"what do you mean testing the NTC seperately?"
Checking a electronic component separately means measuring it desoldered from PCB, isolated
of its circuit, of any other component. That is a regular procedure to positively find its state.
Measuring a component in-circuit is considered prevaluation of its state, it may or may not
reveal its condition, depending of its type and its surroundings in the circuit. Decision to
order spares is made only after that. Only in rare situations operational measurements gives
stronger indication in respect to separately measured component value.
Temperature sensor NTC should affect heaters temperature control only, elecric (which you said
didn't shut down) or gas. It should not have anything to do with gas ignition failure, that is
another problem. Now, it seems to me, we are still dealing with (at least) two faults. Confusion
allways lurks in these cases.
Some points of interest....
1. What resistance did the original NTC show off-circuit?
2. In gas starting sequence, were you able to detect gas escape from supply lines (ie. pressure
drop by valve(s) opening)?
BTW. As you replaced the potentiometer, you expectedly have it in mid-position? At least you
should start by that before further adjustment.
These resistors have no polarity.