Viessmann gas valves often have a plastic "restrictor" plug installed in the OUTLET of the gas valve.

When replacing a gas valve, the replacement gas valve will ship with replacement instructions, the valve (With the outlet containing a solid plug), and gaskets.

The directions do describe removal of the shipping plug AND removal of the existing restrictor plug from the OLD VALVE, then installing into the NEW gas valve.

Problems that might be observed IF the part is not installed will be flame failure after ignition.

Usually, the burner will light, but as the combustion fan starts to change speeds, the O2 (Measured in the combustion gasses) will start to drop (Down to 1.8 to 2.2%), as the flame smothers out.

Turning the gas selection shutter (Propane conversion position) results in functional operation of the boiler. This is what one customer discovered while trying to resolve problems with their system.

If the gas supply is correct, the venting and condensate match the installation manual, and the gas valve has been replaced at some point, it is well worth checking for this part.