C1735

What Does Code C1735 Mean?

Diagnostic Trouble Code C1735, “Air Suspension LR Corner Up Timeout,” indicates that the vehicle’s Air Suspension Control Module (ASCM) or Ride Height Control Module (RHCM) has detected a failure in the Left Rear (LR) air suspension system to reach its commanded “up” or target ride height within a specified calibration timeframe during an inflation event. The ASCM/RHCM monitors the actual ride height via a dedicated Left Rear Ride Height Sensor (LR RHS). When the module commands the air compressor to deliver air to the LR air spring, it expects a corresponding, measurable increase in the LR RHS signal within a predefined period. If the sensor reading fails to achieve the target height within this set duration, the module interprets it as an “up timeout” for that specific corner and registers C1735. This code points to an inefficiency or failure within the LR corner’s air delivery and retention system, preventing it from adequately lifting or maintaining pressure.

Common Symptoms

• Visibly Uneven Vehicle Stance: The Left Rear corner of the vehicle may appear lower than the other corners, or lower than its intended ride height.
• Harsh Ride Quality: If the air spring cannot properly inflate, the suspension may be riding on its bump stop, leading to a very stiff and uncomfortable ride.
• Persistent “Air Suspension Fault” Light: A warning indicator on the instrument cluster, often accompanied by a message such as “Check Air Suspension System.”
• Excessive Compressor Operation: The air suspension compressor may run more frequently or for extended periods in an attempt to raise the LR corner, without success.
• Audible Hissing Sounds: Air leaks from the Left Rear air spring or associated air lines may be audible when the vehicle is parked or the compressor is running.
• Reduced Load-Carrying Capacity: The vehicle’s ability to maintain ride height under load may be compromised, especially on the affected side.

What Causes the Code C1735?

• Air Leak in LR Air Spring: The most prevalent cause, where a rupture, crack, or degradation of the rubber diaphragm or seals in the Left Rear air spring allows air to escape, preventing it from holding pressure.
• Compromised Air Line to LR Air Spring: Damage, kinks, chafing, or a loose connection in the air line supplying air to the Left Rear air spring, leading to a pressure leak or insufficient airflow.
• Faulty LR Air Suspension Solenoid Valve: The solenoid valve responsible for controlling air flow to the LR air spring may be stuck closed, partially open, or failing to seal properly, preventing adequate inflation or causing a leak.
• Weak Air Suspension Compressor: While C1735 is corner-specific, an overall weak or failing air suspension compressor might not be able to generate sufficient pressure or volume of air to lift the specific corner within the timeout period, especially if there’s a marginal leak.
• Malfunctioning LR Ride Height Sensor: An electrical fault within the Left Rear Ride Height Sensor or its associated wiring could cause it to send inaccurate or no signal to the ASCM, leading the module to falsely believe the corner is not rising.
• Wiring and Connector Issues: An open circuit, short to ground, or high resistance in the electrical wiring to the LR solenoid valve, LR ride height sensor, or their respective connectors.
• Internal ASCM/RHCM Fault: Though rare, an internal failure of the Air Suspension Control Module could result in incorrect commands or misinterpretation of sensor feedback.

How to Diagnose and Troubleshoot

Diagnosis of C1735 requires a systematic approach, often utilizing a diagnostic scan tool with bidirectional control capabilities and a digital multimeter (DMM).

1. Preliminary Visual Inspection:
   • Inspect the Left Rear air spring for obvious signs of collapse, cracks, dry rot, or moisture/debris accumulation, which could indicate a leak.
   • Trace the air line from the LR air spring towards the compressor/valve block, checking for kinks, chafing, cuts, or loose connections.
   • Examine the electrical connectors at the LR air spring (if applicable), LR ride height sensor, and the air suspension valve block for corrosion or damage.
2. Scan Tool Diagnostics:
   • Connect a diagnostic scan tool capable of communicating with the ASCM/RHCM.
   • Read and record all stored Diagnostic Trouble Codes (DTCs), including any pending or historical codes. Look for related codes that might point to a height sensor or compressor issue.
   • Access live data for the air suspension system. Monitor the “Left Rear Ride Height Sensor” voltage or height reading (in mm/inches) and compare it to other corners. Note any discrepancies.
   • Using bidirectional controls, command the LR air spring to “inflate” and “deflate.” Observe the corresponding changes in the LR height sensor data and listen for the compressor running and air movement at the LR corner.
   • Monitor overall system pressure (if available as a PID) during inflation attempts.
3. Air Leak Detection:
   • With the vehicle safely raised and supported (ensuring the suspension is extended), or by commanding the system to inflate, liberally spray a soapy water solution around the entire Left Rear air spring, its top and bottom mounting points, and all air line connections. Look for bubble formation, which indicates an air leak.
   • Listen carefully for any distinct hissing sounds emanating from the LR area, especially immediately after the compressor stops running.
4. Electrical System Checks (using DMM):
   • LR Ride Height Sensor:
     • Disconnect the sensor connector. With the ignition on, verify the presence of a reference voltage (typically 5V) and a good ground at the sensor connector using a DMM.
     • Backprobe the signal wire of the sensor. As you manually articulate the suspension arm (within safe limits), the voltage signal should change smoothly and linearly from minimum to maximum range (e.g., 0.5V to 4.5V). Compare these readings to manufacturer specifications.
   • LR Solenoid Valve:
     • Disconnect the solenoid valve connector. Measure the resistance across the solenoid coil terminals. Compare to the manufacturer’s specification (typically a few ohms). An open circuit or excessively high resistance indicates an internal fault.
     • With the scan tool commanding inflation, use a DMM to check for voltage and ground pulses at the solenoid connector from the ASCM/RHCM. This confirms the module is attempting to actuate the valve.
   • Wiring Harness Integrity: Perform continuity and resistance checks on all wires between the LR components (height sensor, solenoid valve) and the ASCM/RHCM connectors to identify any open circuits, shorts, or high-resistance connections.
5. Compressor Output Test:
   • If no leaks are found and the LR specific components check out, verify the compressor’s ability to generate and maintain adequate pressure. Some scan tools allow direct activation of the compressor. Monitor system pressure PIDs or use a calibrated pressure gauge if accessible.

Recommended Repairs and Solutions

Addressing C1735 typically involves replacing the component identified during diagnosis that is either leaking or failing to actuate properly.

1. Replace the LR Air Spring: This is the most common repair. If a leak is found in the air spring bladder itself, its integrated seals, or the mounting points, the entire Left Rear air spring assembly must be replaced.
2. Repair/Replace Air Lines and Fittings: If an air leak is pinpointed to a damaged air line or a faulty connection fitting, replace the affected section of the line or the fitting. Ensure secure and leak-free connections using appropriate tools.
3. Replace the LR Air Suspension Solenoid Valve: If electrical tests or leak detection pinpoint the solenoid valve as faulty, replace the valve. Often, these are integrated into a larger valve block or directly into the air spring assembly.
4. Replace the LR Ride Height Sensor: If electrical diagnostic procedures confirm that the LR ride height sensor is providing inaccurate, erratic, or no signal, it requires replacement. Ensure proper calibration after replacement.
5. Replace the Air Suspension Compressor: If the compressor is found to be weak, running excessively without building sufficient pressure, or failing to activate, it will need to be replaced. While C1735 is specific to one corner, a failing compressor can hinder the entire system’s ability to achieve commanded heights.
6. Repair Wiring Harness: Repair any identified open circuits, shorts, or high-resistance connections in the wiring. Utilize proper automotive wiring repair techniques, including soldering and heat-shrinking, to ensure durable and reliable connections.
7. ASCM/RHCM Replacement: Only consider replacing the Air Suspension Control Module as a last resort, after meticulously ruling out all other potential causes. This typically requires professional programming and calibration using specialized diagnostic equipment.

Important Mechanic’s Tips:

• Always follow manufacturer’s specifications for depressurizing the air suspension system before disconnecting any air lines or components to prevent personal injury and system damage.
• After replacing any air suspension components, it is crucial to perform a system calibration or “learn procedure” using a compatible scan tool. This ensures the ASCM/RHCM accurately knows the vehicle’s reference ride heights.
• When replacing air springs, it is often recommended to replace them in pairs (e.g., both rear air springs) to maintain balanced suspension performance and prevent an early failure of the older, unreplaced spring due to age and wear differences.
• Inspect the desiccant dryer, which is usually integrated with the air compressor. A saturated dryer can introduce moisture into the system, potentially leading to freezing in colder climates, which can block air lines or damage solenoid valves.
• Always clear all diagnostic trouble codes after completing repairs and perform a thorough test drive to confirm the issue is resolved and the code does not return.