[billybek]

Clean the flame sensor with sand cloth.
Check all the sta-con connectors from ignitor/sensor to board.
Check all connectors to/from the board.
Check voltage across the main gas valve when energized.
Check to see if the pressure switch is closing. Tap gently on this during the ignition cycle with the back of a screw driver.
Edit: Make sure the fitting for the hose connection to the pressure switch is not plugged. Pull the hose off and clean the opening off the ventor fan motor. Those products of combustion are wet and can plug the port.
Check for a good ground to the unit.



Sounds like it is not sensing a flame after ignition.
Flame sensor shouldn't be too much money and I bet the ignition board is not all that expensive.
As mentioned above there should be a flashing code led on the ignition board and usually a chart of codes on the burner section panel.
Trouble shooting at a distance is kind of like throwing parts at the problem but start with the easy stuff and go from there.
If polarity wasn't the problem, I would switch it back.
You have the installers guide and wiring diagram?