JLG 450 A runs on propane but engine malfunction indicator light stays on

JLG 450A boom lift ran fine in December 2014 but not used until April. Started fine on gas but then quit 10 min later and would not restart. I put a tank of propane on and it started and ran fine but the engine malfunction indicator light stays on. I caned the fuel filter and I got it to start for a few minutes but it will not start on gas again. Any Ideas? When running on propane the engine malfunction indicator light stays on. Ed