U356D code meaning
Anode Header Drain Valve
Network system Generic code U35xx codes Check engine light
What U356D means
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.
U356D at a glance
- Check engine lightYes, usually on
- Emissions testNot an emissions code
- Limp modePossible
- Repair difficultyWorkshop only
- Typical labor1 to 4 hours
- Clears after repairAfter a drive cycle
Symptoms of U356D
- Fuel cell system warning, MIL on
- Reduced power, uneven cell voltages
- Higher hydrogen consumption if the valve stays open
Common causes of U356D
- PossibleHeader drain valve or its line frozen
- PossibleValve sticking or leaking from deposits or a worn seat
- Less likelyHeader drain line to the cathode exhaust blocked
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Anode header drain valve
What each part of U356D means
| U | Network system: communication between the control modules (CAN bus). |
| 3 | Reserved range, usually generic. |
| 6D | The exact fault in that area. |
What to do next
- Note U356D and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about U356D
What does U356D mean?
U356D means "Anode Header Drain Valve". U356D is set when the header drain valve responds electrically yet does not empty the header as intended. It is a generic code, so it means the same on every make.
Is it safe to drive with U356D?
Drive gently and only to a workshop. With U356D the car can go into limp mode (reduced power) to protect itself.
Will U356D fail an emissions test?
U356D is not an emissions code, so on its own it usually does not fail an emissions test. Other stored codes or a lit check engine light still can.
How hard is U356D to fix?
Difficulty: workshop only. A typical repair takes 1 to 4 hours of work. This is a job for a workshop.
Will U356D clear itself after the repair?
After the repair the car has to complete a drive cycle (a mix of city and highway driving) before the test runs again. Until then the readiness monitor shows "not ready", which can fail an inspection.