C1906

What Does Code C1906 Mean?

DTC C1906 signifies an detected open circuit within the electrical control pathway for the Left Rear (LR) Ride Control Shock Actuator. This code is typically set by the vehicle’s Suspension Control Module (SCM) or Ride Control Module (RCM), which then communicates the fault to the Powertrain Control Module (PCM) or Engine Control Module (ECM) via the Controller Area Network (CAN) bus. The ride control system, often referred to as an adaptive or active suspension system, utilizes electronically controlled shock absorbers or dampers to continuously adjust the vehicle’s damping characteristics based on road conditions, driving style, and driver-selectable modes. The LR shock actuator is an integral component, usually a solenoid valve or a magnetorheological fluid control unit, responsible for altering the fluid flow or magnetic field within the shock absorber to modify its firmness or softness. When the SCM/RCM detects an unexpected interruption or excessive resistance (an open circuit) in the electrical wiring leading to or within the LR shock actuator, it determines that it cannot effectively command or monitor the actuator’s function. This results in the inability to control the damping force for that specific corner of the vehicle, often causing the system to revert to a default setting for the affected shock (e.g., full soft, full firm, or disabled) and illuminate a suspension warning light.

Common Symptoms

• Suspension Warning Light Illumination: A dedicated suspension fault light, check suspension message, or general check engine light (if the fault is critical or affects vehicle stability).
• Degraded Ride Quality: The vehicle may exhibit a noticeably harsher, softer, or bouncy ride sensation, particularly on the left rear side, due to the shock actuator defaulting to a fixed, non-optimal setting or losing all damping control.
• Reduced Vehicle Handling and Stability: Impaired cornering stability, increased body roll, or a general feeling of imbalance during maneuvers, especially when the adaptive system is expected to provide enhanced control.
• Uneven Vehicle Stance or Sagging: While C1906 primarily relates to damping, if the ride control system is integrated with an air suspension component, a severe fault could indirectly affect ride height or lead to subsequent issues that do. However, for an actuator, this is less direct.
• No Noticeable Symptoms (in some cases): Depending on the specific system’s fail-safe mode and the driver’s sensitivity, the change in ride quality might be subtle or only apparent under specific driving conditions.

What Causes the Code C1906?

• Open Circuit in the Wiring Harness: The most prevalent cause involves a break, fray, or severe corrosion within the electrical wiring leading to the Left Rear shock actuator. This could be due to physical damage, rodent activity, or long-term exposure to road salts and debris.
• Faulty Left Rear Shock Actuator: An internal electrical failure within the actuator itself, such as a broken solenoid coil winding or a motor circuit, resulting in an open circuit condition when measured.
• Corroded or Loose Electrical Connectors: Poor electrical contact at the multi-pin connector for the LR shock actuator, or at the corresponding connector at the SCM/RCM, due to corrosion, dirt, or improper seating.
• Damaged Shock Absorber Assembly: If the actuator is an integrated, non-serviceable component of the shock absorber, physical damage to the shock body can sometimes compromise the internal electrical pathways of the actuator.
• Faulty Suspension Control Module (SCM/RCM): Less common, but an internal fault within the SCM/RCM could prevent it from properly delivering power or command signals to the actuator, or from correctly monitoring its circuit, incorrectly flagging an open.

How to Diagnose and Troubleshoot

Diagnosing C1906 requires a systematic approach using a diagnostic scan tool and a Digital Multimeter (DMM). Always begin with a thorough visual inspection.

1. Initial Scan Tool Analysis:
   • Connect an OBD-II scanner capable of communicating with the SCM/RCM. Read all stored DTCs and any associated freeze frame data. Note any other suspension-related codes.
   • Check live data parameters related to the LR shock actuator, such as commanded current, actual current, or resistance feedback (if available). While C1906 indicates an open, observing live data for other actuators can help identify expected values.
2. Thorough Visual Inspection:
   • Inspect the Left Rear shock absorber and its associated wiring harness. Look for obvious signs of damage, such as cut or frayed wires, melted insulation, rodent damage, or pinched sections.
   • Examine the electrical connector at the LR shock actuator for corrosion, bent pins, loose terminals, or signs of moisture intrusion. Follow the wiring harness back as far as possible towards the SCM/RCM, checking for any similar issues.
3. Actuator Circuit Continuity Test (DMM):
   • Disconnect the LR shock actuator’s electrical connector.
   • Using a DMM set to ohms (Ω), measure the resistance across the two terminals of the actuator itself. A healthy actuator should typically show a specific resistance value (e.g., 2-20 ohms, depending on the manufacturer and type). An “open” reading (OL or infinite resistance) indicates an internal fault within the actuator.
   • Next, test the wiring harness side. Leave the actuator disconnected. Identify the power and ground/control wires for the actuator at the harness connector.
   • With the ignition OFF, test for continuity between the corresponding terminals at the SCM/RCM connector and the LR actuator harness connector. Use the vehicle’s wiring diagram to identify the correct pins. An open circuit in the wiring will show an OL reading.
   • Also, test each wire for continuity to chassis ground and to battery positive. There should be no continuity (OL) to either, indicating no short circuits.
4. Voltage and Power Supply Test (DMM):
   • With the ignition ON and the LR actuator still disconnected, use the DMM set to DC volts to measure for proper power supply and signal voltage (if applicable) at the harness connector going to the actuator. Refer to the vehicle’s wiring diagram for expected voltage values. Some actuators receive constant battery voltage and are ground-switched by the SCM/RCM.
5. Wiggle Test:
   • If all static tests pass, but the code persists, gently wiggle the wiring harness and connectors while monitoring the DMM for intermittent opens or shorts, or while observing live data parameters with a scanner if possible. This can expose intermittent connections.
6. SCM/RCM Check:
   • Only after thoroughly ruling out the actuator and wiring, consider the SCM/RCM as a potential cause. This often involves testing outputs from the SCM/RCM or comparing its behavior to known good values, which may require specialized diagnostic tools or a known-good module for testing.

Recommended Repairs and Solutions

Once the root cause of C1906 has been pinpointed through diagnosis, the following repairs are typically recommended:

• Repair or Replace Damaged Wiring Harness: If the diagnostic procedure identified an open circuit in the wiring harness, the damaged section should be professionally repaired. This involves using appropriate gauge wire, soldering connections securely, and protecting them with heat-shrink tubing and electrical tape. In cases of extensive damage, replacing the entire harness section may be more practical and reliable.
• Replace Left Rear Shock Actuator or Shock Absorber Assembly: If the internal resistance test of the actuator itself shows an open circuit, the actuator is faulty. Many modern adaptive shock actuators are integrated into the shock absorber and cannot be replaced independently. In such cases, the entire LR shock absorber assembly must be replaced. Ensure the replacement part is the correct type for the vehicle’s adaptive suspension system.
• Clean and Secure Electrical Connectors: For issues stemming from corrosion or loose connections at the actuator or SCM/RCM, carefully clean the connector terminals using an electrical contact cleaner and a small brush. Ensure all pins are straight and make solid contact. Apply dielectric grease to prevent future corrosion and ensure the connector is fully seated and locked.
• Replace Suspension Control Module (SCM/RCM): If, and only if, all other components (wiring, connectors, actuator) have been rigorously tested and confirmed to be functioning correctly, a faulty SCM/RCM may be the culprit. Module replacement is generally a last resort, as it is often costly and may require specific programming or calibration procedures by a dealership or a specialized shop to function correctly with the vehicle’s network.
• Clear DTCs and Test Drive: After any repair, clear the C1906 and any related codes using a scan tool. Perform a thorough test drive under various conditions to confirm the repair has resolved the issue and that the ride control system is functioning properly across all modes. Observe for any return of symptoms or warning lights.
• Wheel Alignment Check: If the entire shock absorber assembly was replaced, it is strongly recommended to perform a wheel alignment check, particularly for the rear axle, to ensure proper handling and tire wear, even if no adjustments were made to other suspension components.