Viessmann 1b Fault Code: Causes, Fixes & Repair Costs
What does the Viessmann 1b fault code mean?
Fault code 1b on Viessmann boilers indicates that the burner control unit has gone into lockout because the fan has failed to come to a complete standstill between heating cycles, or because the electrical plug-in connection to the fan is unstable. Before attempting to ignite the burner, the boiler's control system checks that the fan is fully stopped and spinning at the correct speed. If it detects the fan is still turning, running at the wrong RPM, or receiving an erratic signal from the speed sensor, it refuses to fire as a safety precaution and displays this fault. This code applies across the Viessmann range, though the exact diagnosis can vary slightly between models — always cross-reference with your specific boiler's manual.
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 motor failing to stop between cycles Common
The fan motor may be slow to wind down, or the tachometer (speed sensor) built into the fan is returning inaccurate RPM readings. The control board sees a non-zero speed signal when it expects zero and immediately locks out the burner as a safety measure.
- Loose or corroded plug-in connector to the fan Common
The electrical connector on the fan's wiring loom can work loose over time due to vibration, or corrode in the damp environment inside the boiler casing. An intermittent connection causes erratic speed signals, which the PCB interprets as a fan fault.
- Faulty or worn fan motor Sometimes
Fan motors have bearings and windings that degrade with age. A motor that is starting to seize, running erratically, or drawing incorrect current will produce unreliable tachometer feedback and trigger the 1b lockout.
- PCB fault affecting fan speed monitoring Rare
If the printed circuit board's fan-control circuitry is damaged or failing, it may misread the fan's speed signal even when the fan itself is operating normally. This is less common but should be considered if the fan checks out fine.
How to fix it
- Reset the boiler once DIY safe
Press and hold the reset button on your Viessmann boiler for a few seconds (the exact location varies by model — check your user manual). The boiler will attempt to restart. If it runs normally, monitor it closely over the next few hours. If the 1b code returns, do not keep resetting repeatedly as this can cause additional wear on components.
- Visually check any accessible wiring connectors near the fan DIY safe
If the fan's wiring connector is visible without removing sealed panels or gas-side components, look for obvious signs of a loose plug, damaged insulation, or corrosion on the connector pins. Do not probe inside the boiler casing or touch any gas-side components. This is a visual check only.
- Check that your boiler's flue outlet is unobstructed DIY safe
A blocked or restricted flue can place extra load on the fan. From outside your property, confirm the flue terminal is clear of debris, bird nests, or ice in cold weather. Never attempt to access or modify the flue internally.
- Call a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
A qualified engineer will use diagnostic tools to test the fan motor, check the tachometer signal, inspect and re-seat or replace the wiring connectors, and test the PCB's fan-control circuitry. If the fan is found to be faulty, they will replace it with a compatible Viessmann part. If the PCB is misreading the signal, they will advise on repair or replacement. Do not attempt to open the sealed boiler casing, handle the fan assembly, or touch any gas or electrical components yourself.
Parts you may need
- Viessmann fan motor assembly · from £130
- Fan wiring harness / plug-in connector · from £25
- Tachometer / fan speed sensor · from £35
- Viessmann PCB (printed circuit board) · 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 £150–£350, depending on the underlying cause.
Frequently asked questions
Can I fix Viessmann fault code 1b myself?
The only safe DIY steps are a single reset attempt and a visual check of any externally accessible wiring connectors. All internal work — testing or replacing the fan motor, reseating connectors inside the casing, or working on the PCB — must be carried out by a Gas Safe registered engineer. The fan is part of the combustion air system, and Viessmann specifies that only a qualified engineer should work on these components.
How much does it cost to fix a Viessmann 1b fault in the UK?
Most people with this fault pay somewhere between £150 and £350. If the fix is simply reseating a loose connector, you're likely looking at a callout/diagnostic fee of £60–£100 plus a short labour charge. A full fan motor replacement (supply and fit) typically runs £200–£300 depending on the model. If the fault turns out to be a faulty PCB, replacement costs are higher — usually £350–£500+ fitted — but this is less common and worth getting a specific quote for.
Why does my Viessmann boiler keep showing 1b after I reset it?
If the code keeps returning after a reset, it means the underlying problem — most likely a worn fan motor, a faulty tachometer, or a loose electrical connector — has not been resolved. Repeatedly resetting the boiler without addressing the root cause can put extra stress on components and potentially make the repair more complex. One reset is reasonable; after that, book a Gas Safe engineer to investigate properly.
Is fault code 1b dangerous?
The lockout itself is a safety feature, not a danger. The boiler is correctly refusing to fire the burner until it can confirm the fan is operating as expected, which prevents unsafe combustion conditions. However, you should not attempt to bypass the lockout or repeatedly force restarts. Leave the boiler off and arrange for a Gas Safe engineer to inspect it promptly, especially in cold weather when you need heating.