Network, generic (extended)

U301F Control Module Wake-up Circuit B Low

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

U301F reports wake-up line B held below its valid level, most often by a short to ground. Since B is a second, independent wake path, the module can still be roused through line A or the network; what it loses is the wake source wired to B. With an edge-triggered input such as the WAKE pin of NXP's TJA1043, which only reacts to a level change that remains stable for a short filter time, a grounded line simply never delivers that change. Its threshold sits only a few volts under battery voltage, so a partly shorted or corroded wire can count as low as well. No manufacturer documents U301F publicly. Unlike U301E, the circuit is not open, so a resistance check to ground with both connectors unplugged is the quickest way to find the fault.

Likely causes

Wake-up wire B chafed and touching ground
Moisture or corrosion pulling line B down
Source on line B with its output stuck low

Symptoms

  • Function tied to wake source B unavailable
  • No other complaint while the module is awake

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 bwakeup wiring

Related codes

U301EU3020U301BU3021

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