Worcester Bosch [C6 224] fault codewhat it means and how to fix it

Start with what Worcester actually publish, because the two halves of this code belong to different faults. In Worcester's own Greenstar i fault table the C6 alert code appears twice, as C6 215 "fan speed too high" and C6 216 "fan speed too low", and the remedies for both are the fan harness and connector, fan operation, and the connections of the control box. The number 224 is published too, but not with C6: Worcester pair it with the temperature alert code, as E9 224 "flue or high limit thermostat activated (safety circuit)", and the Greenstar 8000 Life table gives 224 as "maximal flue gas thermostat or heat exchanger maximal thermostat has activated".

So if what you are reading on the display is C6, you have a fan fault and the rest of this page applies. If what you are reading is 224, you are looking at the overheat safety circuit and the fan is not your problem. Worcester's user guide explains why this confusion is so easy: the alert code and the three-digit number are two separate fields shown one under the other, and people quote whichever they noticed.

With that established, the fan fault itself: the fan on a condensing boiler both supplies combustion air and clears flue gases, and the control board monitors its actual speed against the commanded speed on every cycle. C6 means the two do not agree. The boiler will not fire when it cannot verify that flue products are being removed, which is exactly the behaviour you want. The usual physical cause is a fan reaching the end of its service life, and it normally warns you first with weeks of rattling or droning on startup. The alternative cause — a restricted flue or air intake — produces an identical reading and carries a genuine combustion risk, which is why the flue is always checked as part of diagnosing this code.

1. 1

   Look at the flue terminal outside. Check for nests, leaves, or anything built near it since the boiler last worked — a new fence panel or shed roof is enough.

   Look only. Never push anything into a flue terminal; you risk driving a clearable obstruction deeper into the flue.
2. 2

   Listen at startup. Rattling, droning or grinding before the lockout means worn bearings, and telling the engineer that saves them diagnostic time.

3. 3

   Reset once. If it returns within a cycle or two, leave it and book someone.