Audi A6 C5 Allroad Air Suspension Compressor Control Module Testing

The Audi A6 C5 Allroad uses an electronically controlled air suspension system managed in part by a compressor relay and control circuit mounted in the luggage compartment. When this control module or its associated relay fails, the car typically drops to its lowest ride height overnight, refuses to raise on startup, or shows erratic behaviour between levels. It is not always the compressor itself that has worn out — the control electronics that trigger and monitor compressor operation fail independently and are a well-documented weak point on vehicles from this era.

How the Compressor Control Circuit Works

The Allroad air suspension system on the C5 platform is coordinated by the Electronic Damping Control module (EDC/ECAS), which monitors ride height sensors at each corner and sends commands to raise or lower the suspension by opening solenoid valves and activating the compressor. The compressor itself is a simple piston-driven pump, but it relies on a relay and a separate thermal cutout circuit to prevent overheating. The control module monitors compressor run time, thermal limits and system pressure. When the electronics in this circuit fail — often due to heat cycling, corrosion on the relay contacts, or capacitor degradation — the compressor may not receive a run signal at all, or may run without stopping and trigger a thermal cutout.

This makes diagnosis more complex than simply testing whether the compressor motor spins. You need to verify that the control signal is actually reaching the compressor, that the relay is switching correctly, and that the module has not stored a permanent fault that locks the system out. For comparison, see Repair or Replace the Audi A6 C5 Allroad Air Suspension Module.

Fault Codes Associated with This Module

The following fault codes are typically retrieved via VAGCOM/ODIS from the suspension control module on the C5 Allroad:

Code	Description
01315	Suspension control module — no communication (module not responding on CAN)
01044	Control module incorrectly coded (after replacement without adaptation)
00778	Air suspension compressor relay — open circuit or implausible signal
00772	Level control system — compressor overtemperature cutout triggered
00776	Level control — compressor run time exceeded (system pressure not reached)
01130	Air suspension system — level control interrupted, failsafe active

Code 00778 in particular points directly at the relay circuit rather than the compressor motor. If the relay is switching correctly under live data but the compressor still does not run, the fault moves to the compressor motor winding or the thermal fuse embedded in the compressor body.

Testing Procedure Before Replacing the Module

Before ordering a replacement module, work through these checks in sequence:

Check ride height sensor output: Use live data to confirm all four height sensors are providing plausible values. A failed height sensor can prevent the system from activating the compressor because it believes the car is already at the correct level.
Inspect the compressor relay: The relay is located in the left luggage compartment alongside the compressor assembly. Remove it and test coil resistance and contact switching. Burned contacts are a common cause of no-start commands reaching the compressor.
Check the thermal cutout: The C5 Allroad compressor has a thermal fuse integrated into the motor housing. If the compressor was overloaded — typically because it was running against a leaking air spring — this fuse blows permanently. Resistance between the motor terminals should be approximately 0.5–1.5 Ω. Open circuit means the thermal fuse has gone.
Test the compressor directly: With the relay bypassed by a direct 12V feed to the compressor motor terminals, the motor should run freely if the motor itself is undamaged. If it runs when supplied directly but not through the relay circuit, the control module or relay is the fault.
Inspect module connector pins: The suspension control module connector, usually a large multi-pin plug behind the left rear trim panel, is prone to corrosion and pin retraction. Clean and reseat all connections before condemning the module itself.

OEM Part Numbers — Compressor Control Module and Relay

Several part variants were used across the C5 Allroad production run. Matching the correct number to your specific vehicle configuration is essential — there are differences between quattro variants, production years and equipment levels that affect coding requirements.

4Z7 616 007 A — Air suspension compressor assembly with relay, early production
4Z7 616 007 B — Revised compressor assembly, fitted from late 2001 onwards
4Z7 698 007 — Compressor repair kit with relay and thermal cutout
4B0 907 553 A — EDC suspension control module (ECAS unit)
4B0 907 553 B — Updated ECAS control module, later production variant
4Z7 616 013 — Compressor relay standalone replacement

Always cross-reference the part number printed on your existing module against the vehicle identification number before sourcing a replacement. The difference between an A and B suffix variant can require recoding if the wrong unit is installed. A related diagnostic guide is Audi A6 C5 Allroad Air Suspension ECU Faults Diagnosis Guide.

Affected Vehicles and Production Notes

The C5 Allroad was produced from 2000 to 2005 and shared the B5/C5 platform with the standard A6. Air suspension was standard fitment on all Allroad variants. The compressor control circuit faults are most commonly reported on vehicles from 2001 to 2004, typically those which have had one or more air spring failures in the past — a leaking spring forces the compressor to run longer cycles, accelerating thermal wear on the relay contacts and the compressor motor thermal fuse. The 2.5 TDI and 2.7T petrol variants are equally affected; the suspension electronics are not engine-dependent.

Frequently Asked Questions

Can the Audi A6 C5 Allroad drive with the air suspension failed?

The system enters a failsafe mode that locks the suspension at its lowest ride height. The car can be driven carefully at low speeds, but ground clearance is severely reduced and the handling characteristics change. Extended driving in failsafe mode is not recommended, particularly on uneven roads where the car may ground out.

Does a replacement ECAS control module need coding on the C5 Allroad?

Yes. The ECAS module requires basic coding to match the vehicle’s equipment configuration and then a ride height adaptation procedure must be completed with VAGCOM or ODIS after installation. Without adaptation, the system will store fault code 01044 and may not control ride height correctly even if the hardware is functioning. For additional context, read Audi A6 C5 Allroad Suspension Control Module Testing Procedure.

What is the typical lifespan of the C5 Allroad air compressor?

On vehicles where the air springs have remained intact, the compressor can last the life of the car. However, once an air spring begins leaking — a common failure on these cars after ten years — the compressor runs far more frequently than designed and typically fails within 12 to 24 months of the spring developing a slow leak. Replacing the compressor without also addressing leaking springs usually results in rapid repeat failure.

Can a used OEM compressor module be installed on the Allroad?

Yes, a tested used OEM ECAS module can be installed provided the part number is correctly matched to the vehicle. The unit will require adaptation after fitting. When sourcing a used module, confirm it was removed from a running vehicle and has not been water damaged — these modules are located near the boot floor where water ingress from leaking rear light seals is possible.

How do I know if the compressor motor is failed versus the control module?

The direct power test described in the testing procedure above gives a clear answer. Apply 12V directly to the compressor motor terminals, bypassing the relay and control circuit entirely. If the motor runs freely, the motor is intact and the fault is in the relay or control electronics. If the motor does not run even with direct power, the thermal fuse inside the compressor has blown and the compressor assembly needs replacement.

Find tested OEM electronic parts and modules at:
https://giantparts.eu/

Related diagnostic guides and OEM parts

Browse the category: Suspension Control Modules & ECUs