What Does C1840 Mean?
The air suspension control module has detected a failure in the air suspension disable switch circuit. This switch allows the technician or owner to disable the air suspension system (typically for service, towing, or jacking). When this circuit fails, the module cannot determine the switch position. The system may default to a disabled or limited-operation mode, and the air suspension warning light will illuminate.
Common Causes
35%
Air suspension disable switch has failed internally (contacts worn, corroded, or broken)
25%
Wiring to the disable switch is damaged, particularly in trunk or cargo area routing where luggage can crush or abrade harnesses
20%
Disable switch connector has corroded or loosened, especially if located in the trunk where moisture can accumulate
10%
Switch was inadvertently left in the OFF position, and the circuit fault is being set due to a monitoring issue
10%
Control module input circuit for the disable switch has failed
Diagnostic Steps
1
Locate the air suspension disable switch (typically in the trunk, cargo area, or near the jack storage) and verify it is in the ON/ENABLE position — some vehicles use a toggle switch, others a rocker.
2
Toggle the switch several times and check the scan tool live data for the switch status PID — it should toggle between enabled and disabled. If it does not change, the switch or circuit has a fault.
3
Disconnect the switch connector and measure switch continuity in both positions with a DVOM — the switch should show open in one position and closed in the other.
4
With the switch disconnected, check the wiring from the switch connector back to the air suspension module for continuity and verify no shorts to ground or power exist.
5
Inspect the switch connector and surrounding area for water damage, corrosion, or signs of physical damage from cargo loading.
Estimated Repair Cost
$30 - $300
Parts + labor, varies by vehicle and location
The air suspension control module has detected a failure in the air suspension disable switch circuit. This switch allows the technician or owner to disable the air suspension system (typically for service, towing, or jacking). When this circuit fails, the module cannot determine the switch position...
The most common cause of C1840 (Air Suspension Disable Switch Circuit Failure) is: Air suspension disable switch has failed internally (contacts worn, corroded, or broken)
Typical repair costs for C1840 range from $30 to $300, depending on the vehicle, location, and whether you do it yourself or go to a shop.
If the system defaults to disabled mode, the air suspension will not actively maintain ride height. The vehicle can still be driven, but ride quality will be reduced and the vehicle may sit at a default height. Verify the switch is not simply turned off before performing any repairs.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to C1840 to identify the root cause.
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Quick Info
Category
Chassis
System
Air Suspension System
Difficulty
Type
Manufacturer
Recommended Tools
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