U356FAnode Header Drain Valve Heater Relay Control Circuit
- Generic OBD-II code · Network
- How the car detects it
- 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.
- What you may notice
- Fuel cell warning message
- Start problems after a frosty night
- Battery discharged if the heater stays on
- Likely causes
- LikelyOpen, short or corrosion in the relay coil wiring or connector
- PossibleHeater relay faulty
- RareRelay driver in the fuel cell control unit faulty
- Repair
- Medium · Usually a job for a workshop.Typical cost: €80 – €600
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.