is this a downfow unit?
Aux limit and draft trip will be a reset button in the middle of a switch like this
Possible code fault...but it sounds more like a pressure switch fault. have you checked pressure switch power in and out when unit starts? And have you checked the priXXXXX XXXXXmit on the front wall behind the gas valve?
First go here.... http://www.colemanac.com/PDFFiles/035-21118-001-D-1206.pdf
check all safeties based on location. Make sure we hit all of them. Then we can go from there
With meter///check for 24 in one side and then 24 out the other.
No meter......Remove 2 wires from limit and use a paper clip end to stink in the bottom of each wire and tape so no metal shows...really anyway to just get the metal wires to connect. This bypasses the limit (testing purposes only).
The pressure switch depends on if you 2 or 3 wires to it.
If 2 wires...remove wires and hold apart...start furnace....after exhaust fan kicks on touch the 2 wires together and hold them that way...see if fault clears.