what is the cause when the dash says "car too low?"

What is your question?

Bought 2003 s500 4matic Car was very low on all 4 wheels, owner said it needed back air shocks. Dash says "car too low" Took it to a mechanic, they said suspension compressor and suspension relay don't work. They also said it could be something else, since computer scan doesn't let them go past that point. Should I change the relay first? Could it be something else? Where are the suspension compressor and relay located on the car? What do you advise I do first?