to check the easy things first.

If the P0420 doesn't reappear, I wouldn't worry about it. In the mean time, the following could be at issue due to the P0440:

The gas cap is not installed or working properly
The purge solenoid has failed
The canister is plugged and not working properly

You can try the following to see if it will clear permanently:

Remove and reinstall the gas cap, clear the code, and drive for a day
    and see if the codes come back.
Inspect the EVAP system for cuts/holes in tubes/hoses
Inspect for damaged or disconnected hoses around the Evap purge solenoid
Check and/or replace the sensor
Check and/or replace the purge valve
Have a professional use a smoke machine to detect leaks
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