C1255

What Does Code C1255 Mean?

DTC C1255 indicates an ABS Inlet Valve Coil RR Circuit Open. This code is specifically registered by the Anti-lock Braking System (ABS) module, also known as the Electronic Brake Control Module (EBCM), when it detects an electrical open circuit condition within the solenoid coil of the rear right (RR) inlet valve. During normal operation or a system self-test, the EBCM continuously monitors the electrical resistance and current draw through each ABS solenoid valve coil. The inlet valve for each wheel is responsible for controlling the flow of hydraulic brake fluid to that specific wheel caliper. When an open circuit is detected in the RR inlet valve’s solenoid, meaning infinite or unacceptably high resistance, the EBCM interprets this as a failure of the solenoid coil or its associated wiring. This condition prevents the EBCM from accurately controlling hydraulic pressure to the rear right wheel during an ABS event, compromising the system’s ability to prevent wheel lock-up.

Common Symptoms

• ABS Warning Light Illumination: The most immediate and common symptom is the illumination of the ABS warning light on the instrument cluster, signaling a fault within the anti-lock braking system.
• Traction Control Light Illumination: On vehicles equipped with integrated traction control, the traction control warning light (often an exclamation mark within a triangle or a car with wavy lines) may also illuminate, as the systems are interdependent.
• Reduced or Inoperative ABS Functionality: The ABS system will likely be disabled, meaning the wheels could lock up under hard braking conditions, leading to a loss of steering control.
• Potential Brake System Warning Light: In some cases, a general “Brake” warning light may also illuminate, indicating a critical fault impacting braking system integrity.

What Causes the Code C1255?

• Open Circuit Within the Solenoid Coil: The most direct cause is an internal electrical break in the winding of the rear right ABS inlet valve solenoid coil itself. This typically requires replacement of the ABS Hydraulic Control Unit (HCU) as solenoids are rarely individually serviceable.
• Damaged Wiring Harness: An open circuit in the electrical wiring leading from the EBCM to the rear right ABS inlet valve. This can be caused by chafing, cuts, rodent damage, or corrosion within the wiring insulation.
• Corroded or Damaged Connector Terminals: Poor electrical contact at the connector points, either at the EBCM connector or at the ABS HCU connector, due to corrosion, bent pins, or pushed-out terminals.
• Faulty Electronic Brake Control Module (EBCM): While less common, an internal fault within the EBCM’s driver circuit responsible for activating the rear right inlet valve can cause this code. This is usually diagnosed after ruling out the solenoid and wiring.

How to Diagnose and Troubleshoot

Diagnosing C1255 requires careful electrical inspection and measurement. Always begin with safety precautions, including disconnecting the vehicle battery when working with electrical components.

1. Initial Code Verification and Freeze Frame Data: Connect an OBD-II scanner capable of accessing ABS module data. Confirm C1255 is present. Record any freeze frame data, as it may provide clues about the conditions under which the fault occurred. Check for any other related ABS or brake system codes.
2. Visual Inspection:
   • Carefully inspect the wiring harness leading from the EBCM to the ABS Hydraulic Control Unit (HCU) and specifically towards the rear right section of the HCU. Look for signs of chafing, cuts, rodent damage, or pinched wires.
   • Inspect the electrical connectors at the EBCM and the HCU for corrosion, bent pins, loose connections, or signs of water intrusion. Ensure the connectors are fully seated and locked.
3. Resistance Test of the Solenoid Circuit (Key Off):
   • Locate the ABS HCU and its main electrical connector. Consult the vehicle’s specific service manual for the wiring diagram and pinouts for the rear right inlet valve solenoid.
   • Disconnect the main harness connector from the ABS HCU.
   • Using a high-impedance digital multimeter (DMM) set to the ohms (Ω) scale, measure the resistance across the two specified terminals (or between the control wire and the power/ground wire, depending on the circuit design) that correspond to the rear right ABS inlet valve solenoid within the HCU.
   • An “OL” (Open Line) reading or a resistance significantly higher than the manufacturer’s specification (typically 1-10 ohms) indicates an open circuit within the solenoid coil itself or its internal HCU wiring. This would point to a faulty HCU.
4. Continuity Test of Wiring Harness (Key Off):
   • If the solenoid resistance test was within specification, the issue likely lies in the wiring harness.
   • Keep the EBCM/HCU connector disconnected.
   • Identify the corresponding pins at the EBCM side of the harness (again, consult the service manual).
   • Using the DMM set to continuity mode, test for continuity between the identified pin at the EBCM connector and the corresponding pin at the HCU connector for both wires of the RR inlet valve circuit. An “OL” or no beep indicates an open circuit in the harness.
   • Also, test each wire for continuity to ground and to battery positive. There should be no continuity in either case.
5. EBCM Output Test (Advanced – Bi-Directional Scan Tool): If the wiring and solenoid test good, and if available, use a bi-directional scan tool to perform an ABS component test or output command for the rear right inlet valve. Monitor for feedback or listen for the solenoid click (though an open circuit would prevent activation). This helps confirm EBCM functionality.

Recommended Repairs and Solutions

The repair for C1255 depends directly on the root cause identified during diagnosis:

• Repair or Replace Wiring Harness: If the diagnosis points to an open circuit in the wiring harness, the damaged section of wiring must be professionally repaired. This involves cutting out the faulty section, soldering in a new wire of the correct gauge, and insulating with heat-shrink tubing. For severe damage, replacing the entire harness segment may be necessary.
• Clean or Repair Connector Terminals: If corroded or damaged connector pins are found, carefully clean the terminals using electrical contact cleaner and a small brush. If pins are bent or loose, use a terminal pick to carefully reshape or reseat them. If damage is extensive, replace the connector housing or pigtail. Apply dielectric grease to prevent future corrosion.
• Replace ABS Hydraulic Control Unit (HCU) / EBCM Assembly: If the internal resistance test of the rear right inlet valve solenoid confirms an open circuit within the HCU, the entire ABS HCU (which typically contains the solenoids) or the combined EBCM/HCU unit must be replaced. Individual solenoids are rarely replaceable components within the HCU. After replacement, a full brake system bleed is mandatory. Many vehicles require a scan tool to perform a specific automated ABS bleed procedure to cycle the new solenoids and ensure all air is purged from the modulator unit.
• Replace Electronic Brake Control Module (EBCM): If all other components (solenoid, wiring) test good, and the EBCM is suspected of having an internal driver fault, the EBCM will need to be replaced. Depending on the vehicle, this may require programming or configuration after installation.
• Post-Repair Procedures: After any repair, clear the diagnostic trouble codes (DTCs) from the ABS module. Perform a thorough test drive, including activation of the ABS (if safe to do so), to confirm that the code does not return and that the ABS system functions correctly.
• Mechanic’s Tip: Always consult the vehicle-specific service manual for precise wiring diagrams, component locations, pinouts, and resistance specifications. ABS components are safety-critical; ensure all repairs are performed with meticulous attention to detail and appropriate torque specifications. Brake fluid is corrosive; exercise care to avoid spills on painted surfaces.