Worcester Bosch [A8] fault codewhat it means and how to fix it

A8 is the communication fault across every Worcester generation, and only the wording changes. The Greenstar CDi Classic table gives it as "break in communication to FX/Sense controls — check electrical connections"; the earlier CDi combi and system manuals say the same about the TD200 and RT10 controls of their day; the Highflow manual calls it "EMS communication error".

On the Greenstar i ErP the letters gain a cause code and a narrower meaning — A8 361, "HCM invalid: the inserted HCM does not correspond with the control box or HMI type", which is an internal mismatch rather than a wiring one. What all the bus versions share is the two-terminal EMS connection, marked B B on the boiler and on the control, over which a Worcester programmer or thermostat talks to the board. Break that pair, wire it to the wrong terminals, or fit a control the board does not recognise, and A8 appears.

Worcester's service booklet for the CDi Classic walks the engineer through it in order: continuity of the B wires between controller and boiler, then any branch box and bus module in between, then the programmer, then the bus module, and only then the control board. The boiler itself is usually not the problem. It keeps making heat, because the fault is in the instruction path rather than the burner, and the practical effect is that the room control has stopped talking to it.

1. 1

   Check the room controller. If its display is blank or it shows its own communication warning, the break is at that end — a wall plate that has come loose is worth pressing home.

2. 2

   Reset the boiler using the procedure for your range: three seconds on the reset button for a CDi Classic, a brief single press on a Greenstar i or Compact.

3. 3

   If the code clears and returns, leave the controller alone and book an engineer. Bus wiring runs at low voltage but it terminates inside the boiler casing, which is not for you to open.