U0121 — A dead battery, A faulty ABS module, A problem with ABS module circuit, A problem with the CAN bus

Causes

• Dead or very low vehicle battery / poor battery connections
• Blown fuse or faulty ABS module power supply
• ABS control module failure
• Open/short or high-resistance in ABS module wiring or connectors
• CAN bus wiring fault (short to ground/power, open, or missing termination)
• Faulty gateway or central electronics module interrupting bus

Symptoms

• ABS warning lamp illuminated (may also trigger traction/stability lights)
• Loss of ABS/traction control or limited braking system functionality
• Other modules/reporting devices show 'module not present' for ABS
• No ABS data available on diagnostic tool / ‘communication error’
• Possible multiple network-related warning lamps

What to check

• Read and record U0121 and any other network codes with a factory-level scanner
• Check battery voltage at rest and during cranking (>12.4 V at rest; >11.5 V while cranking recommended)
• Inspect battery terminals and ground straps for corrosion and tightness
• Check relevant fuses and relays for ABS and central electronics
• Scan for presence of ABS module on the CAN bus (module should appear in scanner)
• Measure CAN_H and CAN_L voltages at an accessible connector with ignition ON

Signal parameters

• Battery/resting supply voltage: typically 12.4–12.8 V (acceptable >12.0 V)
• Cranking voltage: should remain above ~11.5 V
• CAN idle voltages (approx): CAN_H ≈ 2.4–2.6 V, CAN_L ≈ 2.4–2.6 V
• CAN dominant state (message active): CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V
• CAN bus termination resistance: approx. 60 Ω measured between CAN_H and CAN_L (two 120 Ω resistors in parallel)
• Open or shorted CAN will typically show abnormal voltages or infinite/very low resistance

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