Viessmann CF Fault Code: Causes, Fixes & Repair Costs
What does the Viessmann CF fault code mean?
The CF code indicates that the boiler's control system has detected a problem with the outdoor weather compensation sensor — either a short-circuit, an open-circuit, or a broken cable. Rather than shutting down entirely, most affected models switch into a fallback 'Constant Mode', operating as though the outdoor temperature is fixed at a preset value. This protects your heating supply in the short term but reduces efficiency and comfort, as the boiler can no longer automatically adjust its output based on real outside conditions. On the Vitodens 100-W, the code can also be associated with DIP switch S2 being set incorrectly to 'ON'. On Vitotronic 050 and 200-H controllers, the fault typically activates when the sensor cable is detected as broken, and the system defaults to 0°C as its reference point.
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
- Short-circuit inside the outdoor temperature sensor Common
The NTC thermistor inside the external sensor (typically rated at 10 kΩ) can develop an internal short due to moisture ingress, frost damage, or age-related corrosion. This sends a false zero-resistance signal to the control unit and is the most directly cited trigger for the CF code.
- Broken or damaged sensor cable Common
The two-core cable running from the outdoor sensor box to the boiler is exposed to UV, frost, and weathering over many years. Physical damage — whether from gnawing rodents, a cable clip driven through the sheath, or simple weathering — can break the circuit and cause the controller to log a CF fault.
- Loose or corroded wiring connection Sometimes
Plug-in terminals at the sensor end or at the control board connection can work loose over time or develop surface corrosion, creating intermittent resistance that the control unit interprets as a sensor fault. This is worth checking before assuming the sensor itself has failed.
- Incorrectly positioned or physically damaged sensor Sometimes
Viessmann specifies that the outdoor sensor should be mounted on a north or north-west facing wall, between 2 m and 2.5 m above ground level, away from windows, doors, air vents, and overhanging gutters. Poor positioning or physical weathering damage can produce erroneous temperature readings that confuse the controller.
- DIP switch S2 misconfiguration (Vitodens 100-W) Rare
On Vitodens 100-W models specifically, the CF fault mode can be triggered or worsened when internal DIP switch S2 is set to the 'ON' position when it should not be. This is an internal control board setting and requires an engineer to investigate and correct.
How to fix it
- Inspect the outdoor sensor unit and its cable from outside DIY safe
Locate the small plastic sensor housing mounted on an external wall. Look for cracking, discolouration, water staining, or physical damage to the casing. Trace the cable from the sensor back towards where it enters the building and check for any visible cuts, kinks, or sections where the outer sheath is worn through.
- Check the sensor cable connection at the boiler DIY safe
Without opening the boiler casing or touching any gas or electrical components, check that the sensor cable's plug or terminal block where it connects to the boiler is firmly seated. If you can see visible corrosion or the connector appears loose, note this to report to the engineer — do not attempt to rewire it yourself.
- Attempt a boiler reset DIY safe
On most Viessmann models, you can reset the boiler by briefly turning the heating dial to the reset symbol and back, or by holding the 'Mode' and 'OK' buttons simultaneously for a few seconds (refer to your model's user guide for the exact method). If the CF code clears and does not return within a day or two, the fault may have been a temporary glitch. Do not reset more than two or three times — repeated resets without a fix can mask a genuine fault.
- Engineer to test outdoor sensor resistance Gas Safe engineer
A Gas Safe or Viessmann-trained engineer will use a multimeter to measure the NTC thermistor's resistance and compare the reading against Viessmann's published temperature-resistance curve. A reading significantly outside the expected range confirms sensor failure and justifies replacement.
- Engineer to inspect and repair sensor wiring Gas Safe engineer
If sensor resistance appears acceptable, the engineer will test continuity along the full cable run and check terminal connections at both ends for corrosion or poor contact. Wiring repairs or terminal replacement will be carried out as needed.
- Engineer to replace the outdoor temperature sensor if faulty Gas Safe engineer
If the NTC sensor has failed, the engineer will source and fit a compatible Viessmann outdoor temperature sensor, ensuring it is mounted correctly on a north or north-west facing wall at the appropriate height and away from any heat sources or vents.
- Engineer to check DIP switch settings and control board (Vitodens 100-W) Gas Safe engineer
On Vitodens 100-W models, the engineer will verify internal DIP switch S2 configuration. If both the sensor and wiring test as healthy, a PCB fault may be suspected and the control board will require professional diagnosis.
- Call a Gas Safe registered engineer if you cannot resolve the fault yourself Gas Safe engineer
If the visual checks and reset do not clear the CF code, book a visit from a Gas Safe registered engineer — ideally one with Viessmann accreditation. Attempting to open the boiler casing, handle internal wiring, or fit components without the relevant qualifications may invalidate your warranty and is potentially dangerous.
Parts you may need
- Viessmann outdoor NTC temperature sensor · from £35
- Sensor cable (2-core, pre-terminated) · from £20
- Sensor terminal/connector block · from £12
- Vitotronic/Vitodens PCB control board · from £320
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
Will my heating still work while the CF code is showing?
In most cases, yes — the boiler enters a 'Constant Mode' fallback and continues to provide heating and hot water. However, it is no longer adjusting its output based on actual outdoor temperatures, which means it will be less efficient and your home may feel less comfortable than usual. You should still have the fault investigated promptly, as running in constant mode for an extended period wastes energy and can place extra wear on the system.
How much does it cost to fix a Viessmann CF fault code in the UK?
For the most common cause — a failed outdoor temperature sensor and straightforward cable — expect to pay roughly £100–£200 all-in, covering the engineer's call-out, labour, and the sensor part. Wiring repairs alone are often included within the call-out fee. If the fault turns out to involve the PCB control board, costs can rise considerably — replacement boards for Viessmann models can run to £300–£500 or more including fitting — but this is an uncommon outcome for a CF code.
Can I bypass or ignore the outdoor temperature sensor on my Viessmann boiler?
Technically, many Viessmann controllers will continue to run without the outdoor sensor connected, defaulting to constant-temperature mode. However, Viessmann does not recommend operating this way long-term. You lose the weather compensation function that optimises efficiency, your energy bills will likely rise, and some warranty conditions require the system to be maintained in full working order. It is always better to have the sensor repaired or replaced.
Where should the Viessmann outdoor temperature sensor be mounted?
Viessmann specifies that the sensor should be fixed to a north-facing or north-west-facing external wall, between 2 metres and 2.5 metres above ground level. It should not be positioned directly above windows, doors, extractor vents, or air-conditioning units, and should not sit immediately below a balcony or gutter where dripping water could affect readings. Incorrect placement can cause the boiler to misread outdoor conditions and may contribute to a CF fault or inaccurate weather compensation.