There will be a sensor that monitors the temperature of the water and turns the boiler off as a safety measure if it gets too hot - probably called an overtemp stat.
The sensor may be operating incorrectly.
The control board may be seeing the correct signal from the overtemp stat but misinterpreting it as an overtemp.
The sensor and the control board may be working correctly but the circulating pump may not be moving the water away from the heat exchanger quickly enough or at all.
The pump overrun control may be defective - circulating pumps are powered for a short time after the main burner shuts down to remove residual heat from the heat exchanger - this may not be happening and the water in the heat exchanger is becoming too hot.
Peccavi, March 2009
