Finally got around to checking the DPFE, been living with the P0401 for a while. It was putting out ~.5 V instead of the indicated 1V, so replaced it.

Driving for a while the CEL went out, thinking time to celebrate (and have it smogged)

Threw the scanner on this morning (still no new CEL light after a few hundred miles of driving). and my scanner is still reading a P0401. If it was really still a problem, shouldn't the CEL be on? If it's all clear it shouldn't be sending the code?

I cleared it with the scanner and going to let it be driven for a the day and see what comes.

 

Must be a pending code and not a hard code yet. It takes normally three pending DTCs to set the CEL/SEL.

 

Did you clear the original code when the DP was replaced?

The monitors must be cleared (completed drive cycles) and no CEL before you can get a smog pass.

 

I didn't clear it, but I guess that kind of goes to the heart of the matter. Does the old code sit there forever, or does it self clear once the CEL goes off? It should have enough miles on it end of today to see if it has a new code (and still no CEL).

 

The old code will remain in memory until you either clear it or the battery is diconnected for 10min.

 

Some newer vehicles have some DTCs that can't be cleared with a tool or even by disconnecting the battery. They must be clear by the module or whatever that set the DTC.