Today I tested my KF 4, 912S by climbing to 15,500'. OAT was -3C at this altitude. Everything performed great but I quit climbing because I was cold. I then started my descent and 30 mile return to my home airport when my Dynon flashed a red "low fuel pressure" warning. Needless to say, I forget that I was cold. I think I was down to about 8-10,000'. I turned on the electric fuel pump and the warning disappeared. I then tested by turning off the elec. pump and the warning returned immediately. After turning on the elec pump, I toggled the Dynon into the page showing the actual fuel pressure, and repeated the test. The fuel pressure immediately started dropping when I turned off the elec. pump. I suspected that the mech. pump had failed. I returned home on the electric pump, landed, but then could not replicate the low fuel pressure warning when on the mech pump only. Full power run up on the ground, on the mech pump only, ran great. Any suggestions? Did the cold temps cause a problem with my 100ll av gas? I sumped the tanks and gascolator but did not find any water. Bruce Estes