Trane 80xl, 3 flashes error light - draft pressure problem, help?

Xsta

I have a Trane 80XL and the light on it is giving the '3 flash' error indicating a draft pressure problem. Any help with troubleshooting?
So far I turned off the power to the furnace, reset the gas ignitor switch, turned the power back on, turned the heat on at the thermostat and then I get the 3 flashes. The blower motor won't seem to kick in automatically. I can turn it on and it runs fine but it won't kick in on it's own. I checked the tubing to the fan (recommended in an earlier post on this site) - it's clean and clear. the furnance is only from 2002. Vents are clear, filters are clear... If I turn off the power to the furnance and turn the blower motor switch off, the light will flash normal (but the heat won't kick in) until I flip the blower motor swtich to on, then it reverts to the 3 flashing mode.
any suggestions?

Thanks!

redline

Does the blower motor have a seperate switch?