Buderus CY 241 Fault Code: Causes, Fixes & Repair Costs
What does the Buderus CY 241 fault code mean?
CY 241 is a Buderus EMS system fault that points to a short-circuit condition in the outdoor (ambient) temperature sensor circuit. On Buderus boilers running the Logamatic EMS control system, the 'CY' code family flags sensor-related faults reported by the RC35 or equivalent programming unit, while the sub-code '241' narrows this down specifically to a short circuit — either within the NTC thermistor itself or in the wiring that connects it to the controller. Because this is classified as a blocking (non-lockout) fault rather than a locking fault, the boiler will typically continue to operate in a reduced or emergency mode rather than shutting down completely. You may notice weather-compensated heating behaving oddly or defaulting to a fixed flow temperature, as the controller can no longer read the outdoor temperature accurately. This is not usually an emergency, and a standard weekday engineer visit is normally appropriate.
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
- Damaged or corroded outdoor sensor wiring Common
The cable running from the outdoor NTC thermistor to the boiler or RC35 controller is exposed to the elements year-round. UV degradation, frost, or physical damage can cause a short circuit in the cable itself, while corrosion at the terminal connections — particularly where the cable enters the boiler casing — is a very common trigger for this exact code.
- Moisture ingress into the sensor housing Common
The outdoor sensor's protective cover can be dislodged by strong winds, allowing rainwater inside. Once moisture reaches the sensor element or its internal contacts, it can create a short-circuit path that matches the 241 sub-code condition. Condensation inside the housing during temperature swings can have the same effect.
- Failed NTC thermistor (sensor end-of-life) Common
Outdoor temperature sensors are NTC thermistors that degrade over time. When the thermistor fails internally, its resistance drops out of the expected range and the EMS controller interprets this as a short circuit. Buderus outdoor sensors have been reported to fail within eight years, sometimes needing two replacements over the life of the boiler.
- Incorrect sensor placement causing inaccurate readings Sometimes
If the sensor is mounted where it receives direct sunlight or heat reflected from a garage wall or south-facing masonry, it can feed misleading temperature data to the controller. While this is more likely to cause erratic behaviour than a hard short-circuit code, a dirty or partially obstructed sensor can exacerbate the issue.
- RC35 or EMS control module fault Rare
In a small number of cases, the short-circuit condition is misreported because the RC35 controller itself has developed a fault — either a failed input channel or a loose internal connection. This is considerably less common than a straightforward sensor or wiring fault and should only be suspected once the sensor and its wiring have been confirmed as good.
How to fix it
- Check whether the fault code is flashing or steady on the display DIY safe
On Buderus EMS boilers, a flashing fault code indicates a locking fault that requires a reset once the cause is fixed, while a steady (permanent) code is a blocking fault where the boiler continues to run. CY 241 is typically a blocking fault shown steadily. Do not press the reset button repeatedly — doing so more than two or three times in quick succession can damage the ignition transformer and is not a substitute for finding the root cause.
- Visually inspect the outdoor sensor cable from the sensor head to where it enters the building DIY safe
Walk the full cable run of the outdoor NTC sensor. Look for visible cuts, kinks, or sections where the insulation has perished or been pinched. Check the terminal connections at both the sensor end and where the cable connects to the RC35 or boiler — corrosion or a loose spade connector here is a very common cause of CY 241. You do not need to open the boiler to do this check.
- Check the sensor housing for moisture or a missing cover DIY safe
Examine the outdoor sensor's plastic housing on the exterior wall. If the cover is missing, cracked, or has dirt packed around the sensor element, note this for the engineer. Removing obvious surface dirt from around the sensor face is safe to do yourself. Do not attempt to open the sensor housing or tamper with its internal wiring.
- Check that the sensor is not positioned in direct sunlight or near a heat-reflecting surface DIY safe
A north- or east-facing external wall away from heat sources is the correct location for a Buderus outdoor sensor. If yours is on a south-facing wall, above a tumble dryer vent, or close to a garage door that radiates heat, flag this to your engineer — relocation may be needed alongside the electrical repair.
- Arrange for a Gas Safe registered engineer to test the sensor resistance with a multimeter Gas Safe engineer
An engineer will disconnect the outdoor sensor and measure its resistance at the ambient temperature, comparing the reading against the NTC resistance-temperature chart in the Buderus service manual. A reading that is far below the expected value (indicating a short) confirms sensor failure. If the resistance is within tolerance, the fault lies in the wiring or at the controller end.
- Have the engineer replace the outdoor NTC thermistor if it has failed Gas Safe engineer
The outdoor sensor is a relatively inexpensive and straightforward part to replace. Buderus recommend using genuine OEM parts to protect the boiler warranty. The engineer will route and terminate the new cable correctly and verify the RC35 is reading a plausible outdoor temperature before clearing the fault.
- Have the engineer inspect and test the RC35 control unit if the sensor and wiring prove sound Gas Safe engineer
If sensor resistance and cable continuity are both good, the RC35 controller's sensor input channel may itself be faulty. The engineer can check internal connections, reseat any relevant jumpers, and — if the board is confirmed faulty — arrange a replacement. Only use genuine Buderus control modules to ensure EMS compatibility.
- Call a Gas Safe registered engineer if you have not already done so Gas Safe engineer
If the steps above have not resolved CY 241, or if you are not confident performing the visual checks yourself, book a Gas Safe registered engineer. Because this is typically a non-lockout fault, a standard weekday appointment is usually sufficient and avoids the higher out-of-hours call-out charge.
Parts you may need
- Buderus outdoor NTC temperature sensor (EMS compatible) · from £45
- Outdoor sensor cable/extension lead · from £18
- Buderus RC35 programming unit / Logamatic EMS controller · from £220
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–£250, depending on the underlying cause.
Frequently asked questions
Will my boiler keep working with a CY 241 fault code showing?
In most cases, yes. CY 241 is a blocking fault on the Buderus EMS system, meaning the boiler is designed to carry on running in a reduced mode rather than shutting down entirely. Weather compensation will no longer work correctly because the controller cannot read the outdoor temperature, so the boiler may default to a fixed flow temperature. Heating and hot water often continue, but you should still get the fault investigated promptly to avoid unnecessary wear on the system.
Can I replace the outdoor sensor myself to fix CY 241?
The outdoor sensor itself carries only low-voltage signal wiring, so physically handling it is lower risk than working on gas components. However, correctly diagnosing whether the fault lies in the sensor, the cable, or the RC35 controller requires a multimeter resistance test against the manufacturer's NTC chart — a task best left to a qualified engineer. If the cable run enters the boiler casing, any work at that end must be carried out by a Gas Safe registered engineer. Given that the sensor and labour together typically cost £100–£250, professional diagnosis is usually the most cost-effective route.
How much should I expect to pay to fix a Buderus CY 241 fault?
For a straightforward outdoor sensor replacement, most UK homeowners pay in the region of £100–£250, covering the engineer's call-out, labour, and the sensor part. The sensor itself typically costs around £30–£55 for a genuine Buderus part. If the RC35 programming unit or EMS controller turns out to be the root cause, the cost rises considerably — a replacement controller can be £200–£350 for the part alone before labour. Labour rates in London and the South East run notably higher than the UK average, so factor that in if you are in those areas.
Why does my Buderus outdoor sensor keep failing?
Outdoor NTC thermistors are exposed to harsh conditions all year round, so a service life of around eight to ten years is not unusual. The most common reasons for repeated failure are moisture getting into the sensor housing (especially if the cover has been cracked or blown off), a poor mounting location that leaves the sensor exposed to driving rain, or a cable route that traps water and corrodes the terminals. When having a replacement fitted, ask your engineer to confirm the sensor position meets Buderus guidelines — a north- or east-facing wall, away from heat sources and sheltered from direct rainfall where possible — to maximise the life of the new part.