Viessmann dA Fault Code: Causes, Fixes & Repair Costs
What does the Viessmann dA fault code mean?
The dA fault code indicates that your Viessmann boiler has detected a short circuit within the thermistor (temperature sensor) of one of its heating circuits. Viessmann groups this code alongside the d6, d7, and d8 codes, which all relate to sensor or thermistor faults across different heating circuits. When the boiler's control board reads an out-of-range signal from a heating circuit sensor — consistent with a short circuit rather than an open circuit — it logs dA and shuts down as a precaution. This is an electronic and sensor-related fault, not a pressure or ignition issue. Note that the precise circuit affected (heating circuit 1, 2, or 3) can vary by model, and Viessmann advises confirming the diagnosis with a Viessmann-trained installer for your specific boiler.
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
- Faulty or shorted heating circuit thermistor Common
The most common cause. The thermistor itself has developed an internal short circuit, causing it to send an abnormal resistance reading to the PCB. This typically happens due to age, heat stress, or moisture ingress at the sensor head. The boiler interprets this signal as a dangerous out-of-range temperature reading and shuts down safely.
- Damaged or deteriorated wiring to the sensor Common
The wiring loom connecting the heating circuit thermistor to the PCB can develop faults over time — particularly where cables run close to hot components. A pinched, chafed, or corroded wire can create a short circuit on the signal line, mimicking a failed sensor even when the sensor itself is intact.
- Loose or corroded sensor connector Sometimes
The plug-in connector at the thermistor or at the PCB terminal can loosen or corrode, leading to intermittent or permanent short-circuit signals. This is often seen on older boilers where connectors have been disturbed during previous servicing.
- PCB fault Sometimes
If the thermistor and its wiring both test as healthy, the fault may lie within the PCB itself. The control board continuously monitors sensor signals, and internal component failures — such as a damaged input circuit on the board — can generate false short-circuit readings. This is less common but should be investigated if sensor replacement does not clear the code.
- Electromagnetic interference (EMC) Rare
Strong electromagnetic interference from nearby equipment can occasionally disrupt the boiler's electronic circuits and trigger sensor-related fault codes. This is a rare cause but worth mentioning if the boiler is installed close to industrial equipment or large motors.
How to fix it
- Check the basics: power supply and circuit breaker DIY safe
Before assuming the worst, confirm the boiler has a stable power supply. Check your consumer unit for any tripped breakers and reset if necessary. Also confirm that no other electrical appliances have recently developed faults, which could indicate a wider electrical issue.
- Attempt a single boiler reset DIY safe
Many sensor codes can be caused by a temporary glitch. On most Vitodens models, press and hold the reset (R) button on the front panel for approximately two seconds until the boiler attempts to restart. If the boiler fires up and runs normally, monitor it over the next hour. If the dA code returns immediately or after a short time, do not reset again — repeated resets will not fix a genuine sensor fault and can mask the underlying problem.
- Call a Gas Safe registered engineer to inspect and test the thermistor Gas Safe engineer
A qualified engineer will access the heating circuit thermistor and use a multimeter to measure its resistance at a known temperature. A shorted thermistor will show near-zero resistance rather than the expected value from the manufacturer's specification table. If the sensor is confirmed faulty, it will be replaced — a relatively straightforward and cost-effective repair on Viessmann boilers.
- Have the engineer inspect wiring and connectors throughout the sensor circuit Gas Safe engineer
If the thermistor tests as healthy, the engineer should trace the entire wiring loom from the sensor back to the PCB. They will look for signs of chafing, pinching, heat damage, or corrosion, particularly at plug-in connectors. A continuity and insulation resistance test will identify any short circuit in the cable itself.
- PCB testing and replacement if required Gas Safe engineer
Should both the sensor and wiring prove to be in good condition, the engineer will carry out voltage and continuity checks across the relevant input terminals on the PCB. Minor faults such as cracked solder joints can sometimes be repaired on-site, but in most cases a faulty board will require full replacement. On older boilers, the engineer should discuss whether the cost of a new PCB makes financial sense compared to a new boiler installation.
Parts you may need
- Heating circuit thermistor / temperature sensor · from £35
- Sensor wiring loom or connector repair kit · from £20
- Viessmann PCB (control board) — model dependent · 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–£350, depending on the underlying cause.
Frequently asked questions
Is the Viessmann dA code dangerous — do I need to turn off the boiler?
The dA code will typically cause the boiler to lock out and stop operating as a safety measure, so in most cases the boiler has already shut itself down. You do not need to take any additional emergency action. There is no gas leak or combustion risk associated with this code — it is an electronic sensor fault. You can leave the boiler powered on while you arrange an engineer visit. However, if you smell gas at any point, leave the property, do not operate any switches, and call the National Gas Emergency line on 0800 111 999 immediately.
Can I fix the dA fault myself by replacing the thermistor?
While replacing a thermistor is not a gas-related task in itself, we strongly recommend against a homeowner attempting this repair. Accessing the sensor requires opening the boiler casing, working around live electrical components, and correctly identifying the right sensor on the right heating circuit — all of which carry real risks and will void your boiler warranty. A Gas Safe engineer can test the sensor before replacing it, ensuring the correct part is fitted and that the fault is not actually caused by wiring or the PCB. The sensor itself is inexpensive (around £30–£40), but diagnosis is the skilled part.
How much does it typically cost to fix a Viessmann dA fault code?
For most people, this fault is resolved by replacing the heating circuit thermistor and/or repairing a wiring fault. Including the engineer's time, you should budget roughly £100–£350 for this outcome. If the PCB turns out to be the root cause, replacement costs typically add £400–£650 in parts and labour, which can make this a more significant decision — particularly on boilers over eight to ten years old, where the cost of a new PCB starts to approach the cost of a new boiler installation.
My Viessmann boiler is showing dA but it cleared after a reset — is it fixed?
A single successful reset can clear a temporary glitch, but if the fault was caused by a genuinely failing thermistor or damaged wiring, the dA code will almost certainly return — sometimes immediately, sometimes after a few heating cycles. Keep an eye on your boiler over the next day or two. If the code reappears, arrange an engineer visit rather than resetting repeatedly. Persistent resetting will not resolve the underlying fault and could, over time, stress other components unnecessarily.