Juncker gas boiler shuts off after ignition

Hello, The gas boiler switches off briefly after ignition. Then the temperature rises to 95, then it drops to 65..after that it can no longer be ignited. The safety valve was replaced and water supplied. Earlier I heard a bang/rattle in the boiler. Does anyone know the problem?

Does the device not display an error code?

Does the device not show an error code?

At the beginning EA, I noticed that the gas valve of both Juncker gas boilers was closed.
I opened it and then it ignited. I heard a short bang after I turned one radiator to 5. Since then it goes out shortly after ignition.

EA is to be expected when the gas valve is turned off. Strange that no further error is displayed. But there is enough water?

EA is to be expected when the gas valve is turned off. Strange that no further error is displayed. But there is enough water in it?

Yes, there is enough water in it. As I said, it only shows initial values like p1 to p5, then 95, 65; I assume these are temperature values.

Does it get a flame then? I may only be spreading half-knowledge, but at least I can share my experience with our gas condensing boiler from Vaillant. In recent months, I have frequently experienced misfires up to complete failure. In the end, it turned out that the gas valve, which regulates the mixture, was shot. On top of that, the lambda sensor was slightly fouled and therefore could no longer determine the correct mixture. No idea if that helps you, but maybe search in this area?