Viessmann dC Fault Code: Causes, Fixes & Repair Costs
What does the Viessmann dC fault code mean?
The dC fault code on a Viessmann boiler indicates that the control unit has detected a short-circuit in the room temperature sensor on one of the heating circuits. It sits in a family of related codes — dA covers a sensor short-circuit on heating circuit 1 (without mixing valve), dB covers heating circuit 2 (with mixing valve), and dC typically relates to a further heating circuit, often a third circuit or additional mixed zone depending on your system layout. Rather than triggering a full lockout, the boiler usually continues to run in a degraded 'constant mode', where it operates at a fixed output temperature instead of responding to room-temperature feedback. This means your heating may still work but will not respond intelligently to the conditions in your home. A reset may temporarily clear the code, but if it returns, the underlying sensor or wiring issue must be investigated.
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 in the room sensor wiring Common
A pinched, damaged, or corroded cable running between the room thermostat or sensor and the boiler control unit is the most frequent trigger. Even minor insulation damage — for example where a wire runs behind a radiator or through a door frame — can cause the control unit to read a short-circuit and flag the dC code.
- Failed room temperature sensor Common
The sensor component itself may have developed an internal short-circuit due to age, moisture ingress, or manufacturing defect. When the sensor's resistance drops to near zero, the control unit interprets this as a short and enters constant mode.
- Loose or corroded plug-in connections Sometimes
The connection between the sensor cable and the boiler's terminal block or control unit can work loose over time, especially in systems subject to vibration or temperature cycling. A poor contact can mimic a short-circuit and trigger the dC code intermittently.
- PCB fault Rare
In a minority of cases the control board itself is responsible, either through a failed input channel or internal component damage. This is usually only confirmed after sensor and wiring have been checked and replaced without resolving the fault.
How to fix it
- Reset the boiler DIY safe
On most Vitodens models, press and hold the Reset (R) button for approximately two seconds until the display clears. If the dC code disappears and does not return within a short period of normal operation, the fault may have been a transient electrical event. Do not reset more than two or three times — repeated resets without fixing the underlying problem will not help and can mask a developing fault.
- Check that your room thermostat or sensor is securely mounted and undamaged DIY safe
Walk to wherever your room thermostat or Viessmann room sensor is installed and look for obvious physical damage — a cracked casing, a loose backplate, or a wire that has been pulled partially free. Gently press the connector into its socket if it feels loose. Do not open the device or cut any wires.
- Inspect the visible wiring run between the room sensor and the boiler DIY safe
Trace the cable from the room thermostat back towards the boiler, looking for sections that are pinched under skirting boards, caught in door frames, or showing signs of heat damage near radiators. Note any problem areas so you can describe them to the engineer — do not attempt to splice or repair the wiring yourself.
- Contact a Gas Safe registered engineer to test and replace the sensor or wiring Gas Safe engineer
A qualified engineer will use a multimeter to measure the resistance across the room temperature sensor and its wiring. A reading close to zero confirms a short-circuit. They will replace the faulty sensor or damaged cable section, reseat all terminal connections at the control unit, and perform a reset and function test to verify the repair. If the fault persists after sensor and wiring work, the engineer will assess whether the PCB requires replacement.
Parts you may need
- Viessmann room temperature sensor · from £35
- Sensor wiring harness / cable · from £25
- Boiler control unit (PCB) · 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–£280, depending on the underlying cause.
Frequently asked questions
Will my heating still work when the dC code is showing?
In most cases, yes — but in a limited way. The boiler typically switches to a constant-output mode, meaning it heats to a fixed flow temperature rather than adjusting based on the room sensor's feedback. You may notice the house feels too warm or too cool, and the boiler will not respond to room temperature changes as intended. Central heating and hot water usually remain available, but efficiency suffers. Get the fault investigated promptly so normal control is restored.
Can I clear the Viessmann dC code myself without an engineer?
You can safely attempt a boiler reset (press and hold the R button for two seconds) and check for any obviously loose connections at your room thermostat. However, properly diagnosing a sensor short-circuit requires resistance testing with a multimeter, and replacing a sensor or wiring must be done by someone competent in electrical work on heating systems. Because the fault sits within the boiler's control system, a Gas Safe registered engineer — ideally one trained by Viessmann — should carry out the repair if the code returns after a reset.
How much does it cost to fix a dC fault on a Viessmann boiler?
For most households the repair costs between £100 and £280 including labour. A straightforward room temperature sensor swap typically comes in at £80–£150 all in, while wiring repairs or more involved diagnostic work can push the cost towards £200–£280. If the PCB turns out to be faulty, parts alone can cost £200–£400, so a full repair including labour could reach £500 or more — though PCB faults are uncommon as the root cause of this specific code. Getting at least two quotes is always worthwhile for the higher-cost scenarios.
What is the difference between the dA, dB and dC codes on a Viessmann boiler?
All three codes point to a short-circuit in a room temperature sensor, but on different heating circuits. dA relates to heating circuit 1 (a direct, unvalved circuit), dB to heating circuit 2 (a circuit with a mixing valve), and dC to a further heating circuit — typically circuit 3 or an additional mixed zone. Which code appears on your display simply tells the engineer which circuit's sensor and wiring to test first. The diagnosis and repair process is essentially the same for all three.