U0100: Lost Communication With ECM/PCM A

Common causes

• A weak battery or loose, corroded battery terminals
• A blown fuse or a power or ground problem at the engine computer
• Damaged wiring or a loose, corroded or wet connector on the network
• A failed engine computer (less common)

Code meanings can vary slightly by manufacturer. Use these as a starting point and confirm with your vehicle's service information or a mechanic before replacing parts.

Symptoms

• Engine that cranks but won't start, or stalls
• Several warning lights on at once
• Gauges that drop out or act erratically
• Transmission stuck in one gear

Is it safe to drive?

If the car won't start, stalls, or has several warning lights or gauges acting up, don't drive it; have it towed. If it runs normally and the code was only stored (for example after a dead battery), clear it and watch whether it returns; if it does, get it diagnosed.

How to check it with a scanner

1. Plug an OBD2 scanner into the port under the dashboard and read all stored codes and the freeze-frame data (the conditions when the code set).
2. Test the battery and charging system, and clean and tighten the battery terminals and main ground connections.
3. Check the fuses and relays that power the engine computer.
4. With the key off, inspect the engine computer's connectors for corrosion, water or loose pins.
5. Use a scanner that can see all modules to check which ones are communicating; a shop can then test the network wiring.
6. After a repair, clear the code and drive normally for a few days to confirm it doesn't come back.

A basic code reader shows the code; scanners with live data let you watch sensor readings, which is what you need for most of the checks above. See OBD2 scanners under $100.

Questions

Related codes

• P0562: Low battery or charging voltage
• P0700: Transmission computer has stored a fault
• All trouble codes
• How to choose a car diagnostic scanner