WTF PO420, then went away

[dahoy]

I have had the PO420 error (bad cat drivers side) for a few days. The ses light came on after I had ran it pretty good for about 15 minutes. This morning it went away...this is weird seeing as how this error is normally an indicator that something has broke. Should I take it to the dealer? I am all stock mods wise right now.

[eboggs_jkvl]

Error codes are constantly checked for validity. If it meets preset parameters and fails tests, then a light will come on and the code gets stored in the PCM. If the conditions that caused the original light change and the failure code doesn't continue to meet the requirements, the light will go off BUT the code will remain on file in the PCM.

Say you leave the gas cap loose and set a light. Well, you find the trouble, screw down the cap and after 3 restarts of the engine the light should go off but the cap code can be read from the PCM.

So, whatever set off your light has had something change and it now passes the testing. It'll probly come back but for now it's still working well enough to keep the light off.

E:)

[dahoy]

The funny thing is that I had about 7 restarts, so for the last three I guess it has been clear, I have been counting restarts, and after the third one, it still stayed. I am going to swap O2 sensors also, I think it is bad.

[killer01ws6]

My buddy at the dealer told me when he replaced my drivers side cat ( I had a ses light, then it went out) that the cats have to set a code to the PCM like 5 or 6 times in a row to get a SES light.
Now when they went in to look at the history they can see it has been coming and going even if the SES is not coming on.

Hope this helps.

[drptopz]

It's not a certian number of starts that reset the light, it is a certian number of drive cycles. The PCM must see a number of items to be classified as a drive cycle (such as low starting ECT, going into closed loop, atleast x number of minutes in closed loop, atleast a ect of "X", ect.) Just starting the car and shutting it off 3 times will not neccessarily reset the light once the error is gone.