Network, generic (extended)

U356C Anode Header Drain Valve Control Circuit

Check engine lightOn
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

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.

Likely causes

Open, short or corrosion in wiring or connector of the header drain valve
Valve solenoid coil open or shorted
Driver stage of the fuel cell control unit faulty

Symptoms

  • Fuel cell system warning, MIL on
  • Power limited, especially after parking on a slope
  • Often no noticeable effect at first

Repair estimate

DifficultyShop_onlyWorkshop job
Parts and labour€150–1,000
Time0.5–3 h

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

Parts involved

anode header drain valvevalve wiring

Related codes

U356DU356AU356FU3568

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