Worcester Bosch CC 800 Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch CC 800 fault code mean?
The CC fault code (displayed as CC 800 on some Worcester Bosch models) indicates a problem with the optional outdoor temperature sensor. This sensor, when fitted, continuously monitors the outside air temperature so the boiler can adjust its flow temperature intelligently — a function known as weather compensation. When the boiler detects an open circuit or short circuit on the sensor circuit, or receives readings that fall outside expected parameters, it raises the CC code. Importantly, CC is a blocking fault rather than a full lockout: in most cases the boiler will continue to fire and provide heating and hot water, but weather compensation will be disabled, which means the boiler loses the ability to modulate efficiently based on outdoor conditions. Not every Worcester Bosch boiler will have an outdoor sensor fitted — if yours has never had one installed, this code can appear simply because the control board is configured to expect a sensor that isn't there.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Faulty or failed outdoor temperature sensor Common
The most frequent cause. The outdoor sensor is mounted on an external wall and is exposed to the elements year-round. Over time, moisture ingress, UV degradation, or simple component wear can cause the sensor element to drift out of range or fail entirely, producing an open or short circuit that the boiler's PCB detects as a CC fault.
- Damaged or corroded sensor wiring Common
The cable running from the outdoor sensor back through the wall to the boiler can develop faults — particularly at points where it passes through masonry or is exposed to condensation. A break in the cable (open circuit) or two conductors touching (short circuit) will produce exactly the same CC signal as a failed sensor itself.
- Outdoor sensor not installed but boiler configured to expect one Sometimes
If a previous engineer set the boiler's parameter menu to use an outdoor sensor, but the sensor was subsequently removed or was never fitted, the control board will permanently flag CC because it is looking for a signal that doesn't exist. Reconfiguring the parameter is a Gas Safe engineer task.
- Faulty PCB (control board) Rare
If the sensor and its wiring both test correctly but the CC fault persists, the PCB itself may be misreading the sensor input. PCB faults are less common but should be considered once the sensor circuit has been ruled out.
How to fix it
- Check whether an outdoor sensor is actually fitted to your property DIY safe
Look for a small, usually white or grey plastic sensor housing mounted on a north- or east-facing external wall, with a thin cable disappearing through the wall into the boiler. If you cannot locate one, it is likely the boiler's configuration needs adjusting — note this for the engineer.
- Reset the boiler DIY safe
Press and hold the reset button for approximately three seconds (on dial-controlled models, rotate the dial to the reset position and hold). A single reset is worth trying in case the fault was a momentary signal dropout. Do not reset more than two or three times — repeated resets without resolution will not fix an underlying sensor fault and may mask useful diagnostic information.
- Visually inspect the outdoor sensor and its cable (external only) DIY safe
Without opening the boiler or touching any internal components, take a look at the outdoor sensor casing and the visible run of cable outside. Check for obvious physical damage, the sensor housing cracked open, or the cable pinched or chewed. Do not attempt to disconnect or test the sensor yourself — note what you see and report it to the engineer.
- Call a Gas Safe registered engineer to diagnose the sensor circuit Gas Safe engineer
A qualified engineer will use a multimeter to measure the resistance of the outdoor sensor across its terminals and compare it against the manufacturer's temperature-resistance curve. They will also continuity-test the wiring back to the PCB terminal block to identify any open or short circuit in the cable run.
- Engineer to replace the outdoor temperature sensor if found to be defective Gas Safe engineer
The outdoor sensor itself is an external component and the swap is relatively straightforward, but the work involves accessing boiler wiring terminals and potentially adjusting control parameters — Gas Safe registration is required. Genuine Worcester Bosch sensors and quality compatible alternatives are both widely available.
- Engineer to inspect or replace the PCB if the sensor circuit tests healthy Gas Safe engineer
Should the sensor and wiring prove to be in good condition, the fault lies with how the PCB is interpreting the signal. PCB replacement is a more involved and costlier repair; the engineer may also check for firmware or parameter configuration issues before condemning the board.
Parts you may need
- Worcester Bosch outdoor temperature sensor · from £45
- Replacement sensor cable / extension lead · from £18
- Worcester Bosch PCB (control board) · from £270
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £100–£280, depending on the underlying cause.
Frequently asked questions
My boiler is still working — do I need to fix the CC fault urgently?
Because CC is a blocking fault rather than a lockout, your boiler will usually continue to heat your home and provide hot water. However, with weather compensation disabled the boiler can no longer fine-tune its flow temperature based on outdoor conditions, which typically increases gas consumption. It is also worth fixing promptly because a wiring fault left unattended can deteriorate further. We would recommend booking an engineer within a few days rather than treating it as an emergency, unless your heating stops working altogether.
How much does it cost to fix a Worcester Bosch CC fault?
For the most common repair — replacing a failed outdoor temperature sensor — most homeowners pay in the region of £100 to £200 all in, covering call-out, the part, and labour. If the wiring needs re-running the cost may edge toward the higher end of that range. If the PCB turns out to be the root cause, expect to pay roughly £250–£450 depending on the engineer's call-out rate and whether a genuine Worcester Bosch board is used. PCB replacement is much less common for this particular fault code.
What is the sub-code CC 800 and are there other CC sub-codes?
Worcester Bosch Greenstar 8000 series models typically display a two-part fault code on their screen — in this case CC followed by 800. The number 800 identifies the outdoor sensor circuit fault specifically. On older or simpler Greenstar models the display may show CC alone without a numeric suffix; the underlying fault meaning is the same. If your display shows a different number alongside CC, make a note of it for the engineer as it can help pinpoint whether the issue is an open circuit, a short circuit, or a configuration mismatch.
Can I use my Worcester Bosch boiler without an outdoor sensor fitted?
Yes — the outdoor sensor is an optional accessory that enables weather compensation, not a component the boiler needs to fire safely. If you decide you no longer want the sensor, a Gas Safe engineer can reconfigure the boiler's parameter settings to tell the control board no sensor is present, which will clear the CC fault permanently. Alternatively, replacing the faulty sensor restores full weather-compensation functionality and can noticeably reduce your gas bills over a heating season.