C1A93 — Manufacturer-specific control module communication fault

Causes

• Faulty or unpowered control module
• Damaged or corroded connectors or wiring in communication bus (CAN high/low)
• Shorts to battery or ground on communication lines
• Missing or incorrect termination (open or double termination)
• Low vehicle battery voltage or poor ground connections
• Software/firmware mismatch or module programming/configuration error

Symptoms

• Illuminated warning lights or multiple network-related DTCs
• Loss of function from the affected module (e.g., doors, ABS, infotainment)
• Scan tool shows module not responding or not present
• Intermittent faults or sporadic message loss
• Unable to reprogram or communicate with module via diagnostic tool

What to check

• Read all stored codes and freeze frame data from all modules; note which module reported C1A93
• Check battery voltage (engine off and cranking) and main battery connections
• Visually inspect connectors and wiring harnesses for corrosion, damage, or water ingress at the affected module and along the bus
• Scan for other network codes (U-codes) that indicate bus-wide faults or other modules failing to respond
• Backprobe the module communication connector and verify CAN_H and CAN_L voltages with key on (see signal parameters)
• Measure bus termination resistance (expected ~60 ohms across CAN_H and CAN_L with ignition off)

Signal parameters

• Idle (recessive) CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (each around battery midpoint)
• Dominant state: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V (approximate differential ≈ 2 V)
• Bus termination: typically two 120 Ω resistors in parallel → ~60 Ω across CAN_H and CAN_L
• Common CAN bus baud rates in vehicles: 125 kbps, 250 kbps, 500 kbps (verify vehicle-specific rate)
• Bus should show regular CAN frames/messages when observed with a scope or bus analyzer; repeated error frames or long dominant periods indicate a fault
• Low-voltage conditions: VBAT below ~11.5 V can cause communication issues

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