Worcester Bosch [D4 341] fault codewhat it means and how to fix it

The Greenstar i ErP table lists D4 341 under "Temperature fault" as "primary flow temperatures rising too fast", with the engineer directed to check the primary system water pressure, air in the system, circulation within the heating system, and the wiring and operation of the pump and the respective sensors. It is a gradient fault rather than a limit fault.

E9 219 and E9 224 fire when a temperature is too high; D4 fires when the temperature is rising at a rate that a properly circulating boiler could not produce, because the burner is heating a small, nearly static volume of water. It is therefore usually the earliest warning of a circulation problem, appearing before the overheat codes do. The Compact family use the same letters for a related measurement: D4 271, "temperature difference between flow and safety sensors exceeds limit — the temperature of the primary flow and the safety sensor should be within 15 °C of each other", a blocking code rather than a lockout.

Two sensors a few centimetres apart disagreeing by that much again means water is not moving between them. Both tables also carry a cause code with no letters, 212, "safety or flow temperature rises too fast — air lock or reduced water content", which is the blocking stage of the same condition. Whichever version your display shows, the boiler is not broken in the burner or the electronics. It is short of moving water, and the reasons for that are nearly always pressure, air or the pump.

1. 1

   Read the pressure gauge cold. If it is below 1 bar, top the system up to about 1.5 bar with the filling loop. Low pressure is the first item on Worcester's own check list for this code.

2. 2

   Bleed any radiator that is cold at the top, then check the pressure again and top up if it has fallen.

3. 3

   Reset the boiler — a brief press of the reset button on a Greenstar i or Compact — and listen for the pump. If the code returns quickly with the pressure correct and no air, the pump or a sensor needs an engineer.

   Do not keep resetting a boiler that overheats within a minute of firing. Each cycle stresses the heat exchanger, which is the most expensive part in it.