P0455 Cadillac -

Question

I have change the evap vent solenoid and a new gas cap. The light went off for 4 days and I took my truck through the car wash and the light came back on.

Answer 1

P0449 Evaporative emission (EVAP) vent solenoid control circuit

P0455 Evaporative emission Large leak detected

This probably has noting to do with going to the car wash, Once the Evap code is cleared or any CK ENG LT cleared, it has to run through a monitor and meet certain criteria. So four days sounds just about right for the monitor to clear for the EVAP system.  P0455 can set a vent solenoid control circuit P0449, There been a lot of fixes where the FTP (fuel tank pressure ) sensor was at fault setting both these code condition.