Code C0684 on the 2020 Mercedes-Benz GLC indicates a malfunction in the air suspension compressor. The GLC with AIRMATIC suspension uses this compressor to generate the air pressure needed to inflate and adjust the air springs at each corner.
AIRMATIC Compressor Function
The air suspension compressor is an electric pump that pressurizes air and supplies it to the air springs and pressure reservoir. It operates when the system needs to raise ride height, compensate for load changes, or maintain pressure lost through normal leakage. The compressor has thermal protection and may cycle off if overworked.
Symptoms and Warnings
With C0684 present, an air suspension warning appears on the dashboard and in the COMAND system. The vehicle may sit lower than normal, especially after sitting overnight. You might notice the suspension doesn't adjust properly or takes longer than normal. In severe cases, the vehicle may sag on one or more corners.
Diagnostic Evaluation
Compressor failures can result from internal motor wear, relay problems, thermal overload from system leaks, or electrical supply issues. Before condemning the compressor, check for air leaks that would cause the compressor to overwork and overheat. Monitor compressor run time and temperature - continuous operation indicates a leak somewhere in the system.
Repair Implications
If the compressor has failed, replacement involves removing the unit from under the vehicle (typically mounted near the rear). Mercedes compressors are matched to system specifications, so OEM or OEM-equivalent parts are recommended. After installation, the system requires initialization through XENTRY/Star Diagnostic to establish proper operating parameters.