C1790

What Does Code C1790 Mean?

Diagnostic Trouble Code C1790 signifies a detected electrical malfunction within the output circuit for the left rear (LR) air spring or shock absorber solenoid. This code is typically registered by the Air Suspension Control Module (ASCM), which is responsible for monitoring and actuating the components of the electronically controlled air suspension system. The ASCM continuously monitors the electrical characteristics of the solenoid circuits, including resistance, current draw, and voltage feedback. When the ASCM detects a deviation from specified parameters—such as an open circuit (infinite resistance), a short circuit to ground, a short circuit to voltage, or an out-of-range resistance value—within the LR air spring/shock solenoid circuit, it registers C1790. This indicates that the module cannot reliably control the solenoid, which is crucial for regulating air pressure in the air spring (for ride height adjustment) or adjusting the damping rate of an electronically controlled shock absorber (for ride comfort and handling dynamics) at the left rear corner of the vehicle.

Common Symptoms

• The vehicle’s left rear corner may sit noticeably lower or higher than other corners.
• A distinct difference in ride quality, often a harsh or overly soft feel, specifically originating from the left rear.
• Illumination of a dedicated air suspension warning light or display of a “SERVICE AIR SUSPENSION” message on the instrument cluster.
• Audible hissing sounds indicating an air leak near the left rear suspension, potentially exacerbated or caused by a stuck-open solenoid.
• The air suspension compressor may run excessively in an attempt to compensate for the perceived imbalance, leading to premature wear.

What Causes the Code C1790?

• Internal Solenoid Failure: The most frequent cause is an electrical fault within the left rear air spring or shock absorber solenoid itself, such as a broken coil winding, internal short, or carbon buildup.
• Wiring Harness Damage: An open circuit, short to ground, or short to voltage within the wiring harness connecting the ASCM to the LR solenoid. This can be due to chafing, corrosion, rodent damage, or impact.
• Faulty Electrical Connector: Corrosion, loose terminals, bent pins, or poor seating within the electrical connector for the LR solenoid, leading to intermittent or complete loss of electrical continuity.
• Water Intrusion: Water entering the solenoid connector or internal components can cause short circuits or corrosion, leading to circuit failure.
• Air Suspension Control Module (ASCM) Malfunction: While less common for a circuit-specific code, an internal failure within the ASCM’s output driver circuit dedicated to the LR solenoid could cause this code.

How to Diagnose and Troubleshoot

A systematic diagnostic approach is critical for accurately pinpointing the root cause of C1790:

1. Retrieve and Document DTCs: Use an advanced OBD-II scanner capable of accessing chassis (C-codes) to confirm C1790 is present. Note any other related air suspension codes that may provide additional context. Clear the codes and cycle the ignition to see if C1790 immediately reappears, which can indicate a hard fault.
2. Visual Inspection: Begin with a thorough visual inspection of the left rear air spring/shock assembly. Pay close attention to the electrical connector for the solenoid, inspecting for signs of corrosion, loose terminals, frayed or damaged wiring, or physical impact. Trace the wiring harness as far as possible towards the ASCM, checking for chafing, pinching, or heat damage.
3. Solenoid Resistance Test: With the ignition OFF and the LR solenoid electrical connector disconnected, use a Digital Multimeter (DMM) to measure the resistance across the two terminals of the solenoid itself. Refer to the vehicle manufacturer’s specifications for the expected resistance range (typically a low ohmic value, e.g., 2-10 ohms). An “OL” (open circuit) reading or a significantly out-of-spec value indicates an internal solenoid failure.
4. Wiring Harness Continuity and Short Test:
   • Continuity: With the ignition OFF and both the solenoid connector and the ASCM connector disconnected, identify the specific wires for the LR solenoid circuit in the wiring diagram. Use the DMM to check for continuity between the corresponding pins at the ASCM connector and the LR solenoid connector. Ensure continuity is present (near 0 ohms).
   • Short to Ground: With the wires still disconnected from both the solenoid and ASCM, check each wire for a short to ground by placing one DMM lead on the wire terminal and the other on a known good chassis ground. Any reading near 0 ohms indicates a short to ground.
   • Short to Voltage: With the DMM set to voltage mode, turn the ignition ON (engine OFF). Check for voltage on each wire in the disconnected harness (both ends disconnected). The presence of unexpected voltage indicates a short to voltage.
5. Voltage and Ground Supply Check (at Connector): Reconnect the DMM to the vehicle-side harness connector (disconnected from the solenoid). With the ignition ON, verify the presence of the specified voltage supply (e.g., battery voltage or regulated control voltage) and a reliable ground signal at the connector pins that feed the solenoid. Missing voltage or an intermittent ground indicates a wiring or ASCM output driver issue.
6. ASCM Output Test (if applicable): If available with an advanced diagnostic tool, attempt to command the LR air spring/shock solenoid directly through the ASCM. Listen for the solenoid clicking or observe system response. If the solenoid tests good electrically but does not respond to commands, it further points to a wiring issue or ASCM driver fault.

Recommended Repairs and Solutions

Based on the diagnostic findings, the following repairs are typically recommended:

• Replace the LR Air Spring/Shock Solenoid/Assembly: If the solenoid resistance test indicates an internal failure, the most common solution is to replace the entire air spring or shock absorber assembly, as the solenoid is often an integrated, non-serviceable component of the unit. Ensure that the replacement part is compatible with the vehicle’s specific make and model.
• Repair or Replace Damaged Wiring or Connectors: If the wiring harness or connector is found to be damaged, corroded, or shorted, it must be repaired or replaced. Use high-quality, automotive-grade wiring and connectors, ensuring proper crimping, soldering, and waterproofing (e.g., using heat shrink tubing with sealant). If severe, replacement of the relevant section of the wiring harness may be necessary.
• Address Water Ingress: If water intrusion is identified as the cause of damage, in addition to repairing the wiring/connector, identify and seal the source of the water entry to prevent future issues.
• ASCM Replacement and Programming: If all wiring, connectors, and the solenoid itself test unequivocally good, and no other external factors are found, the ASCM may be faulty. This is generally a last resort. After replacing the ASCM, it will typically require programming or calibration to the vehicle’s specifications using specialized diagnostic equipment.
• Post-Repair Procedures: After any component replacement or wiring repair, clear all stored DTCs. Re-inflate the air suspension system, if applicable, and perform any necessary ride height calibrations or sensor relearn procedures as specified by the manufacturer. Conduct a thorough test drive to ensure the code does not return and that the air suspension system operates correctly, maintaining proper ride height and delivering expected ride quality.