Furnace is 5 yrs old, have reset it , thermostat goes blank, blower continues to blow but only cold air. When try to restart it, it may run for 5 minutes, then goes off and blows cold air again. Replaced fuse, shut off circuit breaker and restarted, but continues to not run. Filter is clean, no debri anywhere. It runs on propane. The flashes look like 33 but not sure. Any suggestions?