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Limit switch jumps to manual

Emerson 5D51-78 Fan/Limit Control, 9 months old, jumps to manual. I set the switch to auto, burner ignites, heats plenum, fan kicks in, thermostat temperature is reached, burner shuts down, fan runs and pulls switch on over to manual. I can switch it back to auto and shut fan off but, eventually it cycles back to manual. What's happening, contacts welding together?