U356DAnode Header Drain Valve
- Generic OBD-II code · Network
- Turns on the check-engine light
- Can trigger limp mode
- Needs a drive cycle to confirm a fix
- How the car detects it
- U356D is set when the header drain valve responds electrically yet does not empty the header as intended. GM splits each opening into two phases: first water runs out of the header, then nitrogen-rich gas is bled off. The end of the water phase is recognised from stack voltage or from an estimated gas flow across the valve, and the bleed phase is sized by the hydrogen content of the anode. If the water phase takes unusually long, GM concludes the stack is tilted or the header overfilled and calls on the valve more often. A valve stuck closed lets water build up and starve the cells farthest along the header; one stuck open keeps venting hydrogen into the cathode exhaust. Ice is a further risk at outlet manifolds. No maker publishes U356D. A warning and less power follow; circuit faults are U356C.
- What you may notice
- Fuel cell system warning, MIL on
- Reduced power, uneven cell voltages
- Higher hydrogen consumption if the valve stays open
- Likely causes
- PossibleHeader drain valve or its line frozen
- PossibleValve sticking or leaking from deposits or a worn seat
- RareHeader drain line to the cathode exhaust blocked
- Repair
- Workshop only · Usually a job for a workshop.Typical cost: €300 – €1,500
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.