Found system boiler supply temperature sensor 35°F off the actual temperature of the boiler causing boiler to run long cycles. Replaced temperature sensor with customer provided universal replacement sensor and boiler immediately shut off on temperature. There is potential that on the coldest nights and all the radiators getting up to temperature that the expansion tank may be too small but it is currently functional and in good shape. Did not note any other issues on the boiler at this time. Recommend watching unit over the next couple of days to check that pop off has stopped leaking water.