Some confusion in terminology use - the 'proof of fire' sensor is the thermo-couple on your Quad, the thing that sticks out into your burn pot, as that is what senses if the pellet feed has ignited, and that the fire has reached specific temps. When it's working normally, this should turn on a green light in your control box once the fire pot temp gets to 200 F, then the green light should turn to red once the fire pot reaches 600F temp, indicating normal operating temps have been reached.
If you have a flashing yellow / amber light in the control box, this indicates your thermo-couple is defective. In order for the thermo-couple to read correctly, it needs to be fully seated and touching the inside end of the ceramic cover that slides over it, so you should check that as a simple fix.
Do you get the green, then the red light in the control box ? Does it look like the blob of solder that connects the ends of the two twisted thermo-couple wires is cracked? If so, that is likely your problem. You can take the thermo-couple out and test it with a multi-meter, to see if it is generating enough milli-voltage necessary to accurately sense the fire pot temps, but if the yellow / amber light is flashing in the control box, that is definitive that your thermo-couple is defective.
Just to clarify - in the normal start-up sequence your yellow / amber light will flash once, but it won't continue to flash, unless the box is sensing that your thermo-couple has failed.
Trouble shooting a stove issue is often a process of elimination in figuring out what the problem is, or often a combination of problems occurring, that can cause your stove to shut down. AsBkins noted, this is often related to obstructed air flow due to fly ash build-up in the exhaust pathway, so that has to be ruled out as a cause as well.
But post back on what your control box lights are doing, and then we can continue from there as needed, OK?