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Generic SAE code Network communication

U0100: Lost Communication With ECM/PCM "A"

Another module has stopped receiving messages from the engine control module.

High risk

Have it checked soon. The engine may stall or not start.

Short answer · Can I keep driving with U0100?

If the engine runs normally, it may have been a short, intermittent loss. If it stalls or will not start, do not rely on the car until it is checked.

On this page

What does U0100 mean?

U0100 is stored by modules such as the transmission, ABS or instrument cluster when messages from the engine control module (ECM/PCM) stop arriving.

What the car does when the code is set

Other modules cannot receive data from the engine or transmission module. The engine may not start, the gearbox may go into limp mode and several warning lights can come on at once.

How the system works

The engine and transmission control modules share data with other modules over the network, such as engine speed, torque and gear. Each module expects regular messages; when they stop arriving for a set time, a lost communication code is stored.

UNetwork
0Generic code (SAE)
1Network communication
00Specific fault

Common symptoms

Check Engine LightAmber lightHigh riskTransmission Warning LightAmber lightHigh riskCar Won’t StartCheck soonNormal riskEngine Stalls While Driving Or IdlingCheck promptlyHigh riskCar In Limp ModeCheck promptlyHigh riskHarsh Or Delayed Gear ShiftsCheck soonNormal risk

Possible causes

Listed without ranking. The cause on your car depends on make, engine and mileage — see the maker notes below.

  1. Power or ground fault at the moduleA blown fuse, failed relay or poor ground switches the module off the network.
  2. Wiring fault in the network linesDamaged or corroded CAN wires between modules.
  3. Low battery voltageVoltage drops during cranking can cause temporary lost communication.
  4. Failed control moduleLess common than wiring and power faults, but possible.
  5. Water damageModules and connectors exposed to water corrode.

How it is diagnosed

  1. Note how the car behavesIf the engine does not start or the gearbox is in limp mode, the module may really be offline. If everything works, the code may be an old, intermittent event.
  2. Check fuses, relays and groundsUse the fuse diagram for the engine or transmission control module.
  3. Check battery voltageCharge or test the battery before deeper diagnosis.
  4. Scan all modules and test the networkA workshop checks whether the module answers the scan tool and measures the CAN lines.

Questions about U0100

Can U0100 cause a no-start?

Yes, if the engine control module is really offline.

Does lost communication always mean a bad module?

No. Power, ground and wiring faults are more common causes than a failed module.

Terms on this page

ECU (Engine Control Unit)Transmission Control Module (TCM)OBD-IICar Battery