Viessmann 38 Fault Code: Causes, Fixes & Repair Costs
What does the Viessmann 38 fault code mean?
Fault code 38 on Viessmann boilers indicates that the boiler's control system has detected the fan has not come to a complete stop between heating cycles, or that there is an unreliable electrical connection at the fan's plug-in connector. As a safety measure, the burner is prevented from igniting until the fan is confirmed to be at rest and all connections are stable. On the Vitodens 100-W this typically places the burner into fault/lockout mode (sometimes linked to DIP switch S2 being set to ON). On Vitotronic 300-K combined with Vitotronic 200 systems, code 38 can instead indicate a communication fault between the two control units, and up to four related sub-codes may be logged on those models. Because the meaning can vary slightly by model, always cross-reference your boiler's specific documentation or ask a Viessmann-trained installer to confirm the diagnosis.
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
- Fan failing to reach standstill between cycles Common
Viessmann's control system checks that the fan has fully wound down before permitting the next ignition attempt. A fan motor with worn bearings or a failing capacitor may keep spinning longer than expected, causing the control board to flag the fault and lock out the burner as a precaution.
- Loose or damaged plug-in connector at the fan Common
The wiring harness connecting the fan to the PCB can work loose over time due to vibration, or the connector pins can corrode. A poor connection causes the control board to receive erratic or absent fan-speed signals, which it interprets as the fan not being at standstill.
- Faulty fan motor Sometimes
The fan motor itself may have failed electrically or mechanically. When the motor cannot spin up or down correctly, the boiler has no safe way to verify proper ventilation before ignition and will refuse to fire.
- PCB or control unit misreading fan signals Rare
If the fan and its wiring both check out, the PCB or control unit may be incorrectly interpreting the fan-speed feedback signal. On Vitotronic 300-K systems this can also manifest as a communication fault between the two control units rather than a physical fan problem.
How to fix it
- Reset the boiler once or twice DIY safe
Viessmann boilers can often be reset by turning the heating dial to the reset symbol and quickly returning it to the original position, or by pressing and holding 'Mode' and 'OK' together for a few seconds (method varies by model — check your user guide). If the fault clears and does not return within a short time, monitor the boiler closely. If code 38 reappears after one or two resets, do not keep resetting — move on to the next step.
- Check that the boiler has power and that your gas supply is active DIY safe
Confirm the boiler is receiving mains power and that other gas appliances in your home (e.g. a gas hob) are working normally. If there is a wider gas supply issue, contact your gas supplier rather than attempting further boiler checks.
- Do not attempt to access internal components Gas Safe engineer
The fan, wiring harness, PCB, and all internal boiler components must only be inspected and worked on by a Gas Safe registered engineer. Removing panels or disturbing electrical connections without the correct qualifications is unsafe and will invalidate your Viessmann warranty.
- Book a Gas Safe registered engineer — ideally a Viessmann-trained installer Gas Safe engineer
An engineer will: inspect the fan motor and check it is spinning up and winding down correctly; reseat or replace the plug-in connector and wiring harness to the fan; test fan-speed sensor feedback to the PCB; and, if the fan is mechanically sound, assess the PCB or control unit for signal-reading faults. On Vitotronic 300-K systems they will also check for communication errors between the two control units. Viessmann recommends using a Viessmann-trained installer for accurate diagnosis — you can find one via Viessmann's customer services on 01952 675000 or through the Gas Safe Register.
Parts you may need
- Viessmann boiler fan / flue gas fan assembly · from £180
- Fan wiring harness / plug-in connector · from £25
- PCB / control unit (model-dependent) · 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 £200–£420, depending on the underlying cause.
Frequently asked questions
Is Viessmann fault code 38 dangerous — can I keep using my boiler?
The boiler locks out as a safety precaution because it cannot confirm the fan is working correctly before allowing ignition. You should not attempt to override the lockout or repeatedly reset the boiler. Leave it off and arrange for a Gas Safe engineer to investigate. Using a boiler with a confirmed fan fault risks poor combustion or carbon monoxide build-up.
Why does code 38 mean something different on my Vitotronic 300-K compared with a Vitodens 100-W?
Viessmann uses the same numeric codes across a wide range of models, but the underlying fault the code points to can differ by product family. On the Vitodens 100-W, code 38 specifically relates to the fan not reaching standstill or a plug-in connection issue. On Vitotronic 300-K systems paired with a Vitotronic 200 controller, the same code signals a communication fault between the two control units, and up to four sub-codes may be stored. A Viessmann-trained installer can read the full fault log to pinpoint the exact cause on your specific model.
How much does it cost to replace a fan on a Viessmann boiler in the UK?
A fan replacement including parts and labour typically costs between £200 and £400, depending on your boiler model and your engineer's call-out rate. If the fault turns out to be the PCB or control unit rather than the fan, expect costs closer to £300–£500. These larger component costs are best confirmed with a written quote before work begins.
Could an annual boiler service prevent fault code 38 from appearing?
Yes, regular servicing helps catch early signs of fan wear, loose connectors, and wiring issues before they cause a lockout. A Viessmann boiler service in the UK typically costs £80–£125. Keeping up with annual services also helps maintain any manufacturer warranty you may have — Viessmann's standard warranty is five years for boilers registered within 30 days of installation by an approved installer.