The way you have it wired is so wrong!!
You are keeping power on the gas valve via 12V thru t-stat contacts and not thru circuit board.
If main flame where to go out the gas valve would stay open due to direct power from t-stat, until you turned t-stat off.
The fault light is because the circuit thru sail switch/high limit switch is dead......but with direct power to gas valve furnace continues to run.
The problem with main flame going out after 5 seconds when the wiring was correct is because main flame is not being proved back to circuit board which then shuts off power to gas valve so it doesn't stay open without a proven flame.
Flame proving is done via the spark electrode.
*Main flame has to engulf electrode.....heats electrode and induces a milivolt signal back to circuit board proving main flame
*Electrode has to be clean
*Electrode has to be properly positioned...should be fixed when installed but should have a 1/8" gap between electrode and ground rod. The ceramic should not have any cracks.
*The wire between electrode and board could be grounded or bad
*The exhaust could be blocked causing unbalanced air pressure
*Gas pressure could be low causing improper flame height/temp (should be 11" WC with at least 50% of other propane appliances in service)
*Connector on circuit board not making proper contact
Here is a service manual for Hydro-Flame Furnaces........pg. 10 shows wiring for 8500 Series.
LINK
