Hi, had a similar problem a couple of days ago. Was driving along and the the hdc inactive and air suspension inactive warnings came on, accompanied by the abs light. I adopted the Coporal Jones approach "don't panic" and drove gently home.
When inspected from the outside, car seemed level, so I went through the proceedure outlined below and cleared down the error. Re started the car and "touch wood", the error did not re-occur. As I had had the wheels tracked on Saturday, I raised the suspension to off road seeing and had a quick look at the wiring for the abs sensors and ride height sensors, nothing immediately obviously wrong, but I suspect, given that the HDC inactive light is a regular feature on early morning start ups (despite a new battery), I may have to replace a ride height sensor or two. If anybody has any advice to offer on this subject I would appreciate it, like how do you know which one would need to be replaced etc.
Anyway, the proceedure below, I lifted from the "interweb" and seems to work (so far) on my "hobby", which is a 2005 TD6.
Have fun
Regards
Ed
Resetting ALL Defect Codes
Ignition key must be “off”.
• Press and hold the Check Control button in the instrument cluster (RIGHT button), and turn the ignition key to the first position (KLR).
* You will first see "Check Control OK", some seconds later you will see "Test Nr.01"
* Push the left button on the instrument cluster to see the car's identification number. Add the 5 digits (not 6) together and write down that number, you will need it for unlocking all the functions. Example: A123457 will lead to 2+3+4+5+7 = 21
* Push the right button until you see "Test Nr.19"
* Push left button until you see the number that is equal to the one you calculated above
* Push right button again
* Using the left button select 21. Reset all defect codes
* Press the Right button to select.