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U356F Anode Header Drain Valve Heater Relay Control Circuit

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

U356F reports that the relay feeding the heater on the anode header drain valve cannot be driven properly because its coil circuit is open or shorted. That valve hangs on the stack's anode outlet header, where condensate gathers and can freeze during a cold soak. GM targets exactly this zone with heated outlet manifolds so ice does not block them during subzero starts. Whether the header valve carries its own heater and when it is switched is not described in public maker documents; the code only implies a relay separate from the one for the sump drain valve (U356E). Such a fault can leave the heater dead, risking an iced valve and a delayed or failed cold start, or keep it powered, which costs battery energy. In warm weather the effect is rarely noticeable. U356F has no public manufacturer source.

Likely causes

Open, short or corrosion in the relay coil wiring or connector
Heater relay faulty
Relay driver in the fuel cell control unit faulty

Symptoms

  • Fuel cell warning message
  • Start problems after a frosty night
  • Battery discharged if the heater stays on

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€80–600
Time0.5–2 h

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

Parts involved

anode header drain heater relayrelay wiring

Related codes

U356EU356CU356D

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