C1A12

Code

Generic C — Chassis

ABS control module CAN communication fault

Brand: Generic

Type: C — Chassis

Views: UK: 8 EN: 14 RU: 13

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Page language: EN

Causes

Open or short in CAN wiring (CAN_H or CAN_L)
Intermittent or missing power / ground to the ABS module
Faulty ABS control module or CAN transceiver
Bus termination fault (open or incorrect resistance)
Short to battery or ground on the CAN bus
Software/firmware mismatch or corrupted module programming

Symptoms

ABS warning lamp illuminated
Loss of ABS/traction control functionality or disabled ABS
Other modules report lost communication or related U-codes
Intermittent faults that clear and return
Speedometer or stability control messages missing or erratic
Diagnostic tool cannot communicate with ABS module or shows timeouts

What to check

Read all stored and pending DTCs and freeze frame data from ABS and other modules
Confirm battery voltage and measure ABS module supply and ground
Visually inspect CAN connectors, wiring harness and pins for damage or corrosion
Check CAN_H and CAN_L DC voltages with key ON (typical ~2.5V idle) and measure differential
Measure bus termination resistance across CAN_H to CAN_L (≈60 ohms with two 120Ω terminators)
Scan for other U-codes indicating bus-wide communication loss

Signal parameters

CAN_H idle voltage (approx. 2.5V, nominal ~2.5–3.5V range under bias)
CAN_L idle voltage (approx. 2.5V, nominal ~1.5–2.5V range under bias)
CAN differential voltage during frames (approx. 1–2V differential)
Bus termination resistance (~60 ohms end-to-end)
Module supply voltage (battery voltage with ignition ON)
Module ground resistance (low ohms to chassis)

Diagnostic algorithm

Retrieve all related DTCs from ABS and other network modules; note freeze frame and last occurrence.
Verify vehicle battery condition and check ABS module supply and ground circuits for proper voltage and low resistance.
Visually inspect ABS module connectors and harness for corrosion, bent pins, damaged insulation, or repairs.
With ignition ON (engine off), measure DC voltages: CAN_H, CAN_L at the ABS connector and compare to other nodes on the same bus.
Measure termination resistance between CAN_H and CAN_L; expect about 60Ω (two 120Ω terminators in parallel).
Use a scan tool to monitor CAN traffic; confirm whether ABS module transmits messages or is silent. Note message IDs and frequency.
If DC checks appear normal but communication is intermittent, use a scope to view CAN differential waveform for noise, distortion, dominant/recessive levels, or reflections.
If a short or open is found, isolate segment by disconnecting modules or harness branches and re-test bus continuity/voltage to locate the fault.
Repair identified wiring/connectors (clean, repair, replace harness or pins). Restore correct termination and secure grounds.
If wiring, power and grounds are good but no CAN messages from ABS, consider ABS module CAN transceiver or internal failure. Confirm with known-good module or manufacturer's bench test if available.
Reprogram or update ABS module software if manufacturer bulletin suggests software-related communication faults.
Clear codes, perform relearn procedures if required, and road test to confirm the fault does not recur.