Network, generic (extended)

U301B Control Module Wake-up Circuit A Low

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

U301B means wake-up line A sits below its expected level, typically because it touches ground or is pulled down by a faulty sender. What happens next depends on how the receiving chip works. NXP's CAN transceiver TJA1043, for example, wakes on a change of the WAKE pin in either direction once the new level has stayed stable for a short filter time, and its switching threshold lies a few volts below battery voltage (typically battery minus 2.5 V). A line clamped to ground therefore produces no further edge and acts like an unused pin, so the module stays asleep when it should start. A wire that only sags, for example through corrosion, can drop under the threshold too. No manufacturer documents U301B publicly; compared with U301A, look first for chafing against body metal.

Likely causes

Wake-up wire chafed through and touching ground
Corrosion or moisture pulling the level down at a connector
Sending module output stuck low

Symptoms

  • Module stays asleep, function or charging unavailable
  • Lost-communication codes in other modules

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€50–600
Time0.5–3 h

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

Parts involved

wakeup circuit awakeup wiring

Related codes

U301AU301CU301FU301DU3012

References: nxp.com, law.cornell.edu, fordservicecontent.com.