Viessmann 99 Fault Code: Causes, Fixes & Repair Costs
What does the Viessmann 99 fault code mean?
Fault code 99 on Viessmann boilers signals that the burner has gone into a safety lockout because the fan has not come to a complete standstill when the boiler's control system expects it to be stationary. In plain terms, the boiler monitors fan speed (RPM) throughout its operating cycle — including after a heating sequence ends. If the fan is still spinning when it should have stopped, or if the fan speed signal coming back to the control board is erratic or implausible, the burner shuts down as a precaution. The fault can also be triggered by an unstable or loose plug-in connection to the fan, which causes the control board to receive a garbled fan-speed signal and react as though the fan is misbehaving. This code applies across the Viessmann range, including Vitodens wall-hung condensing boilers.
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 failing fan motor Common
The fan motor is the most frequent culprit. As motors age or develop internal wear, they may continue to spin after power is cut — or run at an incorrect RPM during operation. The boiler's control board detects that the fan hasn't reached standstill in the expected time window and triggers the lockout to prevent the burner operating under unsafe airflow conditions.
- Loose or damaged fan wiring connector Common
Viessmann specifically cites an unstable plug-in connection to the fan as a direct trigger for this fault. A connector that has vibrated loose, a corroded terminal pin, or a chafed section of the wiring loom can all produce intermittent or false fan-speed signals. The control board interprets these as evidence that the fan is still running and locks out the burner accordingly.
- Defective fan speed sensor (tachometer) Sometimes
A small Hall-effect sensor on the fan assembly monitors the blade rotation and feeds an RPM signal back to the PCB. If this sensor deteriorates or fails, it can report incorrect speeds — either suggesting the fan is still spinning when it has stopped, or vice versa — causing repeated lockouts even though the fan motor itself may be mechanically sound.
- Faulty PCB (printed circuit board) Rare
The main control board is responsible for both commanding the fan and interpreting the speed feedback it receives. A PCB that has developed a fault in its fan-control circuitry may send incorrect drive signals or misread the tachometer input, leading it to falsely conclude that the fan is not at standstill. PCB failure is less common than fan or wiring issues but becomes more likely on older boilers.
How to fix it
- Reset the boiler once or twice DIY safe
On most Vitodens models there is a reset button (marked R or with a flame symbol) on the front panel. Press and hold it for approximately two seconds until the fault clears and the boiler attempts to restart. If the boiler fires up and runs normally, monitor it closely over the next hour or so. If fault code 99 returns after one or two resets, stop trying — repeated reset attempts will not fix an underlying fault and can place additional stress on ignition components.
- Check that your gas supply is functioning DIY safe
Confirm that other gas appliances in your home — hob, gas fire — are working normally. If they are not, contact your gas supplier immediately. If gas supply is fine, the fault is internal to the boiler.
- Visually inspect any accessible fan wiring without opening the boiler casing DIY safe
Without removing any panels or covers, take a careful look at any visible wiring near the fan area. If you can see an obviously disconnected plug or a wire that has clearly come away from a connector, note it for the engineer. Do not attempt to open the boiler casing, reattach connectors yourself, or touch any internal components — this must be left to a qualified professional.
- Have a Gas Safe engineer inspect and test the fan assembly and its wiring loom Gas Safe engineer
A registered engineer will remove the casing safely and check the fan plug-in connector and wiring harness for corrosion, loose pins, or damage. Connectors can be reseated or replaced at relatively low cost and this alone often resolves the fault if the motor itself is healthy.
- Have the engineer test fan RPM output and the tachometer sensor Gas Safe engineer
Using Viessmann diagnostic equipment or a suitable multimeter, the engineer will verify that the fan is reaching the correct speed during operation and returning to standstill correctly afterwards. They will also test the tachometer sensor signal. A faulty sensor can be replaced independently of the full fan assembly, which is a less expensive repair than a complete fan replacement.
- Replace the fan motor if it is confirmed as faulty Gas Safe engineer
If the fan motor is worn, failing to stop in time, or running outside its specified RPM range, the engineer will fit a replacement unit. Viessmann Accredited installers carry specialist diagnostic tools and access to genuine Viessmann parts, which ensures correct calibration of the replacement fan within the boiler's combustion management system. This is particularly important because Viessmann boilers require combustion re-calibration after fan replacement.
- Replace the PCB if fan and wiring checks are inconclusive Gas Safe engineer
If the fan motor and all wiring are tested as serviceable but the fault persists, the PCB's fan-control circuitry is likely at fault. PCB replacement is a more substantial repair and should be weighed against the boiler's age — see the FAQ below for guidance on repair versus replacement decisions.
- Call a Gas Safe registered engineer if the fault recurs or you have not already done so Gas Safe engineer
Fault code 99 is a burner lockout that involves internal fan and electronics components — all of which require Gas Safe registration to work on legally and safely in the UK. If the fault has returned after a reset, or you have not yet contacted a professional, book a Gas Safe engineer as your next step. Ideally choose one with Viessmann accreditation for the most accurate diagnosis.
Parts you may need
- Viessmann fan motor assembly (Vitodens) · from £130
- Fan wiring loom / connector harness · from £25
- Fan tachometer / Hall-effect speed sensor · from £35
- PCB (main control board) — Vitodens 100-W / 200-W · 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 £120–£380, depending on the underlying cause.
Frequently asked questions
Can I fix Viessmann fault code 99 myself by just resetting the boiler?
A reset is always worth trying once or twice — it may clear a temporary glitch in the fan's control signal and allow the boiler to restart normally. However, if code 99 returns after resetting, it means there is an underlying hardware fault (most likely the fan motor, its wiring connector, or the tachometer sensor) that a reset cannot address. At that point you need a Gas Safe registered engineer, because all work inside the boiler casing — including anything touching the fan or electronics — is not legal or safe for a homeowner to carry out.
How much does it typically cost to repair Viessmann fault code 99 in the UK?
Most people with this fault pay somewhere between £120 and £380 all-in. A wiring connector fix or sensor replacement at the lower end can cost £80–£150 including the call-out. A full fan motor replacement is usually £200–£350 depending on the Viessmann model. If the PCB turns out to be the cause, expect £300–£450 for parts and labour — that is the more expensive scenario, and in that case it is worth factoring in the boiler's age before committing to the repair.
Is fault code 99 dangerous — do I need to stop using the boiler immediately?
Fault code 99 is a safety lockout, which means the boiler has already shut itself down to prevent unsafe operation. You cannot simply override it and run the boiler normally — the burner will not fire while the fault is active. The fault itself is not directly dangerous in the way a gas leak would be, but you should not attempt to repeatedly reset the boiler hoping it will stay on, and you should call an engineer promptly, especially during cold weather. If at any point you smell gas or a carbon monoxide alarm activates, leave the property, do not touch any switches, and call the National Gas Emergency Service on 0800 111 999.
My Viessmann boiler is over 10 years old and showing code 99 — should I repair or replace it?
Viessmann boilers are well built and routinely last 12–15 years with annual servicing. If your boiler is under 10 years old and the repair is a fan motor or wiring fix (£150–£350), repair almost always makes financial sense. If the boiler is over 12 years old and the diagnosis points to a PCB replacement (£300–£450 or more), it is worth getting a quote for a new boiler at the same time — a modern Vitodens or equivalent A-rated condensing boiler could cut your gas bills noticeably and comes with a manufacturer warranty, reducing the risk of further repair bills in the near future.