Module communication
U0100
Lost Communication With ECM/PCM "A"
The car's computers talk to each other over a network. One of them has stopped answering, or the network itself has a fault. The engine computer is the one not answering. The car may not start.
Fix it soon
Can I keep driving? For a short while, gently. Have it checked in the next few days, and stop if the car feels, sounds or smells different. It depends on which computer is silent. If the engine, transmission or ABS computer is offline, don't rely on the car.
Common causes
- A weak battery or low system voltage (very common)
- A loose or corroded connector or ground
- Water in a module or connector
- Damaged network wiring
- A failed control module
Listed roughly from most to least likely. The same code can have different causes on different cars.
What to do
- Have the battery and charging system tested first.
- Note which systems stopped working. It helps find the silent module.
- Have a shop with a full scan tool check the network.
General guidance. Your owner's manual wins. If a red warning light is on or the car drives differently, stop safely and get help.
Related codes
- U0001High Speed CAN Communication Bus
- U0073Control Module Communication Bus "A" Off
- U0101Lost Communication With TCM
- U0121Lost Communication With Anti-Lock Brake System (ABS) Control Module
- U0140Lost Communication With Body Control Module
- U0155Lost Communication With Instrument Panel Cluster (IPC) Control Module
Track this automatically in Tagwell Auto
Tagwell Auto keeps your car's service dates and mileage, so when a code shows up you know what was done last. With a Bluetooth reader for the car's diagnostic port (called OBD-II), the phone app can read the code itself.