I had this problem on my 98 Concours. Very hard to diagnose because in my case the car would only be low with a load and only after driving for 100's of miles on the freeway ( slowly leaked down over time), once I restarted the car it pumped right back up for another 200 to 300 miles. Just about every rest area I would get out and it was down about 1.5 - 2 inches. The dealer kept saying no codes no problem, I found out this was not true. After they replaced the shocks and air line with no help at all they finally replaced the compressor and that fixed it.