U0121: OBD-II code U0121 flags loss of communication with the ABS/EBCM — the network can no longer see the anti-lock brake control module.

Symptoms

• ABS, traction control, and stability control warning lamps illuminate together and stay on continuously.
• Anti-lock braking and stability interventions are disabled; standard hydraulic braking still works.
• The speedometer may drop out or read incorrectly on vehicles that source vehicle speed from the ABS module.

Common causes

• A failed EBCM — the module itself has gone offline due to internal failure.
• A CAN bus fault (broken wire or short) between the EBCM and the rest of the network.
• Loss of power or ground at the EBCM — blown fuse, corroded ground point, or damaged connector.

What to do?

Confirm the EBCM has power and ground. Check ABS fuses. Measure CAN bus continuity to the EBCM connector. If the module won't wake up despite good supply and bus, replace and program to the vehicle.