Viessmann boiler error codes
21 codes that Viessmann publishes for its boilers, each with what it means, the likely cause, what you can safely check and when to call a Gas Safe registered engineer. Tap your code:
Gas boiler: do only the checks its user manual gives you — read the pressure gauge, top up through the filling loop, press the reset button, thaw a frozen condensate pipe at ground level. Keep the casing on and leave sealed parts alone. Anything else is for a Gas Safe registered engineer. If you smell gas, call the National Gas Emergency Service on 0800 111 999. Codes that need one are marked Gas Safe engineer.
Read this first
The Viessmann Vitodens 100-W (types B1HF and B1KF) shows a fault symbol and a plain number such as 74 or 65; the list of fault messages is in the installation and service instructions for contractors (2023). This page explains 21 of its codes — pressure, overheating, flame, fan and sensor faults — with what the boiler does while the fault is present. Because the source is written for heating engineers, it gives no checks for the householder; where Viessmann’s measure is simple, such as topping up the water for 74, we say what the engineer will do rather than tell you to do it. Viessmann’s own advice for homeowners is to look up the code in the manual and to consult a Gas Safe engineer if the manual says so or if you are not confident.
74 System pressure too low
- What it means
- The heating system pressure is too low. The burner is locked out and the internal pump is off: no heating and no hot water.
- Likely cause
- Water lost from the heating system.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer Call a Gas Safe registered engineer or your installer. Viessmann’s measure for them: top up with water and vent the system; if it happens again, check the pressure sensor against a separate gauge, the expansion vessel pre-charge and the pressure settings.
Source: Viessmann, Vitodens 100-W, types B1HF and B1KF, installation and service instructions for contractors (6167586 GB, 3/2023): fault messages Checked 2026-09-28.
87 System pressure too high
- What it means
- The heating system pressure is too high. The burner is locked out and the pump is off: no heating and no hot water.
- Likely cause
- Too much pressure in the system, the expansion vessel, closed valves or the pressure sensor.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer Call a Gas Safe registered engineer: Viessmann’s measures for this code are written for the contractor and are inside the boiler.
Source: Viessmann, Vitodens 100-W, types B1HF and B1KF, installation and service instructions for contractors (6167586 GB, 3/2023): fault messages Checked 2026-09-28.
75 No water flow
- What it means
- No flow rate: the system is in a fault state.
- Likely cause
- Too little water, air in the system, closed valves, the flow sensor or the circulation pump.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer Call a Gas Safe registered engineer. Viessmann’s first measure for them is to top up with water and vent the system, then check the valves, flow sensor and pump if it repeats.
Source: Viessmann, Vitodens 100-W, types B1HF and B1KF, installation and service instructions for contractors (6167586 GB, 3/2023): fault messages Checked 2026-09-28.
89 Pump blocked
- What it means
- The internal circulation pump is blocked: no heating and no hot water.
- Likely cause
- A blocked or failed circulation pump.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer Call a Gas Safe registered engineer: Viessmann’s measures for this code are written for the contractor and are inside the boiler.
Source: Viessmann, Vitodens 100-W, types B1HF and B1KF, installation and service instructions for contractors (6167586 GB, 3/2023): fault messages Checked 2026-09-28.
62 Overheat cut-out
- What it means
- The high-limit safety cut-out has responded and the burner is in a fault state.
- Likely cause
- Too little water, the expansion vessel pressure, poor circulation, or the 3-way diverter valve.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer Call a Gas Safe registered engineer: Viessmann’s measures for this code are written for the contractor and are inside the boiler.
Source: Viessmann, Vitodens 100-W, types B1HF and B1KF, installation and service instructions for contractors (6167586 GB, 3/2023): fault messages Checked 2026-09-28.
63 Flue gas temperature limiter
- What it means
- The flue gas temperature limiter has responded.
- Likely cause
- The same checks as 62: water level, expansion vessel, circulation and diverter valve. Viessmann says to reset only once the flue system has cooled.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer Call a Gas Safe registered engineer: Viessmann’s measures for this code are written for the contractor and are inside the boiler.
Source: Viessmann, Vitodens 100-W, types B1HF and B1KF, installation and service instructions for contractors (6167586 GB, 3/2023): fault messages Checked 2026-09-28.
345 Temperature limiter: restarts when cool
- What it means
- The temperature limiter has responded. The burner is locked out and enables itself again once the boiler has cooled down.
- Likely cause
- Poor heat transfer, low water, the expansion vessel, circulation or the diverter valve; during hot water, scale in the heat exchanger.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer Call a Gas Safe registered engineer: Viessmann’s measures for this code are written for the contractor and are inside the boiler.
Source: Viessmann, Vitodens 100-W, types B1HF and B1KF, installation and service instructions for contractors (6167586 GB, 3/2023): fault messages Checked 2026-09-28.
64 Flame lost, restarting
- What it means
- The flame was lost while stabilising or running. The boiler keeps working and relights.
- Likely cause
- The gas supply, flue gas being drawn back in, or the ionisation electrode.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer Call a Gas Safe registered engineer: Viessmann’s measures for this code are written for the contractor and are inside the boiler.
Source: Viessmann, Vitodens 100-W, types B1HF and B1KF, installation and service instructions for contractors (6167586 GB, 3/2023): fault messages Checked 2026-09-28.
65 No flame at start
- What it means
- No flame signal, or too weak a signal, when the burner starts: the burner is in a fault state.
- Likely cause
- The gas supply, the gas valve, condensate backing up (a blocked condensate drain), or the ignition and ionisation electrodes.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer Call a Gas Safe registered engineer: Viessmann’s measures for this code are written for the contractor and are inside the boiler.
Source: Viessmann, Vitodens 100-W, types B1HF and B1KF, installation and service instructions for contractors (6167586 GB, 3/2023): fault messages Checked 2026-09-28.
67 69 Flame signal out of range
- What it means
- The ionisation current (how the boiler senses its flame) is outside the permitted range.
- Likely cause
- The gas supply and valve, the ionisation electrode, or for 69 the heat management unit or the fan unit.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer Call a Gas Safe registered engineer: Viessmann’s measures for this code are written for the contractor and are inside the boiler.
Source: Viessmann, Vitodens 100-W, types B1HF and B1KF, installation and service instructions for contractors (6167586 GB, 3/2023): fault messages Checked 2026-09-28.
68 Flame seen before start
- What it means
- A flame signal is already present when the burner starts.
- Likely cause
- A flame-sensing or control-unit fault.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer Call a Gas Safe registered engineer: Viessmann’s measures for this code are written for the contractor and are inside the boiler.
Source: Viessmann, Vitodens 100-W, types B1HF and B1KF, installation and service instructions for contractors (6167586 GB, 3/2023): fault messages Checked 2026-09-28.
346 Flame calibration error
- What it means
- The ionisation current calibration failed.
- Likely cause
- Gas supply pressure, a dirty gas valve strainer or electrode, flue gas being drawn back in, or a blocked condensate drain.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer Call a Gas Safe registered engineer: Viessmann’s measures for this code are written for the contractor and are inside the boiler.
Source: Viessmann, Vitodens 100-W, types B1HF and B1KF, installation and service instructions for contractors (6167586 GB, 3/2023): fault messages Checked 2026-09-28.
347 Flue gas recirculation
- What it means
- Flue gas is being drawn back into the boiler.
- Likely cause
- A leak or back pressure in the flue system: too little fall, a narrowing or a blockage.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer Call a Gas Safe registered engineer: Viessmann’s measures for this code are written for the contractor and are inside the boiler.
Source: Viessmann, Vitodens 100-W, types B1HF and B1KF, installation and service instructions for contractors (6167586 GB, 3/2023): fault messages Checked 2026-09-28.
71 72 Fan fault
- What it means
- 71: fan speed too low. 72: the fan did not reach its idle state.
- Likely cause
- A blocked or faulty fan, or wrong gas-type or flue settings.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer Call a Gas Safe registered engineer: Viessmann’s measures for this code are written for the contractor and are inside the boiler.
Source: Viessmann, Vitodens 100-W, types B1HF and B1KF, installation and service instructions for contractors (6167586 GB, 3/2023): fault messages Checked 2026-09-28.
176 Condensate backed up
- What it means
- Condensate has backed up in the heat cell.
- Likely cause
- A blocked condensate drain. Viessmann’s measure replaces burner parts, so it is an engineer’s job.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer Call a Gas Safe registered engineer: Viessmann’s measures for this code are written for the contractor and are inside the boiler.
Source: Viessmann, Vitodens 100-W, types B1HF and B1KF, installation and service instructions for contractors (6167586 GB, 3/2023): fault messages Checked 2026-09-28.
59 Supply voltage too low
- What it means
- Undervoltage in the power supply. The burner is locked out and the pump is off: no heating, no hot water.
- Likely cause
- Low mains voltage, or the fan unit if the voltage is correct and the fault repeats.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer Call a Gas Safe registered engineer: Viessmann’s measures for this code are written for the contractor and are inside the boiler.
Source: Viessmann, Vitodens 100-W, types B1HF and B1KF, installation and service instructions for contractors (6167586 GB, 3/2023): fault messages Checked 2026-09-28.
184 185 Flow sensor or high-limit cut-out
- What it means
- A short circuit (184) or a lead break (185) in the flow temperature sensor or high-limit safety cut-out.
- Likely cause
- A faulty sensor or its lead.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer Call a Gas Safe registered engineer: Viessmann’s measures for this code are written for the contractor and are inside the boiler.
Source: Viessmann, Vitodens 100-W, types B1HF and B1KF, installation and service instructions for contractors (6167586 GB, 3/2023): fault messages Checked 2026-09-28.
78 Control panel not talking
- What it means
- No communication between the heat management unit and the programming unit (the control panel). The boiler carries on normally.
- Likely cause
- The cables and plugs between the two units.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer Call a Gas Safe registered engineer: Viessmann’s measures for this code are written for the contractor and are inside the boiler.
Source: Viessmann, Vitodens 100-W, types B1HF and B1KF, installation and service instructions for contractors (6167586 GB, 3/2023): fault messages Checked 2026-09-28.
A code that is not listed here
Other Vitodens models (200-W, 111-W, 050-W) and older Vitotronic controls use different numbers and letter codes such as F2 or F4, and Viessmann’s own online fault-code checker says a diagnosis can differ by boiler. For any code not here, ask a Viessmann-trained or other Gas Safe registered installer.
We list only codes the source below states. If your code is not here, it may belong to a model the source does not cover: your model’s manual is the authority.
Related pages
Source
Every code on this page comes from these pages, published by Viessmann and read on the date shown. Meanings are written in our own words; nothing is taken from other makes, other models or third-party code sites.
- Viessmann: Vitodens 100-W, types B1HF and B1KF, installation and service instructions for contractors (6167586 GB, 3/2023): fault messages Checked 2026-09-28.
- Viessmann: Viessmann UK: boiler fault finding — common problems and how to fix them Checked 2026-09-28.
- Viessmann: Viessmann UK fault code checker Checked 2026-09-28.