Hopefully your computer can't read and yours is an 8520. What is series II,III or IV?
Inspect the yellow wire between the circuit board and the flame sensor. Look for any nicks that would allow moisture or contact to ground.
Make sure the flame sensor is clean.
Manual says to inspect the exhaust and clean as #2 item. First item is to insure 11" column of LP supply
Have you cleaned the burner head? It is on right side in horizontal mount. Shut off LP, remove the gas valve ignitor assembly. The mesh burner assembly is connected to it. Use vacuum to clean out fire box. You can also use compressed air to knock debris loose and then vacuum again.
Adjust ignitor and flame sensor to 1/8th inch
