DTC C1A20-64: Pressure Increase Too Slow When Filling Reservoir - Signal plausibility failure: Action

Using the manufacturer approved diagnostic system, test the operation of the suspension air supply unit (vehicle ride height interface)
Refer to the relevant service information and test the air suspension system for air leaks
Check the air suspension intake filter for blockages/restrictions
Using the manufacturer approved diagnostic system, test the operation of the corner valves (vehicle ride height interface)