What would cause the air suspension on a 2003 Lincoln navigator not to work?

there are several causes to the air pump. the first and most obvious is the air pump (compressor) itself. if you do not hear the compressor pumping air or if you do not feel the slight vibration of the compressor when you put your car into drive, then it's probably the compressor that has gone bad. if the compressor is coming on and you do not hear any air leaking, then your problem could be the height adjuster, located on the suspension bags themselves. Bronconvict: another possibility includes the compressor relay switch. This switch controlls the power to the air bag suspension compressor. This switch is known for burning out when the suspension compressor is working too hard. This can also be caused by a bad/corroded GEM module (due to notorious winshield leak). i had to replace "height sensor" on driver side . part was $200.