Error CC 800

Safety precautions:

- Do not attempt to access or repair gas parts or internal boiler electronics unless you are a Gas Safe registered heating engineer. Working on gas or live electrical circuits is dangerous.

- Before doing any checks at the boiler, switch the boiler off at the mains isolation switch and, if possible, isolate the fused spur. Allow the boiler to power down.

- If you choose to use a multimeter, isolate power first and only measure on low-voltage sensor wiring. If unsure, stop and call a professional.

Initial checks a homeowner can do (no special tools required):

1) Check whether your boiler has an accessory outdoor sensor installed. CC 800 will only appear if an outdoor sensor is fitted and connected.

2) Try a simple reset: with the boiler powered, press and hold the reset or ✓ button for about 8 seconds until the display reboots. If the code clears, monitor for reoccurrence. A one-off clear can be transient; if it returns, continue checks.

3) Visually inspect the outdoor sensor and cable outside: look for physical damage, water ingress, paint/sealant over the sensor, or animal damage to the cable. Ensure the sensor is mounted correctly and not buried or obstructed.

4) Trace the cable route to the boiler and check the connector at the boiler’s sensor socket is fully seated and free of corrosion or debris. Some sensors use a small 2- or 3-pin connector; ensure it is plugged in firmly.

If you are comfortable with basic electrical checks and have a multimeter (power isolated):

5) Isolate mains power to the boiler. Unplug or disconnect the outdoor sensor from the boiler wiring block or connector.

6) With the multimeter set to measure resistance (ohms), check for an open circuit: you should see continuity (not infinite). If the meter reads open/infinite resistance, the cable or sensor is open-circuit and is faulty.

7) Measure the sensor resistance. Most boiler outdoor sensors are thermistors (NTC) so their resistance will change with temperature (resistance falls as temperature rises). The exact resistance vs temperature specification varies by part; consult the boiler manual or sensor datasheet if available. If the reading is extremely low (near 0 ohms) or extremely high/infinite, the sensor is faulty.

8) Perform a wiggle/warm test: with the sensor disconnected and power still off, gently move the cable and connector while watching resistance. Huge jumps or open circuits during movement indicate a broken cable or poor connection. Warming the sensor slightly (by hand or with warm air) should change the resistance slightly; no change suggests a failed thermistor.

If the fault is localized:

9) If you find a damaged external cable or a corroded connector in an accessible location, replacing that section of cable or reseating/cleaning the connector may fix the issue. Use the correct cable type and weatherproof junctioning if you attempt any external repairs; if unsure, stop and call an engineer.

10) If the sensor is confirmed faulty and you can access an identical replacement sensor and are competent fitting low-voltage outdoor sensors, you can swap the sensor for an OEM-approved replacement. Make sure the boiler is powered down while fitting and that connections are secure before powering up. Note: replacing parts inside the boiler casing or altering internal wiring should only be done by a qualified engineer.

If checks do not find the cause or code persists after replacing/examining the sensor:

11) Re-seat the sensor connector at the boiler and try resetting again. If the code remains, the issue may be at the boiler’s control board or wiring inside the casing — do not open the boiler; call a Gas Safe engineer.

12) If the outdoor sensor reads correctly but the boiler still reports CC 800, the fault may be a wiring short/earth or a control electronics fault that requires professional diagnostic equipment and replacement parts.

Temporary operation and consequences:

- If you temporarily remove the outdoor sensor to stop the error, the boiler may revert to a default control mode (no weather compensation). This is not recommended as a long-term solution because it can reduce comfort and increase fuel use.

When to call a professional:

- If you are not confident performing the visual and multimeter checks, stop and call a Gas Safe qualified heating engineer.

- Call a qualified engineer if the sensor or cable needs replacing, if the fault is intermittent after checks, or if the error persists after you have verified the sensor and connections.

- If the issue appears to be an internal wiring or control-board fault, do not attempt to repair; call a Worcester-approved service agent or your installer.

Final notes:

- CC 800 is usually fixable by replacing a faulty outdoor sensor or repairing its wiring. It is not typically an immediate safety hazard but it does affect system control and efficiency. Use a qualified Gas Safe engineer for replacement or in-boiler work and for final testing and commissioning.