Junkers Ceraclass error C6, fan or pressure switch?

Hello,
I know that the C6 error has been discussed many times in this forum. Unfortunately, I have not found a clear answer, how to determine whether the reason for the opening of the pressure switch circuit is itself or too low air pressure generated by the fan.
Of course, the unsealed boiler works properly, there were no changes to the chimney system. The fan is dusted, orifice OK, you can feel the blast coming from the fan. Inlet pipes.

The symptom is that after switching on the fan, it takes about 20 seconds, sometimes more before the pressure switch closes the circuit. Sometimes it doesn't happen at all. If the circuit is already closed, the flame goes out after a minute, it looks like the pressure switch has cut the circuit. Then there is another firing attempt and C6 appears.

Is it possible to somehow "track" whether the fan is blowing poorly or the pressure switch does not keep the circuit closed?
I will add that the pressure switch was replaced a year ago.


I would be grateful for your suggestions.

ostaszm wrote:

there were no changes to the chimney system.

Exactly. Please replace the seals. The flue gas enters the air duct and is sucked back into the boiler. This is checked with an exhaust gas analyzer.

Thanks for the suggestion.
But let me ask you.


I absolutely agree that if there were flue gases in the sucked air, the mixture of gas and air would be wrong and the boiler could turn off. It seems to me, however, that then it would not be C6, but EA or F7 telling about no flame. Then the reason for blocking the boiler would be the lack of flame and not the disconnection of the pressure switch circuit.

What negative impact on the closure of the circuit by the pressure switch would the unsealing between the air inlet and exhaust gas outlet ducts have? the pressure switch compares the pressure. It seems to me that such unsealing would cause a situation in which the air blown into the exhaust duct has an easier exit and at the same time the fan is to suck air through the intake duct easier. That should make it easier to move the diaphragm in the pressure switch. It would be a situation similar to unsealing the closed boiler chamber or disconnecting it from the chimney, which does not limit the operation of the boiler.

Of course, that's my guess. So thank you for your advice and I will take it into account.