Lenox Gas Furance Summer Fan Not Working

Lenox furnace are a bear to work on if all the world isn't right that day. The sensor is not proofing to the control module.

If aquastat wirings are ok, and control module had been set up properly, the the furnace should start.
Check if bypassing the control the furnace starts, if it starts then check aquastat wirings.
If it depends on the furnace, check the valve, the safety sensor and the ignitor.
Here is the guide to set up the Aquastat control:
Guide to Setting & Wiring Heating System Boiler Aquastat Controls

Sounds like limit switch issue. Furnace heats up, high temp limit switch opens, fan will stay on. GMP model is older and do not have the codes.
May be over fired, have airflow problem, to cause the limit to open.
Next time before shutting off power, try to find the limit, it has two wires that connect to a 1.5 x 2 inch card mount to the furnace fire wall. tap on it and see if the blower stops. If so replace the limit.

Check the draft inducer air switch ( vacuum diaphragm switch) If the switch is opening when the burner starts, you might have a blockage on air piping coming into the furnace. Also prime the condensate trap on the condensate drain from the combustion fan. Also check the gas pressure, if your gas pressure is dropping, the gas pressure switch will shut it down. If it's too high the moment it comes on could raise the pressure in the furnace dropping out the combustion fan proving switch.

Hi check out the fan control afc see if it is clean , check out pressure switch and the flame sencer

You can sand the flame sensor but it sounds like another problem. Sounds like a limit switch or something in the control circuit, You need to have a service tech look at this. Ask for free estimate on repair. I have worked on hundreds of these units and they are sometimes tricky to work on. Lenox has come up with new controls for the whisper heat furnace to replace the old ones. If the flame sensor is bad the unit will try to light a few times and lock out and you ahve to kull power to the furnace to reset. If this is whats happening clean the sensor nd you should be ok If not you need help from a service tech.Rus

Your furnace is going into a lockout condition, cycling the power resets the circuit board and it starts over. I will need to know approximate age and model of your furnace, it sounds like you have need to clean the flame sensor rod. However i need to know what i am working with.

On is just the blower fan nothing else, check to see if there is a reset button in the tstat or tripped over load inside the furnace, inside the furance is some safetys, you will see wires runing from each other and finaly to the gas valve, push each untill one of them clicks,then your furnace should start back working, but I would still have someone come out and check why it tripped, because as I said it is a safety, and when it tripps it is for a reason.

There are 4 -steps for full run time to kick in a furnace.. step-1, inducer fan runs & prove thru the air tube which closes a switch and tell the circuit board to go to step-2, the igniter lights the pilot & prove its lit by heating the thermocouple, which tells the circuit board to go to step-3, opens the gas valve to dump gas in, which will light from the pilot light, and prove by a flame sensor metal stick on the last burner , which heats up and tell the circuit board Saying, ok , I've got flame.. step-4. when the box heats up, it closes a switch and turn on the main fan...! (Some gas-valves has a brain -or circuit board on top of them and are real sensetive to moisture..)

There will also be some sort of surface mountad switch which activates the blower at a certain temp. With the unit unpluged and the heater off, take a meter and check for continuity thru the switch. there should not be any. Then start up the heater but keeping the blower unpluged, check again after a few minutes. This time your meter should show continuity.