Whenever I turn my heater on in auto it will not come on. I have switched to Run and it comes on but only blows cool air. The pilot light is lit. The A/C works great so I have no idea what this might be, I read that it might be a relay but have no idea which one it is a ~17 yr old Trane unit so it may be going any day but I would like to prolong it as much as possible.

Usually there is a 24 volt AC transformer in the furnace with the secondary winding connected to a red wire running to the thermostat and a blue wire, common, to the gas valve, A/C relay, and fan relay. From the thermostat there will be white wire to the gas valve, yellow to the A/C, and green to the fan. The thermostat is wired to switch the power from the red to the white, yellow, and green as needed with the blue completing the circuit. Most thermostats and furnaces have the contacts labeled R, B or C, W, Y, and G for the corresponding wire colors.

When calling for heat, the blower won't come on until the gas runs long enough to warm the air in the furnace. You need to go to the gas valve. It is between the gas line and the furnace burner. It should have white and blue wires to it. If they have 24 volts to them, and the gas doesn't come on, the gas valve is bad. If they don't have 24 volts, the thermostat or wiring to it is bad.

Ok I tried to troubleshoot according to your steps but the labels are gone and the wiring colors are not correct, there are no blue wires at all and there are different wire combos than what you listed. Is there any basic descriptions of each component you can give or should I just call a repairman, it if is by chance the thermostat how much do one of those usually run? Thanks for the help.

Follow the gas pipe into the furnace. It should come to a square box with a couple of wires and tubes one of which goes to the pilot light. The far side connects to the furnace burner itself. The thing is the gas valve. It should have small blue and white wires going to it. If they are a different color, no big deal, electricity is color blind. With the power on, and the thermostat calling for heat, there should be 24 volts between the wires. If so, and the gas doesn't come on, the valve is bad. You can shut the gas and power off and remove it, take it to a Trane dealer and buy a new one. Hardware stores and home centers should have one that would work, but may require some changes to make a slightly different one work.

If the gas comes on, but the blower doesn't before the gas goes off again, the temperature limit switch isn't working. It is a little box near the gas burner with a couple of wires running to it. The blower doesn't start up until the limit switch is warmed up by the gas burner.

Ok so the pilot light is on but I read the wires on the gas valve and there was nothing, so I appreciate the help you have giving I will be checking out a Trane dealer either todoy or tomorrow thanks labman.

Trane unit heater not working on auto