During a recent trip to the mountains my 1995 F-150 Flareside 4x4 started loosing power (running on gasoline) at 4000 meters above sea level. At 4900 m.a.s.l. the engine just quit. I had to switch to compressed methane and in this way could (just) make it down to 4000 m.a.s.l., when I could switch back to gasoline. This happened twice during the 2500 Km trip, both times above 4300 m.a.s.l. In theory this should not happen because there is a sensor for athmospheric pressure which compensates for any change in altitude. Can any one tell me how to check this sensor for proper operation ?