U0100 Lost Communication With ECM/PCM 'A'
Modules on the vehicle's network have stopped hearing from the engine computer — usually a wiring or power problem rather than a dead computer.
Electrical & idle
Modern vehicles run a CAN bus that lets modules talk to each other. U0100 means another module stopped receiving messages from the engine control module. Despite how alarming it sounds, a genuinely failed ECM is one of the least likely causes — power supply, grounds, connectors and bus wiring account for the large majority, and low system voltage alone can produce it.
Diagnose U0100 on your specific vehicle Add your year, make and model for ranked causes, repair videos and the exact parts you needCan I keep driving?
Driveable
Often no. Expect a no-start, limp mode, or an unexpected stall. Treat it as urgent.
How serious
High. Communication loss can cause a no-start or a stall, and it often brings a cascade of other codes with it.
Symptoms you'll notice
- Engine won't start or stalls without warning
- Multiple warning lights across the dashboard at once
- Gauges dropping out or reading erratically
- Scan tool unable to connect to one or more modules
- A cluster of unrelated-looking codes from different systems
What causes U0100
Ordered by what to check first — how likely each cause is, weighed against what it costs to rule out. Working down the list in order is what stops you replacing an expensive part to fix a cheap problem.
Low system voltage or a weak battery
Check this first. Modules drop off the bus when voltage sags, and a marginal battery produces communication codes that vanish once it's charged. If P0562 is stored too, fix the charging system before anything else.
Corroded or loose connectors
Water intrusion into a connector, or a pin that has backed out, breaks the bus. Frequently found after body repair, an accident, or an aftermarket accessory install.
Damaged CAN bus wiring
The bus is a twisted pair; a short between the two wires, to power, or to ground takes the whole network down. Rodent damage is a surprisingly frequent cause.
Failed ground connection
A corroded module ground produces intermittent communication faults that come and go with vibration.
Aftermarket electrical accessories
Improperly installed alarms, remote starters and stereos tapping into the wrong circuits are a very common cause of network faults. If the code appeared after such an install, start there.
Failed control module
Genuine ECM failure is real but rare, and it belongs at the end of this list rather than the start.
How to diagnose it
- Test battery voltage and the charging system first — low voltage causes this outright.
- Scan for codes in every module; the pattern of which modules can and can't be reached localises the break in the bus.
- Inspect the ECM connectors for corrosion, water intrusion and backed-out pins.
- Check and clean the main ground connections.
- Measure resistance across the CAN high and low lines — a healthy bus with its two terminating resistors reads about 60 ohms with the key off.
- Ask about, and inspect, any recently installed aftermarket electronics.
Parts you might need
Tools for the job
- Multimeter
- Scan tool with all-module access
- Wiring diagram
- Terminal release tools
Related trouble codes
Get the parts list for your vehicle Ranked causes, tutorials and live part prices for your exact year, make and modelFitment matters. Causes and parts above are the common cases for U0100 across vehicles. The specific part numbers, procedures and torque specs vary by year, make, model and engine — always confirm against service data for your vehicle before buying parts or starting work.