Two months ago I noticed furnace blower would switch off after being on a couple of minutes. Once I heard some humming, thought it was the motor. Had a techinician diagnose it, he said it was the blower motor, a $400 repair. I pulled it out myself, took it to a electric motor shop. They put power to it, it worked and they said to replace the capacitor and sold me one for $15. I also oiled the motor via the oil ports. Things worked fine for 2 months, then I noticed it wasn't coming on again. Sometimes it would work sometimes not. No humming this time. Once I tripped the thermostat and it took 20 minutes before it came on. Next time it didn't come on I turned the manual fan switch at the thermostat on and I left the main furnace switch on and used a test light at the junction box on the blower to see if it was getting power and it was. I tried turning the power switch at the furnace on and off and didn't get any sound or response. I pulled the blower out, applied power and it ran the squirrel cage for 10 minutes without overheating and with no scrapes, squeaks, moans or groans. I called the HVAC company and they said the motor may be pulling too much power when installed due to pulling air through ductwork. I can't spend unnecessary funds! I can get the motor replaced at the motor shop for $150, but is it something else??

Could have a dead spot in the motor or could also be a control problem. If you have an Amp probe you can check current draw and compare it to the rate plate on the motor if not you will have to roll the dice or call the service man back and get it fixed correctly.

I would check what voltage is actually getting to the blower motor when it's installed. You might have a contol or wiring problem that results in the motor getting less voltage than it needs to run.

As above, the power may not be making it through the furnace controls to the motor. Likely the relay on the control circuit board is bad. Unfortunately the cheap, little, failure prone relays are hard to diagnose and replace without a whole new circuit board. $$$$$$$$$ Flaky relays are even harder to diagnose.

One thing you could do is check for power in the wiring to the motor when it won't run. I would use my voltage detector for that.

To do simple checks like this you do need some tools. A test light, a meter, or a voltage detector might be the best place to start with. I came across the niftiest gadget for trouble shooting, a voltage detector. They work through the insulation of wires. There are several brands. I have a GB Instruments GVD-505A, less than $15 at Home Depot. Touch it to a hot wire, and the end glows red. Find the doodad that lights it on one side, and not the other, and you have the culprit. You do not have to open up housings and expose electrical contacts. You are looking at where your hand is, not where the meter is. Most people are capable of doing repairs and will get it going and not get hurt if they use a little sense. The voltage detector makes it even easier.