I have a Coleman Evcon gas furnace in my garage, and when the temperature outside gets extremely cold (like 14 deg right now) the furnace will lockout and not re-light. The garage door is pretty shot, so this furnace has been exposed to the elements for a number of years... I have had a professional check it out... I believe he cleaned the flame sensor, but now the only option he's giving me is to replace the entire unit.

Would trying to raise the temp in the garage help? Is it too cold for the flame sensor to work properly?

Any other suggestions?

I can usually get the unit to light by turning the power on/off after the lockout, and after 2 or 3 attempts it usually lights.

Is your flame sensor a thermal couple? Thermo couples look like a slender cooper tube with a 1 inch long aluminium tube on the end, slightly larger. The aluminimum tube sticks into the pilot light to sense the flame. If this is what you have extreme cold can make it malfunction. Just light it two or three times and it should build up enough heat to open the gas main.

Most residential gas valves are not rated to operate in that low of temperature. Sometimes the temp range will be posted on the valve. If not look up the valva at the manufactures site and see if the low temp is causing the problems with the valve.