Finding the current suspension mode
Failures in the Hydractive system (generally problems with sensors or their wiring) can force the suspension to be stuck in hard mode. To diagnose the fault, it would be convenient to know which mode the car is currently in. As H II systems use a 1 kHz chopper to form the current operating the valves, this leads to an interesting way to learn the current status of the suspension: this chopped current actually disturbs the radio reception. Tune your radio to a spot somewhere near 700 kHz where there is no incoming broadcast. When the solenoid valves are energized (that is, the suspension is in soft mode), you will hear a characteristic buzzing sound from the radio. As you drive around, you can observe what triggers the hard mode erroneously.
On H I systems, you can connect a wire to the solenoid valve (it is located below the car, approximately beneath the front seat). If you connect a simple multimeter to this wire, you'll be able to watch the 5-6 V driving the valve in soft mode.
Actually, in addition to diagnosing the suspension, this buzzing sound can also be used to decide which Hydractive system your car is fitted with. Alternatively, you can look for the ECU itself or the H II-only stiffness regulators.