Viessmann db Fault Code: Causes, Fixes & Repair Costs
What does the Viessmann db fault code mean?
Fault code db on a Viessmann boiler indicates that the control unit has detected a short circuit in the room temperature sensor connected to heating circuit 2 — the mixing-valve circuit. Because the sensor is returning an invalid signal, the boiler switches into 'control mode without room influence', meaning it can still run and provide heat, but it ignores room temperature feedback and falls back to a basic, fixed heating curve instead. This can cause rooms to feel over- or under-heated compared to normal. The code sits within a logical sequence on Viessmann controls: da covers a short circuit on the room sensor for heating circuit 1 (no mixing valve), db covers heating circuit 2 (with mixing valve), and dc covers heating circuit 3 (also with mixing valve). If you see db, the fault is specifically on circuit 2.
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
- Failed room temperature sensor on heating circuit 2 Common
The sensor itself has developed an internal short circuit, most commonly due to age, overheating, or physical damage. When the sensor resistance drops to near zero, the control unit flags a short and ignores the reading entirely. This is the most frequent root cause.
- Damaged or pinched sensor wiring Common
The cable running between the room sensor and the boiler's control board may have been trapped, kinked, or chafed — for example behind skirting boards or during any recent renovation work. A damaged conductor shorting against another or against earth will produce exactly the same fault code as a failed sensor.
- Moisture ingress at the connector or sensor housing Sometimes
If condensation or a small water leak has reached the plug-in connector at either end of the sensor cable, the moisture can bridge the contacts and create an electrical short. This is particularly likely if the sensor is located in a bathroom or utility room.
- Fault within a wired Viessmann remote control or room unit Sometimes
If a Viessmann room thermostat or remote programmer is wired into heating circuit 2, a fault within that device or its wiring can present to the boiler as a sensor short circuit, triggering the db code even though the sensor cable itself is intact.
- PCB or control module fault Rare
In rare cases the short circuit is detected not because the sensor or wiring is actually shorted, but because a component on the control board itself has failed and is mis-reading the sensor input. This is an uncommon cause but should be considered if a new sensor and intact wiring still produce the same code.
How to fix it
- Reset the boiler once or twice DIY safe
Locate the reset button (labelled R or with a flame/reset symbol on the control panel) and press and hold it as described in your user manual. If the code clears and does not return within a day or two, the fault may have been a transient glitch. If db reappears, move on — repeated resets will not fix an underlying wiring or sensor fault.
- Check that your room thermostat or remote sensor for circuit 2 is properly seated DIY safe
If you have a Viessmann room unit (such as a Vitotrol) wired to heating circuit 2, check that it is firmly clipped to its wall base and that no wires are visibly loose at the back. Do not open the boiler casing. This is a surface-level visual check only.
- Inspect the external sensor cable for obvious damage DIY safe
Trace the cable from the room sensor back towards the boiler, looking for any section that appears crushed, frayed, or pinched. Also check whether the cable runs through any damp areas. If you spot clear physical damage, make a note — this will be useful information for your engineer.
- Check for moisture around the sensor and its connector DIY safe
Look at both the room sensor housing and any plug-in connector along the cable run. If there is visible condensation, water staining, or corrosion on the connector, gently dry the area with a cloth and ensure any nearby source of moisture is resolved. Do not attempt to open connector housings or resolder connections.
- Have a Gas Safe registered engineer test and replace the sensor or wiring Gas Safe engineer
An engineer will use a multimeter to measure the room temperature sensor's resistance and compare it against Viessmann's specification for that sensor type. If the sensor or cable is confirmed faulty, they will replace it and check the full cable run. They will also verify the heating circuit 2 configuration and control settings before clearing the fault. If the sensor and wiring both test correctly, the control board will be investigated as a possible cause.
Parts you may need
- Room temperature sensor (Viessmann, suits multi-circuit models) · from £35
- Sensor extension cable / replacement cable harness · from £20
- Vitotrol room unit / remote controller (if fitted to circuit 2) · from £85
- Boiler control PCB (rare — only if board is at fault) · from £280
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–£220, depending on the underlying cause.
Frequently asked questions
Can I still use my heating with fault code db showing?
Yes — the boiler continues to run in a fallback mode that ignores room temperature feedback and uses a fixed heating curve instead. You will likely notice that your rooms are not as accurately controlled as usual; they may be slightly warmer or cooler than the thermostat setting suggests. The fault should still be investigated promptly, but it is not an emergency lockout situation.
What is the difference between fault codes da, db, and dc on a Viessmann boiler?
These three codes all point to a short circuit in a room temperature sensor, but on different heating circuits. Da refers to heating circuit 1, which has no mixing valve. Db refers to heating circuit 2, which is a mixing-valve circuit. Dc refers to heating circuit 3, also a mixing-valve circuit. The letter tells you which circuit is affected, which helps an engineer go straight to the right sensor and wiring run.
How much does it cost to replace a room temperature sensor on a Viessmann boiler in the UK?
Most homeowners pay in the region of £100–£220 all-in, covering the call-out, the sensor itself (typically £20–£50 for the part), and the labour to test, replace, and reset the system. If the issue turns out to be a damaged cable run that needs replacing rather than just the sensor, costs may be towards the higher end of that range. In the unlikely event the control PCB is responsible, that is a more significant repair — typically £350–£600 including labour — but this is rare; mention it to your engineer if a new sensor does not clear the fault.
Will the db fault code clear itself after I replace the sensor?
Once a Gas Safe engineer has replaced the faulty sensor and confirmed the wiring is sound, the fault code should be cleared manually via the boiler's service menu. It will not disappear on its own just because the sensor is renewed — the control unit needs to be told the fault has been rectified. Your engineer will do this as part of the repair process, and the boiler should then return to normal room-temperature-influenced control for heating circuit 2.