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Weil McLain A 41 code

Wmb80/120/155 boiler keeps throwing an A41 code. I have cleaned the mounting surfaces, replaced both sensors. Still getting the error after it sits for an extended period of hours.  Unit is 3 years old,worked fine until this season. I did change out the near boiler piping to 1" as it was originally plumbed 3/4". I also added an 80 gallon water tank as storage.the storage tank is on the return side.
  This unit is a closed system that heats a flat plate heat exchanger. The piping and tank are all that I have changed.So not sure if it is related or not. Thanks in advance.


  • 109A_5
    109A_5 Member Posts: 1,341
    Hello @Ericjeeper1965,

    I would re-seat the sensor connectors at the controller board. I would verify the sensor resistance all the way back to the controller board. Sensor resistance at a given temperature is in the manual. Also the proper position of the sensors in the boiler.

    With the additional load of the 80 Gallon tank a performance curve may have to be changed or relearned.

    It seems odd to me the error pops up when the unit is idle for a while. Maybe the lower in-cabinet temperature is influencing a connection somewhere.

    National - U.S. Gas Boiler 45+ Years Old
    Steam 300 SQ. FT. - EDR 347
    One Pipe System
  • Ericjeeper1965
    Ericjeeper1965 Member Posts: 2
    Thanks,I crimped the connections down a bit. We will see if that changes anything.