I know it's not a truck, but my 1999 Cavalier is throwing a P0440 code - EVAP small leak, no flow condition.
Four probably causes.
1) Defective fuel cap
2) EVAP canister or host cracked
3) Purge of vent solenoid defective
4) Vacuum leak at engine.
Anyone have experience with this code? I guess I'll just start at the top and work down.