Network, generic

U0100 Lost Communication With ECM/PCM A

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

U0100 is stored by a module that has stopped hearing the engine or powertrain controller, ECM/PCM A; a second engine controller has its own code, U0115. Usually the transmission controller logs it, often alongside other listeners. GM's 2019 ten-speed TCM rates it Type A (MIL on the first trip) and holds it back while its own bus-off code U0073 is active; GM's 2016 tables set it once frame $0C9 is absent in 12 of 12 one-second samples at 11–32 V (Type B). Ford's 2012 summary has the TCM switch on the MIL at once, because engine speed and torque are gone. The gearbox then falls back to a limp-home program; GM bulletin 08-07-30-021H traces such cases to harness terminals, bus wiring and corroded connectors.

Likely causes

ECM/PCM without power or ground (fuse, main relay, ground point)
Damaged CAN lines or loose, corroded terminals between ECM and TCM
Weak battery or voltage dip during cranking (history code)
ECM internally faulty or software not running

Symptoms

  • MIL on, often several U codes in other modules
  • Transmission stuck in a substitute gear or harsh shifts
  • Gauges for engine speed or temperature drop out
  • Engine may stall or not crank

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€60–900
Time0.5–4 h

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

What to check first

  1. Check engine computer fuses and grounds
  2. Check network wiring
  3. Scan all modules

Parts involved

ecmcan bus wiring

Related codes

U0073U0115U0101U0301U0401

References: gsi.ext.gm.com, gsi.ext.gm.com, newtonnet.co.uk, fordservicecontent.com.