U2100 — CAN-Bus Malfunction

Causes

• Open or short in CAN-H or CAN-L wiring
• Corroded, loose or damaged connectors at ECUs or junctions
• Failed or misbehaving ECU/module driving the bus
• Missing or incorrect termination (open/shorted resistors)
• Short to battery or ground on CAN lines
• Low battery/weak power supply or poor chassis grounds

Symptoms

• Multiple modules show communication faults or fail to respond
• Warning lights on dash (general network, ABS, airbags, traction, etc.)
• Loss of features that rely on networked modules (e.g., instrument cluster, ABS, infotainment)
• Intermittent or permanent faults that clear and return
• Vehicle may enter limp-home mode or refuse to start if critical module is unreachable

What to check

• Read all U-codes and manufacturer-specific fault memory in all modules; note freeze frame and related codes
• Check battery voltage and charging system; ensure stable supply (12.6 V+ at rest)
• Visually inspect CAN wiring, connectors, and junction boxes for damage, corrosion, or water
• Measure CAN bus idle voltages at a known-good ground: CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V
• Measure dominant state: CAN_H ≈ 3.5 V and CAN_L ≈ 1.5 V during active frames (use scope if possible)
• Measure total bus termination resistance (≈ 60 ohms for two 120 Ω terminators)

Signal parameters

• CAN_H recessive ≈ 2.5 V, dominant ≈ 3.5 V
• CAN_L recessive ≈ 2.5 V, dominant ≈ 1.5 V
• Differential voltage dominant ≈ 2.0 V, recessive ≈ 0 V
• Termination resistance ≈ 60 ohms (end-to-end)
• Typical bus data rates: 500 kbps (high-speed), 250 kbps, 125 kbps (low-speed)
• CAN error frame count and TX/RX error counters from affected modules

Similar codes