Gas furnace fan is blowing cold air and won't shut off except by the switch in the attic next to the furnace. Even after turning it off for an hour or more, once I turn the switch back on the fan immediately begins blowing. The auto igniter never attempts to light.

Background - a couple of days ago had the same scenario except when I turned the switch in the attic off, waited a few minutes and turned it back on, the igniter lit, even with the fan blowing, and all seemed to work properly. Not so lucky now. If I'm not mistaken, the fan shouldn't have begun blowing until ignition/fire??? Maybe something was already wrong at that time.

ICP Model: EDM2X60LAT
Thermostat: Honeywell ST9120 G 4004

 

While I'm not sure of the style of your furnace, I do have a few thoughts.
You say the fan comes on first - which fan are you speaking of - the main blower or the inducer fan which is in the combustion area of the furnace.

If it is the inducer fan running, then it is not pulling in the pressure switch. That would be at the end of a small hose which runs from the inducer fan housing. Check for restrictions in the hose and the nipple on the inducer where the hose attaches. The pressure switch needs to operate before the igniter will come on.

 

Your furnace should have a control board with an LED on it. That LED will flash a trouble code.

Many furnaces will default to "blower on" if one of the safety stats trip. You may have a high heat limit problem due to poor/restricted airflow. Have you checked/replaced the filter recently ?

 

Checked the air filters. Both are clean and fairly new.

I cannot find the error lights and have searched on-and-off for two days. The unit is horizontally mounted laying on its left side and backed against a wall. If there is an error light window on the back or left side I would not be able to see it.

Both fans come on at the same time, instantly when I flip the switch in the attic.

At some point in my frustrations, I disconnected both wires from a switch (the one that reads "L150-20F") just to see what would happen. Both fans continued to run. Also, I checked that same switch with an ohm meter when I had it totally disconnected from the system. The ohm meter made a continuous tone (indicating a closed circuit). I shook and thumped the switch a few times and got the same ohm results. Would this confirm "blower on" default, bad switch, or is it meaningless?

Update but still concerned:
It's been at least 4 or 5 hours since the unit properly operated. When I turned it on a few minutes ago so I could answer your questions it actually turned on properly (of course). At this point I don't want to shut it off at least until the house warms a bit.

Some observations when it turned on properly:
Small fan (inducer?) turned on, then the ignitor, then the flame lit, the ignitor stopped glowing, then the big fan came on. All of this within a few seconds. Heat is blowing throughout the house and both fans are still running. I ran a volt meter across the C, G, Y, W, and R screws. Found 24-ish volts for both the W & R screws. Zero volts for C, G, and Y.

When the complete system was not running properly and just the fans were running, I consistently got zero volts across all five screws.

 

International Comfort Products

I've been trying to find info on your furnace. I think the model number you posted above is just the A/C coil part. The model number for the furnace will be inside, behind the burner access cover, on one of the sides near the burner.

 

I see you've done this before.

NTC5125BKE2

Another update. The thermostat was just satisfied and the furnace appears to have shut down properly (i.e., no fans are currently blowing).