Gas and Oil Home Heating Furnaces - Gas Valve Replacement

I have a Weather King gas furnace (serial # wgpah07ever) that is about 8 years old and in a rental unit. When the latest tennant moved in the gas company "red tagged" the hot water tank saying flue gas wasn't venting properly from the hot water tank and the furncace was also not working but gave no reasoning. I called a local technician and he demonstrated that the water tank was in fact venting properly, but said my furnace needed a new gas valve. I was a little suspect since i had the valve replaced less than 3 years ago and because the price they quoted for the valve itself was $120 more than i found from a reputable local distributor. I cleaned the unit and checked the wiring and to my suprise it cycled up and was working without issue for a night. Now, however the unit is not working properly and only seems to be working intermitently. The fan turns on the hot surface ignitor gets glows, but the blower doesn't kick on, there is no ignition and then the unit shuts down. Is this in fact a gas valve problem or something different? How can i determine if the valve is the source of the problem? What is the best way to go about replacing the valve if it is in fact the problem?
Thanks for any help/advice.

As for the water heater issue. can be a couple of things.

-Was the furnace and water heater running at the same time?

Reason could be for that is that the chimney may be too small, and is not able to handle both load, so may be getting a back draft?

-Was it windy?

If the chimney isn't at the right height at the roof, the wind may be driven down it, and pushing the flue back into the basement.

-Was all the exhaust fans, and dryer running?

The house may be lack of make up air, when it is lack of fresh air, the house is starving for fresh air, and it's finding it'a way down the chimney.

As for the furnace, place the meter on the gas valve to see if you are getting power to it. If you are not getting power to the gas valve, then see if the safety switches are closing.

Thanks Jay,
Sorry, I should have been clearer the water heater problem was actaully resolved, but that situation is what led me to discover the furnace issue. My main problem seems to be the gas valve. I know how to use a meter to check and see if the valve is getting power, but how do I check to see if the safety switches are closing? could it simply be a bad heating element? because the heater is blowing some cold air before shutting down?

Thanks again.

The board won't know if the flame is on or not.. The blower is going off a timer.

Most safety (Pressure switch, flame roll..ect) out are in series, so just follow the wire from the gas valve.