Network, generic (extended)

U3551 Hydrogen Tank A Valve Enable Circuit

Check engine lightOn
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

U3551 concerns the enable circuit of the valve on tank A: not the switched control line, but the path that permits the valve to be powered at all, typically a relay, fuse or supply-side switch. Designs keep it apart so a crash can cut hydrogen reliably. In a Honda patent the fuel cell controller opens a collision cutoff relay as soon as the airbag unit reports an impact, and FMVSS 307 demands valve closure no later than 5 seconds after a crash. Toyota's FCHV guide follows the same idea: pulling two fuses disables the vehicle and closes every tank valve. If this path is open or shorted, tank A cannot open. No manufacturer documents U3551. Enable codes on several tanks at once point to a shared supply.

Likely causes

Enable or crash cutoff relay faulty
Blown fuse or missing supply in the valve enable path
Open or short in the enable wiring to valve A

Symptoms

  • No READY or fuel cell does not start
  • Hydrogen system warning

Repair estimate

DifficultyShop_onlyWorkshop job
Parts and labour€50–1,500
Time0.5–4 h

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

Parts involved

h2 tank a valve enablesafety circuit

Related codes

U3550U3552U3554U3557

References: patents.google.com, law.cornell.edu, techinfo.toyota.com.