Janitol furnace dilema

I am working on a 1993 Janitriol model number GMPN100-4. Actually two of them in the same residence. Big ranch house with on on each side.
I watch the Ignitor turn on, the main valve opens, it ignites and stays lit for about 5 seconds, then the flame goes out. It waits about 30 seconds then does the same thing again.
It stops after three tries and then I get a single blink from the control board.
At least I think it is a single blink. It blinks on, waits 1/2 a second then blinks once again, repeat.....
I replaced the flame sensor and it still does the same thing.
Reading the sticker in the furnce, a single blink means Gas flow, Gas pressure, Gas valve, or Flame sensor.
Sure seems like it is the flame sensor to me, but I have swapped it out once.
It wouldnt flame on if it was the Gas flow, Gas pressure or Gas valve would it?
Any Ideas?
Thanks in advance..
Rob W
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Bad thermostat.
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Noon-Air wrote:

Its propane. All of the pressures checked fine. So I swapped out control boards, and that fixed it.
The control board wasnt picking up the flame sensor.
Thanks to everone who made helpful comments.
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