U356CAnode Header Drain Valve Control Circuit
- Generic OBD-II code · Network
- Turns on the check-engine light
- Can trigger limp mode
- How the car detects it
- U356C covers the drive circuit of a second drain valve mounted directly on the anode exhaust header, the channel inside the stack that gathers the gas leaving every cell. GM lists three valves behind the stack: a bleed valve at the top of the anode sump, a drain valve at its bottom and this header valve, which empties into the cathode exhaust. Condensate pooling in the outlet header harms cell durability, above all when the car stands on a steep slope, and GM has also patented heating of these manifolds against ice. Older logic opened the header valve only at shutdown or on a tilt sensor signal; GM's newer scheme lets it serve a share of bleed requests, by default the fifth. If the drive is open or shorted, the water stays put. No maker documents U356C publicly; a warning and limited operation are likely. The sump drain circuit is U356A.
- What you may notice
- Fuel cell system warning, MIL on
- Power limited, especially after parking on a slope
- Often no noticeable effect at first
- Likely causes
- LikelyOpen, short or corrosion in wiring or connector of the header drain valve
- PossibleValve solenoid coil open or shorted
- RareDriver stage of the fuel cell control unit faulty
- Repair
- Workshop only · Usually a job for a workshop.Typical cost: €150 – €1,000
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.