Find the relay first, unplug it, clean it the plug it back in. Try this before buying a compressor or a new relay. I remember in a car my friend was working on, it was on the RH (Passenger side) between the bumper and headlight. This happened ages ago so i cant recall what car that was. if that doesnt work
Fill the water bottle with water and then put a few drops of soap in the bottle.
Squirt the soapy water onto the air fittings to test for leaks. If the fitting bubbles up, then you have a leak in that corner. In that case, tighten the fitting with a wrench, or remove and reinstall the fitting with thread sealant.
Undo the fitting with a wrench. Apply two wraps of Teflon tape around the fitting, then tighten the fitting and test again.
If the fitting still leaks, you may have a cracked fitting. Replace the fitting and reinstall, then test again.
Check the pressure switch in the air system. Often if the pressure switch isn't working, the air supply isn't being filled, and therefore your vehicle won't lift. Test by jacking up the vehicle in the air and setting on jack stands.
Drain the air tank. Generally there is a drain plug at the bottom of the tank.
Turn on the vehicle so that pressure switch should activate. Place a test light on the wiring. It should show a light on both connections. If it doesn't, check the fuse. Replace if necessary
Replace the pressure switch if the fuse is functioning properly but the switch still doesn't turn on.
Testing the Compressor
Test the compressor for power if the pressure switch is working but the tank is not filling up with air.
Place the test light on the power lead to the compressor(s). If it doesn't get power, check the relay running power to the compressor or the fuse running to the compressor.
If the fuse is bad, replace it and test again. If it's good, test the relay with the test light. If it doesn't work, replace it.