I am located in Ontario, Canada. it is around 10 degree F. outside.
My 18 years old furnace stops last evening. I have to turn OFF both AC and thermostat long enough before the furnace would came back normal. It works for couple hours last night, I can hear it cycle OFF and ON on its own before went to bed.
Wake up at 65 degree in the morning, obviously it stops during the night. Have to shutdown AC and thermostat and wait long before it starts again.
Get back home early today and it was stops again.

My observations are:
*The furnace is try running as gas was heating up the unit.
*Seeing the 4 flash code couple times when checking.
*it seems that AC & thermostat reset would not put furnace back normal right the way. Has to wait longer to cool down.
*During my troubleshooting and restart, ignition will starts, frame goes on full, blower will not starts after minutes. I think eventually furnace stops after overheat protections.
*I checked the limit switch (LS1) is close when shutdown (AC off)
*The pressure switches burner 1LP & condensate 2LP are close when shutdown (AC off).
*Put the FAN to ON on thermostat would not starts the blower. (only starts after a long shutdown period)
*The impeller of blower could turns freely by hand.
*Each normal operation would last for 2+ hours before (blower?) stops, gas will frame up again but no blower.
*Blower motor was hot to touch every time after stops.

When measuring on blower motor wires, blue is heat, black in cool, red is park, white is neutral.
*After the last furnace stops, there is 170VAC between the park red wire & neutral white wire and 120VAC between cool black wire & neutral. Zero volt between heat blue wire and neutral. Furnace frame up but blower motor not turning.
*I measured that heat blue wire is open to neutral, 14ohm between red & neutral and 6ohm between cool black & neutral.

Is these symptoms pointing to either the blower motor or the 5uF capacitor is going to die?

Perhaps there is a overheat thermostat inside the blower motor which get open circuit due to overheat.

My big question is "would motor overheat caused by a bad capacitor?"
or the motor is almost dead?


Appreciated your help and expertise.
Thank You,
Philip

last night after the 15 minutes runs, then stop with 6 flash code, Im not able to bring it back after long cooling. So put the furnace to OFF before bed.

This morning I put the thermostat to COOL position, FAN to auto.
Turn on the furnace, back to the thermostat, turn the FAN to ON, blower starts after couple of seconds. Once again the blower will starts at COOL position. Later flip the swithc to HEAT, couple more seconds later gas starts, warm air came out in minutes.
It ran for more than 2 hours non-stop by now.

This furnace has no AC, AC was added later years. It might explain why there is only one speed.

Any suggestion or comments to above problems?
Thanks.