York Gas Furnace Model # P2MPD14N08001A - Combustion Air

Do not run contentiously during heat mode. the fan runs at a slower speed with heat and heat fires first then the fan cycles on during heat cycle then when heat satisfies the thermostat the heat will cycle off and fan will run for a couple more minutes and then cycle off. So set stat on fan cycle to auto.

I'm surprised the building inspector allowed the condensing furnace you described draw combustion air from the garage !! I'm not familiar with any manufacturer that recommends that as normal practice. Virtuall ALL two pipe condensing furnaces require both pvc pipes to be terminated OUTSIDE the structure with specific heights and distances apart. There is also a limit to the length of both the flu and the combustion air piping and each 90 degree pvc elbow adds the resistance equivalant of roughly 10' of straight pipe. If re-routing is required, you have to bear in mind the direction of 'fall' in the piping usually goes back towards the furnace and remember two 45's may give you a greater variance of change in the piping and still be equal to the equivalant of one 90.

Personally, I would never suggest terminating the fresh air (combustion air) intake between floors of a structure.

the furnace should have never been installed without combustion air in the first place, is it in the attic, basement, closet? The amount of combustion air depends on the btu's. you can run ,metal ducting to the space. propane is a heavy gas and needs to be installed carefully so gas cannot puddle and fill up a specific application if not installed properly.

Check condensate water /drain stopped up. It will allow a start after a reset but then it shuts the combustion air fan down and that shuts the unit down this will require a service tech make sure their familiar
with 95% units

no you are pretty well done on this , you could check the air fiters are clean

it is down to cost how old is it , is it worth it over the cost of a new heater you will neede to speak with a plumber to see what is best to do with this heater as they can look at the heater and advice you .

Check if they have a "sail switch" that senses combustion air flow before allowing fire. These are a small vane in the combustion air intake that moves a microswitch when air is flowing.

Sounds like a problem with the circuit board.

about 18 inches

I am assuming that this is a gas dryer not an electric dryer.
Per the 2009 Uniform Mechanical Code Section 902.0
(B) Gas utilization appliances shall not be installed so its combustion, ventilation, and dilution air are obtained only from a bedroom or bathroom unless the bedroom or bathroom has the required volume in accordance with Section 701.2. [NFPA 54:10.1.2]
Section 701.2 basically says you need to provide adequate combustion air. One way is if the volume of the space is 50 cubic feet per 1000 btu. I personally think it is a bad idea to obtain combustion air from a bedroom even if it has adequate volume; it used to be prohibited by the Uniform Mechanical Code. The combustion air requirements are based on the BTUs of the appliance not the CFM. For gas pipe sizing purposes the Uniform Plumbing Code says that clothes dryers have a BTU input rating of approx. 35,000 BTUs. One 4" duct to the attic with the opening of the duct inside the closet within 12 inches of the ceiling would provide enough combustion air per the "One Permanent Opening Method" in UMC chapter 7.

Make up air is a different requirement than combustion air. Chapter 5 of the UMC states that if you have a dryer in a closet you need to provide a make-up air opening of 100 sq inches. The make -up air is to replace the air that is being exhausted not to provide air for combustion. The make -up air could be taken form the exterior of the building or another room within the building. Technically the closet should be insulated and weatherstripped if you get the make-up air and combustion air form the exterior.

Sorry doc john but the fire inspector is not talking about the exhaust duct (environmental air duct). That is required no matter where you have installed the dryer and is a totally separate