What does it mean?
The evaporative system stores fuel vapour from the tank in a charcoal canister and later draws it into the engine to be burned. A purge control valve meters that flow, and the letter B identifies a second purge valve used where one valve cannot cover every operating condition, for example a turbocharged engine that needs a separate path to the compressor inlet when manifold vacuum is unavailable.
P04AB is set when the control module energises the driver for that second valve and sees no load current, which is the electrical signature of an open circuit. The module concludes the valve is disconnected, the winding is open or a wire is broken, and it stops using that purge path. The result is not an immediate driveability complaint but a system that cannot manage vapour correctly and will fail its emissions monitor.
Symptoms
- MIL (check engine lamp) on with the evaporative monitor unable to complete
- No audible clicking from the second purge valve when it is commanded with a scan tool
- Fuel odour around the vehicle in warm weather as the canister loads up
- Slightly longer cranking after refuelling on some applications
- Failed emissions inspection because of an incomplete or failed evaporative readiness monitor
- Additional evaporative purge flow codes stored over time
Likely causes
- Purge valve connector unplugged, partly seated or with a terminal pushed out of its lock
- Broken control or supply wire between the module and the valve
- Open solenoid winding inside the valve after an electrical or thermal failure
- Corroded terminals in the connector, common on valves mounted low or exposed to road spray
- Open fuse or relay in the supply that feeds the purge circuit
- Rodent damage to the harness section serving the evaporative components
- Module driver stage failed open, verified only after all wiring checks pass
How to fix it
- Verify the code is current, then command the valve with a scan tool while listening at the valve for an audible click
- Inspect the valve connector for a backed out terminal, corrosion, a broken lock or a plug that was never fully seated
- Measure the solenoid winding resistance across the valve terminals; an open reading identifies a failed valve
- With the ignition on, check for supply voltage at the feed pin, then check the control wire for continuity back to the module
- Trace the harness for chafing, rodent damage and pinch points, particularly where it passes under the intake or along a subframe
- Repair the wiring or replace the valve according to the measurements, then confirm normal current flow when the valve is commanded
- Clear the code and run the evaporative monitor drive cycle to confirm the system completes