We have a coleman furnace. The model number is 3400a816. The heating element comes on, but the fan will not come on unless you put it on manual.

You have a bad sequencer or with some models the thermostat brings the blower on in heat mode. Is this a new thermostat your trying to use?

No its not a new thermostat.

I am with T-top the sequencer is probably defective.

Part number

3115-3571 Sequencer

Sequencer controls the sequence in which the heating elements & blowers turn on. This one has the ability to control up to 3 heating elements. Replaces 3400-357, 3400-3571 & 3115-357

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Coleman Electric Furnace Parts

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Coleman Electric Furnace Parts