[rvguy]

I think you’ll find it’s more likely to be an igniter issue (gap, bad unit, etc).  The flame sensor only keeps the burner burning after it’s started.  It does not prevent startup. 

Your original post stated “It worked fine prior to nozzle and after.”  Is that correct?  Meaning it was working correctly after you replaced to nozzle, but now it’s not (no ignition)?

Yes.  Had too much white smoke so I replaced nozzle.  Smoke went away aside from a little residual that was probably in the exhaust pipe that needed to burn off, but it was working fine.  Went on a trip a few weeks later and mid trip I noticed I had flipped on diesel one day as we still needed some A/C during the day and it would not fire.

Tried to be super careful when replacing nozzle to not move points and I did clean the photo eye.

They sell a break out box that you can buy for like 250.00. you just unhook your brain box on the side and plug it in, at that time you can run everything manually, 1 part at a time. Comes in very helpful sometimes.  
If you need a new so called brain box  . dont go thru newell, it costs 980.00
call frerichs electronics  308-440-5154  talk to jim.   totally rebuillt unit 500.00  with a warranty  if you send your old box back  you get 100.00 refund for the core.
he has the 12v box and the 24v box if needed