U0100

Lost Communication With Engine Control Module/PCM

The car cannot talk to the engine control module. This usually means a wiring, power, ground, network, or module problem. The engine may run poorly or not start at all.

Severity4/5 High

Can I drive?⚠ With caution

Do not keep driving if the engine stalls, will not start, or enters limp mode. If the car runs normally, drive only a short distance for repair and avoid long trips.

DIY

$20–$150

Shop

$150–$900

Difficulty

advanced

What It Means

U0100 means other modules on the vehicle network have lost communication with the powertrain control module or engine control module. This can be caused by a failed module, loss of battery power, bad grounds, damaged CAN bus wiring, corrosion, or a blown fuse. The code often appears with other network or powertrain codes when the PCM is offline.

Symptoms

•Check engine light on
•No-start condition
•Engine stalls or runs badly
•Limp mode or reduced power
•Multiple warning lights on the dash
•Other scan tool communication codes
•Intermittent starting or no communication with PCM

Common Causes

1. Blown fuse, relay issue, or loss of battery power to the PCM/ECMhigh

2. Poor PCM/ECM ground connectionhigh

3. Damaged, loose, or corroded wiring or connectors on the control module circuithigh

4. CAN bus wiring fault, short, open, or corrosion in the networkmedium

5. Failed PCM/ECM modulemedium

6. Low battery voltage or charging system problem causing module communication lossmedium

7. Water intrusion or connector contaminationlow

Related Codes

U0101 U0102 U0121 P0606 P0685

FAQ

Can U0100 be caused by a bad battery?

Yes. Low voltage or a weak battery can cause the PCM to lose communication.

Will U0100 stop the engine from starting?

It can. If the PCM has lost power or communication, the engine may not start.

Is U0100 always a bad PCM?

No. Wiring, grounds, fuses, relays, and network faults are usually checked first.

Can I clear U0100 and keep driving?

You can clear it, but it will likely return if the fault is still present. Do not ignore a no-start, stalling, or limp mode problem.