Worcester Bosch CC fault codewhat it means and how to fix it
The outdoor temperature sensor for weather compensation is not being read — open circuit, short circuit or disconnected. The boiler keeps heating but loses its weather-adjusted control.
Before you start
What CC means
CC is the outside sensor code on every Worcester Greenstar that can take one. The CDi Classic service booklet heads its procedure "outside temperature sensor fault (open or short circuit)" and the Greenstar i ErP table gives the pair CC 800, "outdoor sensor defect (available when accessory outdoor sensor is connected) — check sensor harness and connector; operation, replace if necessary". The Greenstar i also lists a maintenance version, H29, for the same sensor. The 2014 Greenstar 24/28/30 manual has "CC — outside temperature sensor not detected — check outside temperature sensor and connecting lead for breaks".
An outdoor sensor is an optional accessory: a small housing on a north-facing wall, wired back to two terminals on the boiler, that lets a Worcester weather-compensating control lower the flow temperature on mild days. If you have never had one fitted, CC should not appear, and if it does the board has been told to expect a sensor it cannot see.
Where one is fitted, the sensor is a thermistor like the others in the boiler, and the fault is nearly always the cable — chewed by something, chafed where it enters the house, or a terminal loosened at the boiler end. The CDi Classic booklet has the engineer confirm the wiring to terminals A and F, measure the sensor against the resistance table, check cable continuity, and only then look at the board. The boiler does not lock out for CC. It falls back to a fixed flow temperature and carries on, so the household effect is usually a heating system that feels slightly less well-judged than before rather than one that has stopped.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Damaged outdoor cable | The cable runs outside and is exposed to weather, gardening tools and animals. A break or a nick that lets water in is the most common finding. | Common |
| Loose terminal at the boiler | The two sensor wires go to low-voltage terminals on the boiler's connection strip, and one worked loose during other work is enough to lose the reading. | Common |
| Sensor failed | The thermistor in the outdoor housing drifts or fails with years of temperature swings. It is checked against Worcester's published resistance values. | Occasional |
| Control configured for a sensor that is not fitted | A weather-compensating controller or a boiler setting expecting an outdoor sensor, with no sensor connected, produces the same code. | Occasional |
What to try yourself
Find the outdoor sensor — a small plastic box, usually on a north or north-west wall away from windows and flues. Check the cable where it leaves the box and where it enters the house for visible damage.
If you have no outdoor sensor and never have, note that for the engineer: the boiler or its controller is expecting one and needs a setting changed rather than a part.
Reset the boiler for your range — three seconds on the button for a CDi Classic, a brief press on a Greenstar i or Compact. CC does not stop the heating, so this is a tidy-up rather than a rescue.
Get a Gas Safe registered engineer to look at your Worcester Bosch CC
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
What it costs to fix
Typical repair cost
£0–£200
Time on site
40 minutes
Free if it is a setting. Cable repair or re-termination £90–£140. Replacement outdoor sensor £120–£200 fitted.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Worcester Bosch outdoor temperature sensor
£30–£80Authorised Worcester parts distributor
Price checked 28 September 2026
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Worcester Bosch fault codes
- E9 220Get it checkedThe safety temperature sensor on the heat exchanger has failed electrically — 220 is a short circuit, 221 an open circuit. Nothing is overheating; the boiler has stopped because it can no longer prove that.Related: sensors and thermistors
- A5Usually simple to fixThe cylinder or heat bank temperature sensor has failed — open or short circuit. Heating carries on; the boiler just no longer knows how hot the stored water is, so hot water control suffers.Related: sensors and thermistors
- EA 227Stop — call an engineerThe boiler tried to light and never detected a flame, so it has shut the gas off and locked out. This is the most common Worcester fault code.Related: same brand
- CE 207Get it checkedSystem water pressure has dropped too low for the boiler to run safely. Topping the system back up to 1.2 bar usually clears it in five minutes.Related: same brand
- E9 219Stop — call an engineerThe boiler has overheated and the safety thermostat has tripped. Water is not carrying heat away from the exchanger, and it should not be run until that is fixed.Related: same brand
- C6 224Get it checkedThe fan is not reaching the speed the boiler asked for, so combustion cannot be confirmed as safe and the burner has been prevented from firing.Related: same brand
Own a Worcester Bosch boiler? Save it and its codes come first next time.
Related guides
Common questions about Worcester Bosch CC
Will I have heating with CC showing?
Yes. The boiler abandons weather compensation and runs at its set flow temperature. Worcester do not treat CC as a locking fault on any range.
What is H29 on a Greenstar i?
The maintenance-menu version of the same thing: outdoor sensor defect, filed under the H codes that warn without stopping the appliance. CC 800 is the fault-table entry.
Is it worth fixing?
If you paid for weather compensation, yes — it is the feature that saves gas by running the boiler cooler on mild days. A cable repair is a modest visit. If the sensor was fitted by a previous owner and you do not use a compensating control, an engineer can simply tell the boiler to stop expecting it.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.