Worcester Bosch C6 fault code: what it means and what to do

C6 on a Worcester Bosch boiler means fan speed too low. That is Worcester's own wording, straight out of the error code table in the Greenstar user instructions, and it is worth reading precisely. It does not say the fan has stopped. It says the fan is not turning as fast as the control board expected it to.

That distinction is the whole article. The board sets a target fan speed for the burn it is about to run, then reads the speed actually coming back. If the two do not match, it will not light, because on a room sealed condensing boiler the fan is what pushes the combustion products out through the flue. No proven fan means no proven vent path, and the boiler would rather leave you cold than fire a burner it cannot vent.

Source: Worcester Bosch, Greenstar CDi Classic user instructions 6 720 812 179

What causes it, in the order it usually turns out to be

Because C6 is a speed mismatch rather than a dead component, the fan itself is a long way from being the only answer.

• Resistance the fan is fighting. A partly blocked terminal, debris or a nest in the flue, a flue run that was always long or bendy for the appliance. The fan is healthy, the load on it is not, and the speed the board reads is wrong because of it.
• Wiring and connectors. The fan lives in a hot box that vibrates every time the burner runs. Plugs work loose and contacts corrode, and the speed signal back to the board is the first thing to drop out when they do.
• Water where it should not be. A weeping heat exchanger, a failed seal or condensate finding the wrong route can land water on the fan or its connections. This matters far more than the fan does, and we come back to it below.
• Worn bearings or a failing motor. The genuine fan failure, and it usually gives warning: a fan that drones, rattles or grinds for weeks before the boiler finally locks out.
• The control board reading a healthy fan wrongly. Least common, and only a fair conclusion after everything above has been ruled out.

What you can safely check yourself

Very little, and on this code that is not us being careful for the sake of it. Worcester head their entire error code table, C6 included, with the instruction not to try to rectify any of these faults yourself and to contact a competent, qualified Gas Safe registered engineer. Take them at their word. The fan sits inside the sealed combustion chamber behind the case, and the case is not a homeowner panel.

What you can usefully do is gather evidence.

• Reset once, using the method in your own manual. Worcester's reset procedure differs across the range: on a Greenstar i it is a brief single press, on a CDi Classic it is a three second hold. If it fires and stays running, note it and watch it.
• Listen while it tries. A healthy fan spins up audibly in the seconds after a reset, before ignition. Silence, or a grinding or droning noise, is worth telling the engineer.
• Look at the flue terminal from the ground. Debris, a nest, sheeting blown across it, something stacked underneath it. Note what you see. Do not poke it, do not go up a ladder.
• Look for water marks under and around the boiler, and note whether the lockouts happen in wind or driving rain. Both change what a good engineer looks for first.
• Write down the three digit number under the C6 if your boiler shows one. Worcester's own instructions ask you to quote the alert code and that number together when you call, and it narrows the fault considerably for the technical line.

Then stop, and in particular do not sit there resetting. A fan that cannot prove its speed does not improve for being asked ten more times, and each attempt is another cycle of a boiler trying to light with an unproven vent path.

Where it stops being your job

At the case, without exception. Worcester's own user instructions state that only a competent engineer may remove the appliance case and carry out work, in accordance with the Gas Safety (Installation and Use) Regulations. Anything involving the fan, the flue connections or the sealed chamber is engineer work, because getting it wrong puts combustion products in the room instead of outside it.

It helps to know what a competent visit looks like, so you can judge the one you get. The engineer should check the electrical supply to the fan and the speed signal coming back before condemning the part. They should check the flue for restriction, and check the flue length and number of bends against the limits in the boiler's own installation manual, because a marginal flue is a very common cause of a fan that reads slow. They should look specifically for water ingress. And once the boiler is running again they should analyse the flue gases, because a fan that has been running outside its expected speed may have left the combustion out of tolerance. A visit that fits a fan and leaves without a combustion check has done half the job.

Source: Worcester Bosch, Greenstar i ErP user instructions 6 720 806 946

Before you book, check where you stand on cover

Worcester publish their own call out conditions. A charge is made where the boiler is outside its guarantee period, where evidence cannot be provided that the first year service inspection was carried out, where the engineer finds no fault with the boiler, or where the cause of the breakdown lies with another part of the heating system rather than the appliance. They also state that no boiler fault is found on over 30% of all service calls.

That matters here more than on most codes, because a C6 caused by a restricted flue or a soaked connector is exactly the sort of fault that can end up classed as something other than an appliance fault. Find the Benchmark checklist in the back of the installation manual and see whether the services have been signed off before you make the call.

Source: Worcester Bosch, Greenstar CDi Classic user instructions 6 720 812 179

Repair or replace: what C6 is telling you

On its own a fan is one of the better repairs a boiler can need. It is a discrete, replaceable part on an appliance with an enormous UK installed base, so parts and engineer familiarity are never the problem. On a Worcester with years left in it, the sensible answer is nearly always to fit one and carry on.

The fork opens on three specific findings. First, if the fan is slow because water has been landing on it, the fan is the symptom and not the fault. Water inside the case usually means a heat exchanger, a seal or a condensate route at fault, and on an older Greenstar that is a different scale of work altogether, the kind that starts to overlap what a replacement costs once labour is counted. Second, if C6 keeps returning on a boiler whose flue was always long, bendy or exposed, the installation is talking rather than the appliance, and a new fan will read slow the same way in a year. Third, if this is the third or fourth part in two winters on a boiler out of guarantee, the parts have stopped being the question.

Age is the quiet factor. A lot of the Worcester stock around Northampton and Milton Keynes is CDi Classic and Greenstar i, fitted through the 2010s across whole streets of estate housing, much of it originally carrying a two year guarantee. Those boilers are long out of cover, they are entering the back half of the ten to fifteen years most boilers manage, and they are the ones where a fan is worth weighing against a planned replacement rather than fitting on reflex.

If it is time for a new one

If the engineer has found water where it should not be, or C6 is the latest in a run of lockouts, tell us about the job and we will put you in touch with three vetted local installers. A good replacement quote deals with the flue route and the condensate run as part of the work, not as an extra discovered on the day.