Network, generic (extended)

U302B Ignition Output Accessory/On/Start

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

U302B belongs to the ignition outputs, i.e. a feed that a module switches itself instead of merely reading it. The Accessory/On/Start variant should carry voltage in every power mode except Off and, unlike most accessory feeds, should stay on while the engine cranks. The driving unit, typically a body controller or the power supply unit of a keyless start system, compares its command with what it reads back on its own pin and stores U302B when the two differ: voltage in Off, or none in accessory, run or crank. No manufacturer documents U302B publicly. Tokai Rika's patent US6900552B2 shows such a unit switching separate relay coils for accessory, two ignition stages and the starter through FET drivers. A driver stuck on or a welded relay contact on this feed can flatten the battery overnight.

Likely causes

Welded relay contact keeping the feed live in Off
Faulty driver stage in the switching module, stuck on or not switching
Output wire or a connected load shorted to ground, tripping the driver protection
Blown fuse between the relay and the loads on this feed

Symptoms

  • Battery flat after the car has been parked
  • Several loads on the feed dead although the vehicle is switched on
  • Warning about the power supply or ignition system in the cluster

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€20–700
Time0.5–3 h

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

Parts involved

ignition outputignition relayoutput wiring

Related codes

U300BU302FU302DU302CU3032

References: law.cornell.edu, patents.google.com, gmupfitter.com.