U0100Lost Communication With ECM/PCM A
- Generic OBD-II code · Network
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- How the car detects it
- 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.
- What you may notice
- 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
- Likely causes
- LikelyECM/PCM without power or ground (fuse, main relay, ground point)
- LikelyDamaged CAN lines or loose, corroded terminals between ECM and TCM
- PossibleWeak battery or voltage dip during cranking (history code)
- RareECM internally faulty or software not running
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Usually a job for a workshop.Typical cost: €60 – €900
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.