U0100: Lost Communication With ECM/PCM 'A'

U0100 sets when another control module stops receiving CAN messages from the ECM/PCM. It's a communication failure, not an engine mechanical fault, and it usually prevents the car from starting or keeps it in severe limp mode.

U0100 is a electrical-related diagnostic trouble code classified as critical severity. When your vehicle's ECU detects this condition, it stores U0100 and illuminates the check engine light.

If your vehicle has triggered U0100, you may notice one or more of these symptoms:

• Engine cranks but won't start, or stalls within seconds
• Multiple warning lights illuminated at once (check engine, ABS, traction, airbag)
• Speedometer and tachometer read zero or sweep erratically
• Transmission stuck in limp mode or won't shift out of park
• Scan tool cannot connect to the ECM but reaches other modules
• Intermittent stalling that resolves after an ignition cycle

Here are the most common causes of U0100, ranked by how likely they are to be the culprit:

No - address this immediately. U0100 means one or more modules on the CAN bus have lost contact with the engine control module. The engine usually won't start or will stall. Even if it runs, transmission, ABS, and stability systems may behave unpredictably because they've lost engine RPM and load data. Get it towed.

Follow these steps to pinpoint the root cause of U0100:

1. Load-test the battery and confirm at least 12.4V resting, 9.6V minimum during crank. Bus faults below 11V during crank are common and mask the real cause.
2. Scan every module and log which ones report U0100 plus any companion U-codes. One reporter points to wiring at that module, multiple reporters point to the ECM or the bus itself.
3. Check ECM-related fuses and relays in both the underhood and interior fuse boxes against the wiring diagram, including battery-fed and ignition-fed circuits.
4. Measure ECM power and ground at the module connector, key on: battery voltage on B+ pins, under 0.1V drop on ground pins referenced to the negative battery post.
5. With ignition off and battery disconnected, measure CAN-H to CAN-L resistance at the DLC. Expect about 60 ohms (two 120-ohm terminating resistors in parallel). 120 ohms means one terminator or half the bus is missing.
6. Backprobe CAN-H and CAN-L at the ECM connector, key on. Both lines should idle around 2.5V, with CAN-H rising and CAN-L falling during traffic. An oscilloscope makes this obvious.
7. If the bus is shorted or flooded, disconnect suspect modules one at a time until communication returns to identify the offender.
8. Only after power, grounds, fuses, and bus wiring are verified good, swap in a known-good ECM to confirm before ordering a replacement.

Repair costs for U0100 vary depending on the root cause and your vehicle. Here are typical estimates:

U0100 behaves differently depending on your vehicle. Select your car below for model-specific causes, known issues, and adjusted cost estimates:

These codes are often seen alongside U0100 or indicate related issues: