U0100 Code: Lost Communication With ECM/PCM "A"

Symptoms

• Engine cranks but won't start, or starts and dies
• Several warning lights come on at once (check engine, ABS, traction, battery)
• Speedometer, tachometer or fuel gauge drop to zero or flicker
• Transmission stuck in a single gear or shifting harshly (limp mode)
• Intermittent stalling while driving, often over bumps
• Scan tool cannot connect to the engine module at all

Possible causes

Ranked by how often each cause is behind this code (editorial estimate).

1. Low or unstable system voltage: weak battery, corroded or loose battery terminals, failing alternator 30%
2. Blown fuse or failed relay feeding the ECM/PCM, or a bad PCM ground 22%
3. Corroded, water-soaked or loose connector at the PCM 18%
4. Chafed or broken CAN-high/CAN-low wiring between the PCM and the network 14%
5. Aftermarket device (remote start, tracker, OBD dongle) disturbing the network 8%
6. Internal ECM/PCM failure or corrupted software after a failed reprogramming 8%

Diagnosis step by step

1. Read codes from every module

   Use a scan tool that can reach the transmission, ABS and body modules, not just the engine. Note which modules store U0100 and whether other U-codes are present; if many modules report lost communication with the same PCM, the problem sits at or near the PCM.

   Tools: OBD-II scanner

2. Test the battery and charging system

   With the engine off the battery should rest at about 12.6 V, and it should not drop below roughly 9.6 V while cranking. With the engine running you should see about 13.5 to 14.7 V. Clean and tighten both battery terminals and the main ground strap to the body and engine.

   Tools: Multimeter, Pliers

3. Check PCM fuses, relay and grounds

   Use the wiring diagram to find every fuse that feeds the PCM (constant power, ignition feed, main relay). Test them with a meter rather than by eye, and do a voltage-drop test on the PCM ground circuits under load; more than about 0.1 V drop points to a bad ground.

   Tools: Multimeter, Flashlight

4. Inspect the PCM connectors

   Disconnect the battery, then unplug the PCM connectors and look for green corrosion, water, pushed-back pins or burnt terminals. PCMs mounted near the cowl, fender or under the battery tray are prone to water intrusion.

   Tools: Flashlight, Screwdriver

5. Measure the CAN bus at the diagnostic port

   With the battery disconnected, resistance between pins 6 and 14 of the OBD port should read about 60 ohms (two 120-ohm terminating resistors in parallel). About 120 ohms means one end of the bus is open; near 0 ohms means a short between the two lines.

   Tools: Multimeter

6. Remove aftermarket add-ons and recheck

   Unplug any remote starter, tracker or insurance dongle, clear the codes and drive. If U0100 stays away, the add-on or its installation was loading the network.

   Tools: OBD-II scanner

7. Confirm the PCM itself

   Only after power, grounds, connectors and bus wiring test good should the PCM be suspected. A replacement unit almost always needs VIN programming and, on many vehicles, an immobilizer relearn at a dealer or well-equipped shop.

   Tools: OBD-II scanner

Where the parts are

1. 1Battery
2. 2Alternator
3. 3Engine computer (ECM)
4. 4CAN bus
5. 5Transmission
6. 6Wheel speed sensor
7. 7High-voltage battery

Repair options & costs

Rough US estimates for parts and labor; shop rates vary by region. Not a quote.

Notes by brand

Related codes

Often appears together with

Emissions / smog test impact

Depends

Whether U0100 fails a US emissions test depends on whether it turns on the check engine light, which varies by vehicle. If the testing station's equipment can't communicate with the PCM at all, the vehicle is typically rejected until the fault is fixed.

Frequently asked questions