U0100
NetworkLost Communication With ECM/PCM
One or more of the car's electronic modules has lost the ability to talk to the engine ECU over the data network. The car's various computers are still there, they just can't hear each other.
ⓘ Information only. This page provides general educational information about fault code U0100. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does U0100 mean?
U0100 is a Network (CAN bus, module communications) fault code. It indicates: Lost Communication With ECM/PCM.
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Engine warning light, often appearing alongside multiple other warnings (ABS, airbag, traction)
- • Hard or no starting in some cases
- • Limp mode, reduced power and gearbox locked into a lower gear
- • Random shutdown while driving (rare but possible)
- • Multiple unrelated dashboard warnings appearing at once
Possible causes
Causes commonly associated with U0100, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Weak or failing battery, or poor battery connections, the most common cause by a wide margin
- 2. Damaged CAN bus wiring (chafed harness, broken splice, water in a connector)
- 3. Corroded earth strap or main earth connection at the engine or chassis
- 4. Aftermarket alarm, tracker, or remote-start unit interfering with the data network
- 5. Faulty ECU itself (uncommon, check everything else first)
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Test the battery first. Voltage should be 12.6V or above with the engine off, and 13.7-14.7V with it running. Below that, the modules can't communicate reliably and U0100 sets even though nothing's mechanically wrong
- 2. Inspect battery terminals for corrosion. White or green crusty buildup means the connection is poor, clean and reseat
- 3. Check the main earth connections. There's typically one at the engine block and one or two at the chassis. A loose or corroded earth causes intermittent communication faults
- 4. Look at any aftermarket electronics. Alarms, trackers, or amplifiers wired into the CAN bus are a frequent cause, especially if installed by anyone other than a professional
- 5. If the basics check out, that's where it goes to a workshop with a CAN bus tester
Common questions about U0100
I just changed the battery and now I have U0100. Why? +
Two common reasons: either the new battery is loose-fitted and intermittently disconnecting, or the modules need a relearn after losing power. On some cars (BMW, Mercedes, newer Volkswagens), changing the battery without registering it to the BCM can cause communication faults. Check the battery is secure first; if it is, a workshop can register the new battery.
Can a flat battery really cause this? +
Yes, and surprisingly often. Modern cars rely on stable voltage for the data network to work. A weak battery that drops to 11V briefly during a cold start can be enough to set U0100. Charge the battery, replace it if it's old, and the code often clears itself.
Is the car safe to drive? +
Often it isn't, depending on what's been knocked offline. If the engine warning light is on with other systems in limp mode, treat it as a get-it-checked situation rather than a drive-it-around situation. Confirm with a mechanic.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →