Network, generic (extended)

U356D Anode Header Drain Valve

Check engine lightOn
EmissionsNot relevant
Drive cycleRequired
Limp modePossible

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.

Likely causes

Header drain valve or its line frozen
Valve sticking or leaking from deposits or a worn seat
Header drain line to the cathode exhaust blocked

Symptoms

  • Fuel cell system warning, MIL on
  • Reduced power, uneven cell voltages
  • Higher hydrogen consumption if the valve stays open

Repair estimate

DifficultyShop_onlyWorkshop job
Parts and labour€300–1,500
Time1–4 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

anode header drain valve

Related codes

U356CU356BU356FU3576

References: patents.google.com, patents.google.com.