Fridge Gas Flame Viewer inside the fridge.
It's possible if the gas assembly is not burning properly then the flame will not be seen here. Try a different viewing angle and it might just be seen as my case.
It is time to change the Gas Assembly, if viewing is impossible. As a temporary check for the gas flame, place a hand over the flue output vent. It should be warm.
The Gas Burner Flue jacket is low down, and on my fridge caused the viewing glass, inside the fridge, to obscure the flame. I have since cut a piece out of the jacket, opposite the fridge viewing glass. About the size of a 2pence piece. The difference is amazing with it now showing the flame for easy viewing.