I have a Trane XL80 furnace, the past couple weeks I have had blower running but no heat. System shows 2 flashes: System Lockout (No flame) check line polarity.
If I shut off switch (by fuse) then turn back on, it will ignite (but might take a couple ignites, if after 5 X's and burners do not stay lit, it will lockout again. The burners will light for 3 - 10 seconds then go out. Any suggestions on cause?

If you have a flame sensor clean it or replace.

I agree start with cleaning the flame sensor. If all the burners are not lighting or slow to light it will give you the same code also. The only time you need to replace the flame sensor is if you broke the metal rod off or if the ceramic is broke or cracked on it.

If you have a flame sensor clean it or replace.
I removed sensor, cleaned with scotchbrite pad, replaced. 1st ignition stayed on.
So far, so good. Thanks!

Glad you got it fixed.