Tried to turn on my furnace this morning and got a code flashing on my Coleman Evcon unit. It is three flashing lights that indicate Combustion Air Switch failed to close.

Steady light on - normal operation.
One flash - ignition failure.
Two flashes - combustion air switch stuck closed.
Three flashes - combustion air switch failed to close.
Four flashes - limit switch open.
Five flashes - gas valve energized with no call for heat.
Rapid flashing - reversed polarity.

Note: Color of light is green.

There should be a diagnostic chart on the furnace door by the wiring schematic. If so see what the code reads for three blinks.
By the term "air combustion switch" I guess you are referring to the pressure switch. Most diagnostics blinking three times is just a dirty flame sensor. If this is the case the burners will light and go out within 4 seconds. If this is the problem carefully remove the flame sensor and clean gently with emery cloth, fine sandpaper or fine steel wool. Reinsert the sensor and enjoy your heat.

Have to know the model and serial of the furnace.
locate code list usually inside or on the back of a door.
Operation sequence= call for heat--inducer blower motor--makes pressure switch--signals board to bring on hsi--after a set time gas valve opens and you have ignition--main blower motor comes on after a set time

there should be a diagnostic code that will tell you what 3 blinks mean for that model. look on door inside cover. tell me EXACTLY what it does at start up. small draft motor comes on? you see a glow from hot surface ignighter? gas comes on? flame on? it should run in this order.

jump out pressure switch see if it lights check hose to pressure switch see if they are split or connections are not good ,cut off ends and put back on presure switch.

I have had the same problem with mine for the last 2 years. Remove the motor on the side of the furnace(usually held on by 3 screws) and turn the fan blades inside by hand. after sitting all summer it seems to get stuck. If it moves freely, replace the motor and try to start it again. This fixed mine the last 2 years! Hope this helped.

call a service guy

Small blower motor? Are you talking about the combustion blower if so then check your vents for blockage.. if the vents are blocked won't work... if it has a control board init it will give you a diagnostic code. Flashing light on the board... count the flashes and look up the fault on the chart in your owners manual ot on the furnace door..

Defective combustion air switch. The 3 blinks are an error code. You didn't say, but I'm guessing you have a Coleman Evcon. They had alot of defective Combustion air switches. Your specialists were not that.

You should be seeing 120 volts to the igniter. Anything less requires either a replacement of the transformer, start relay or control module.