Viessmann 4C Fault Code: Causes, Fixes & Repair Costs
What does the Viessmann 4C fault code mean?
The 4C fault code (grouped by Viessmann alongside code 48) indicates that the boiler's control system has detected a broken cable or open circuit on the supply temperature sensor for Heating Circuit 2 — the secondary heating circuit managed by a mixing or blending valve. In response, the boiler closes the mixing valve as a protective measure. An open circuit means the electrical signal from the sensor to the control board has been interrupted, either by a physical break in the wiring, a corroded or loose connector, or a failed sensor. This is the opposite of fault codes 40 and 44, which indicate a short circuit on the same sensor — if you're seeing 4C, the circuit is broken (open); if you see 40/44, the sensor wiring is shorted. Because this fault sits within the secondary heating circuit's temperature monitoring, it is an internal wiring or sensor issue that requires a qualified engineer to investigate beyond a basic reset.
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
- Broken or damaged sensor cable Common
The wiring loom connecting the Heating Circuit 2 supply temperature sensor to the control unit can fracture, fray, or corrode over time — particularly at points where the cable bends repeatedly or passes through the boiler chassis. A clean break anywhere along the cable is enough to register an open circuit on the control board and trigger the 4C code.
- Loose or corroded electrical connector Common
The plug-in connectors and terminals at either end of the sensor cable can work loose over years of vibration, or develop corrosion that interrupts the electrical contact without the wire itself breaking. This is a frequent culprit on older Viessmann systems and can sometimes be intermittent, causing the code to appear and clear unpredictably.
- Failed supply temperature sensor Sometimes
The sensor itself may have developed an internal open circuit, presenting to the control board as a broken cable even when the wiring is intact. Sensors can degrade after prolonged exposure to heat cycling and moisture. A Gas Safe engineer can confirm this quickly using a multimeter to test sensor resistance.
- Wiring fault introduced during recent servicing Rare
If the boiler has recently been serviced or worked on, a connector may have been inadvertently left partially seated, or a cable routed incorrectly and subsequently pinched. This is a less common but worth-noting cause if the fault appeared shortly after maintenance work.
How to fix it
- Check that your gas supply is working DIY safe
Before anything else, confirm other gas appliances in your home (such as your hob or gas fire) are working normally. If there is no gas supply to the property, contact your gas supplier rather than investigating the boiler further.
- Attempt a boiler reset DIY safe
On most Vitodens models, press and hold the reset button (marked R or with a reset symbol on the front panel) for approximately two seconds. The boiler will attempt to restart. If the fault clears and does not return, monitor the boiler over the next day or two. Do not attempt more than two or three resets — repeated resets without resolution will not fix an underlying wiring or sensor fault and can mask a developing problem.
- Check system pressure DIY safe
While you have the boiler in view, glance at the pressure gauge. Correct operating pressure for most Viessmann systems is between 1.0 and 1.5 bar when cold. If it is below 0.8 bar, top it up via the filling loop before resetting. Low pressure alone will not cause a 4C code, but it is worth ruling out any additional faults.
- Do not open the boiler casing or attempt internal inspection Gas Safe engineer
The sensor and its associated wiring are located inside the boiler or within the system pipework. Opening the casing and handling internal components requires Gas Safe registration. Attempting this yourself risks electric shock, could invalidate your Viessmann warranty, and may leave the heating circuit in a worse state if connectors are disturbed incorrectly.
- Contact a Gas Safe registered engineer to diagnose the wiring and sensor Gas Safe engineer
A qualified engineer will trace the cable from the Heating Circuit 2 mixing valve supply sensor back to the control board, checking for breaks, corrosion, and loose terminals. They will use a multimeter to measure the sensor's resistance and confirm whether the fault lies in the wiring, the connectors, or the sensor itself. This is the definitive step to resolve a 4C fault.
- Engineer to replace the faulty component Gas Safe engineer
Once the engineer has pinpointed the cause — whether a damaged cable, corroded connector, or failed sensor — they will replace the relevant part, clear the fault code, and verify that the mixing valve on Heating Circuit 2 is operating correctly before leaving. Ensure you receive a record of the work carried out, particularly if your boiler is still under a Viessmann warranty that requires servicing by an approved engineer.
Parts you may need
- Heating Circuit 2 supply temperature sensor (NTC sensor) · from £35
- Sensor cable / wiring loom · from £25
- Connector terminals / plug kit · from £12
- Mixing valve actuator (if damaged alongside sensor work) · from £85
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
What is the difference between Viessmann fault code 4C and fault codes 40 or 44?
All three codes relate to the supply temperature sensor on Heating Circuit 2 (the circuit with a mixing valve), but they describe opposite electrical faults. Code 4C — grouped with code 48 — means the sensor circuit is open, i.e. there is a break in the cable or the sensor itself has failed and no signal is reaching the control board. Codes 40 and 44 mean the opposite: the sensor circuit is shorted, where two conductors are touching and creating a false low-resistance reading. The repair approach is similar (wiring inspection, sensor test, component replacement), but knowing which code you have helps the engineer diagnose the fault faster.
Can I fix a Viessmann 4C fault myself?
The only safe DIY steps are resetting the boiler (up to two or three times) and checking that your gas supply is on and system pressure is correct. The actual cause of a 4C code — a broken sensor cable, corroded connector, or failed temperature sensor inside the boiler — requires internal access and electrical testing, both of which must be carried out by a Gas Safe registered engineer. Attempting to open the boiler casing yourself may also invalidate your Viessmann warranty.
How much does it typically cost to repair a Viessmann 4C fault in the UK?
Most homeowners pay somewhere between £100 and £280 to resolve a 4C fault. This usually covers the engineer's call-out and diagnostic time (typically £60–£100) plus any parts and fitting — a replacement NTC temperature sensor and associated wiring or connectors generally costs £35–£60 in parts. Labour rates vary by region, tending to be higher in London and the South East. In rare cases where the fault has caused wider wiring damage or the mixing valve actuator also needs attention, costs can be somewhat higher, but a simple sensor or cable replacement is the most common outcome.
Will a 4C fault stop my heating or hot water from working?
The 4C code specifically affects Heating Circuit 2 — the secondary circuit controlled by a mixing valve, which is typically used in homes with a separate zone, underfloor heating, or a low-temperature circuit. Whether your main heating and hot water are disrupted depends on how your system is configured. In many cases, the primary heating circuit continues to function while Circuit 2 is affected, but the mixing valve will close and remain closed until the fault is resolved, meaning that zone will lose heat. You should arrange an engineer visit promptly rather than leaving the fault unresolved.