Has any electrical work been done in the house lately? What is the incoming voltage?
A polarity error has nothing to do with the flame sensor or pressure switch.

 

No Electrical work done and I really doubt it is a voltage/polarity issue, it is just a what the sticker on the unit says for the double flashing red light The unit is about 20 years old but in pretty good shape, just trying to figure out if there is a simple fix.

Everything fires up, it just shuts off after 2-3 seconds. Did a little research and apparently the flame sensor works by conducting a small amount of current through the flame, so either there is an issue somewhere in the conduction path between the flame sensor and whatever is looking for that current or there is no voltage there...anyone have a similar problem?

 

It’s either going to be a voltage supply or board issue. The flame sensor isn’t going to make the unit fail on a polarity error.
Start by going through all high voltage connections between the breaker and furnace. This includes the collections inside the breaker panel. Deadly stuff lives in there so be careful. Don’t take that cover off if your not comfortable about it.
I’ve also found broken wires causing odd issues like this. If you find everything is tight you may have to use your meter to ring out all 3 conductors.

 

Thanks Roughneck, that helps a ton. I used to install HVAC many moons ago, so I am comfortable navigating the high voltage. What does "ring out all 3 conductors" mean?

 

With a meter, to make sure there are no breaks or open conductors.

 

Turns out all I needed to do was take a piece of emory cloth and clean the nozzle face so that the flame sensor could make a good electrical connection through the flame. Problem solved, thanks for the help.

 

Just to close this out, after much troubleshooting, I ended up replacing the 50A50-473 ignition control module with the White Rodgers 50A55-843 Ignition Control Module (needed to install a jumper wire for the Trane unit). Problem solved, for whatever reason, the control module was not reading the current through the flame sensor and flame. $91 on Amazon, which was a pretty good deal.