We change the gas valve in a american standard package unit but after that the heating mode is not working in auto, only in on position; the thermostat is new, please help me.

The control board times the blower motor on with a gas system. If you turn the blower to on at the thermostat and set it to heat will it run and heat the house? If so you have a bad blower control. Not knowing the model its hard to say if it's a bad board,fan limit control or relay.

Thanks T-Top for your answer,I don't have the model, but like you said is going to be an electrical problem, I'll check is an American Standard package unit and like I said before heating is only working in on position and I can hear the "click" when the thermostat is calling for heating in auto position but the blower does not start. Thanks again.