CAD cell mystery

I have an old Vailant Boiler that I've been running and maintaining for the twenty years I've been in the house. It stopped in the spring and I've since replaced just about all components; pump, electrodes, primary control, CAD cell, transformer, nozzle. It runs for 45 seconds and trips. Everything inside the furnace has been cleaned and the gun is also clean. Resistance of the CAD cell (disconnected from the control unit) when running is 5,000 ohms, obviously too high. When I connect a resistor in parallel (3000 Ohms) it all runs perfectly, and when I block the CAD cell with tape, it times out after 45 second as it should. Obviously, the CAD cell is not receiving sufficient light in order to reach the required resistance, and the parallel resistance allows the burner to start (resistance is high enough) and keep running (resistance is low enough). The question I have is - what changed? Why is the CAD cell no longer receiving sufficient light? There is a round, ceramic cup-shaped vessel sitting on a brick inside the furnace, at the back, on it's side, as if to the "catch" the flame, and has become degraded over the years. I thought it was to prevent the flame from being focused on and heating the back of the furnace. Could this vessel be there to increase the intensity of the light inside the furnace when it is running?

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