U0100 fault code: what it means

U0100: Other modules have lost communication with the engine computer (ECM/PCM). The engine may stall or not start.

Network (communication between modules) · Safety risk

Usual causes, most likely first

weak battery or poor earths
corroded ECM connector pins
blown ECM fuse or failed main relay
damaged CAN wiring

U0100 by make

GWM

Lost communication with the engine ECU.

Usual causes: ECU power/earth fault; CAN wiring; faulty ECU

Haval

Lost communication with the engine ECU.

Usual causes: ECU power/earth fault; CAN wiring; faulty ECU

Hyundai

Lost communication with the engine control module.

Usual causes: CAN wiring or connector fault; low battery voltage; ECM failure

Kia

Lost communication with the engine control module.

Usual causes: damaged or corroded CAN wiring; low battery voltage; ECM failure

MG

Lost communication with the engine ECU.

Usual causes: ECU power/earth fault; CAN wiring; faulty ECU

Mitsubishi

Lost communication with the engine computer. On Outlander PHEV many U0100 time-outs are usually from a weak 12V battery.

Usual causes: weak or failing 12V battery; CAN wiring or ECM connector fault; ECM power or earth fault

Subaru

Lost communication with the engine computer (ECM). Often comes with U0101 and other U codes.

Usual causes: weak or failing 12V battery; CAN wiring or ECM connector fault; ECM power or earth fault; failed ECM (less common)

What the repair usually costs

12 V battery replace, standard flooded (small / medium car)NZ$190–NZ$330 · A$170–A$310
12 V battery replace, standard flooded (SUV / ute / 4WD)NZ$290–NZ$500 · A$270–A$460
Hybrid 12 V auxiliary battery (Aqua / Prius C / Axio, S34B20R)NZ$330–NZ$510 · A$310–A$500
Hybrid 12 V auxiliary battery (Prius 30 / Prius alpha / CT200h, S46B24R AGM)NZ$390–NZ$690 · A$360–A$590

Not sure what U0100 means on your car? Ask Credov.

Before you clear it

Write the code down or photograph the scanner first. A code that comes straight back after clearing points to a fault that is still there. A car whose codes were cleared just before you arrived may be hiding one: the scanner's readiness monitors show "not ready" for a while after codes are cleared.

Researched by Credov from manufacturer and workshop sources. A fault code points to where a fault may be; a workshop confirms it with a diagnosis. Browse all fault codes.