Air Suspension Problem

[MrQ4.8is]

I recently purchased an x5 4.8is 2004 model a week later the air suspension inactive has come up, the car is high on the front left and the height adjust buttons green light has gone off but if you rest the fault via diagnostic computer it all starts to work suspension goes up and down but after about 30 miles inactive again the errors read are height invalid and something about a valve (can't find the paper I wrote the fault codes on).

Any advice like where the sensors are on the car what I should try first...

Mr Q