U0100 code meaning
Lost Communication With ECM/PCM A
Network system Generic code U01xx codes Check engine light
What U0100 means
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.
U0100 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modePossible
- Repair difficultyMedium
- Typical labor0.5 to 4 hours
- Clears after repairWhen cleared with a scanner
Symptoms of U0100
- 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
Common causes of U0100
- Most likelyECM/PCM without power or ground (fuse, main relay, ground point)
- Most likelyDamaged CAN lines or loose, corroded terminals between ECM and TCM
- PossibleWeak battery or voltage dip during cranking (history code)
- Less likelyECM internally faulty or software not running
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
ECMCAN bus wiring
What each part of U0100 means
| U | Network system: communication between the control modules (CAN bus). |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 00 | The exact fault in that area. |
What to do next
- Note U0100 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about U0100
What does U0100 mean?
U0100 means "Lost Communication With ECM/PCM A". 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. It is a generic code, so it means the same on every make.
Is it safe to drive with U0100?
Drive gently and only to a workshop. With U0100 the car can go into limp mode (reduced power) to protect itself.
Will U0100 fail an emissions test?
Yes, it can. U0100 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is U0100 to fix?
Difficulty: medium. A typical repair takes 0.5 to 4 hours of work. This is a job for a workshop.
Will U0100 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.