[Devilish]

When a FBH receives a start up command, either by 12v to pin 1 or earth to pin 3, basically this is the FBH response.

Start the air fan for full speed check, and short combustion chamber purge, during this period the FBH will.

Start the water pump.
Check flame detector resistance.
Power the glow plug.

Then
Slow the air fan to idle.
Start the dose pump.
Check for combustion chamber ignition
Spin the air fan up to full speed.

If the FBH detects any fault during the start up procedure, the water pump will run, and the air fan will remain at full speed for a pre set time, then the FBH will shut down.

Common faults are the glow plug failing the flame detector resistance check, or PCB common component failure.

12v to the glow plug for a 'if it glows it works' test means nothing. Post ignition the glow plug becomes a flame sensor, if the glow plug resistance is too high matters not if it glows nicely.