Fault code U0100 – a step-by-step guide to diagnosing and fixing it

Error code U0100 is displayed and the check engine light is on - what does this mean?

DTC U0100 is a communication fault code that is recognized by the OBD system when communication between the engine control unit (ECM/PCM) and other control units in the CAN bus network is interrupted. This EOBD fault code signals a loss of communication with the engine control unit and may have slightly different names for different manufacturers.

What causes can this fault code have?

• Defective engine control unit (ECM/PCM)
• Damaged or corroded CAN bus cable
• Loose or corroded plug connections
• Defective CAN bus termination
• Short circuit in the communication network
• Faulty ground connection
• Faulty fuse in the engine control unit
• Voltage problems in the on-board power supply
• Software error in the control unit
• Water damage to electrical components

What symptoms occur in parallel with fault code U0100?

• Starting problems: The vehicle is difficult or impossible to start
• Check engine light: Permanent display of the check engine light
• Loss of power: Reduced engine power and poor acceleration
• Unsteady idling: Engine runs irregularly at idle speed
• Communication error: OBD scanner cannot establish a connection to the engine control unit
• Limp mode: Vehicle switches to emergency running mode
• Other error codes: Additional U-codes for other control units

How can the problem with error code U0100 be rectified?

• Repairing trouble code U0100 requires a systematic diagnosis of the communication systems.
• Battery check and voltage supply: Measure battery voltage and ensure that all engine control unit fuses are intact, replace defective fuses if necessary
• Check plug connections: Check all plug connections on the engine control unit for tight fit, corrosion and damage, clean or replace corroded contacts
• Diagnose CAN bus cable: Measure continuity and resistance values of CAN high and CAN low lines, repair or replace damaged cables
• Test engine control unit: check ECM/PCM for function and replace with a new or overhauled control unit if defective, then have it programmed
• Check ground connections: Check all ground cables of the engine control unit for corrosion and tight fit, repair defective ground connections
• Carry out software update: Ask the manufacturer for available software updates for the engine control unit and have them installed