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Vaillant boiler error codes

24 codes that Vaillant 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

Vaillant’s ecoTEC plus boilers show a fault as F. and a number, such as F.22 or F.28. The householder’s operating instructions explain only two of them — F.22 (low water pressure) and F.28 (the boiler did not light) — and say that any other code needs a competent person. The full table is in the installation and maintenance instructions written for the engineer; this page explains 24 of its codes (the ones about temperature sensors, water, ignition, flame, fan, gas valve and electronics) in plain words, with the engineer’s checks named so you know what the visit is about. Newer ecoTEC plus models print three digits (F.022, F.028): the lookup finds either form. Glow-worm, part of the same group, uses many of the same numbers — see its own page.

F.22 Low water pressure

What it means
There is no water, or too little, in the boiler, or the heating system pressure is too low, so the boiler switches off for safety. Once the pressure drops below 0.5 bar the display alternates between F.22 and the current pressure.
Likely cause
Water has been lost from the sealed heating system — Vaillant’s troubleshooting table: “Low water pressure in the heating installation.”
What you can safely check

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.

  • Read the pressure on the display. Cold, Vaillant asks for 1.0 to 2.0 bar.
  • If it is low, fill the heating installation through the filling cock the way your installer showed you: open the radiator valves, let water in slowly until the pressure is reached, close the supply, bleed the radiators, check the pressure again and close the filling cock.
When to call a Gas Safe registered engineer
Call a Gas Safe registered engineer if F.22 stays after filling, or if you have to top up again and again. The engineer’s checks are the pump, the water pressure sensor and their wiring.

Source: Vaillant, ecoTEC plus VU, VUW, VUI ..6/5-5 operating instructions (0020245033_02, 2021): troubleshooting Checked 2026-09-28.

F.28 Boiler did not light

What it means
The flame was not detected while the boiler tried to light. After three failed attempts (five on the 2023 models) it stops in fault mode.
Likely cause
The installation manual lists a closed gas stopcock, low gas pressure, a blocked condensate route, air in the gas, and faults in the ignition electrode, ignition lead, gas valve or electronics.
What you can safely check

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.

  • Check that both gas isolator cocks are open — the one on the boiler and the one fitted on site. It is the first thing the operating instructions check when the boiler will not start.
  • Press the fault-clearance (reset) button for one second. The boiler tries to light again.
  • Try this no more than three times.
When to call a Gas Safe registered engineer
After three unsuccessful reset attempts Vaillant says to consult a competent person: for a gas boiler, a Gas Safe registered engineer.

Source: Vaillant, ecoTEC plus VU, VUW, VUI ..6/5-5 operating instructions (0020245033_02, 2021): troubleshooting Checked 2026-09-28.

F.00 F.01 Temperature sensor not connected

What it means
The boiler has lost the signal from a water temperature sensor: F.00 the flow sensor (water going out to the radiators), F.01 the return sensor (water coming back).
Likely cause
The sensor is faulty or not connected: Vaillant’s checks are the sensor, its plug, the wiring loom and the circuit board.
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: every check Vaillant lists for this code is in the installation manual, inside the boiler.

Source: Vaillant, ecoTEC plus VU, VUW ..6/5-5 installation and maintenance instructions (0020244997_07, 2021): appendix E, overview of fault codes Checked 2026-09-28.

F.10 F.11 Temperature sensor short circuit

What it means
A water temperature sensor has short-circuited: F.10 the flow sensor, F.11 the return sensor.
Likely cause
A faulty sensor, or a short in its plug, cable or the circuit board.
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: every check Vaillant lists for this code is in the installation manual, inside the boiler.

Source: Vaillant, ecoTEC plus VU, VUW ..6/5-5 installation and maintenance instructions (0020244997_07, 2021): appendix E, overview of fault codes Checked 2026-09-28.

F.20 Overheat: safety cut-out

What it means
The water in the boiler got too hot and the safety function switched it off.
Likely cause
Vaillant’s checks: whether the flow sensor sits properly on its pipe, the wiring, and whether all the air was purged from the 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: every check Vaillant lists for this code is in the installation manual, inside the boiler.

Source: Vaillant, ecoTEC plus VU, VUW ..6/5-5 installation and maintenance instructions (0020244997_07, 2021): appendix E, overview of fault codes Checked 2026-09-28.

F.23 Temperature difference too big

What it means
The water coming back is much colder than the water going out: too little water is circulating.
Likely cause
Air or too little water in the heating circuit, the pump, a blocked strainer, or flow and return sensors swapped.
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: every check Vaillant lists for this code is in the installation manual, inside the boiler.

Source: Vaillant, ecoTEC plus VU, VUW ..6/5-5 installation and maintenance instructions (0020244997_07, 2021): appendix E, overview of fault codes Checked 2026-09-28.

F.24 Temperature rising too fast

What it means
The water heated up faster than the boiler allows.
Likely cause
The pump, air or too little water in the circuit, the internal air vent, or system pressure too low.
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: every check Vaillant lists for this code is in the installation manual, inside the boiler.

Source: Vaillant, ecoTEC plus VU, VUW ..6/5-5 installation and maintenance instructions (0020244997_07, 2021): appendix E, overview of fault codes Checked 2026-09-28.

F.25 Flue gas too hot

What it means
The exhaust gas leaving the boiler is too hot, so it shut down.
Likely cause
A blocked flue, wind blowing back, a flue run that is too long, too little water in the heating circuit, the pump, or the safety cut-out wiring.
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: every check Vaillant lists for this code is in the installation manual, inside the boiler.

Source: Vaillant, ecoTEC plus VU, VUW ..6/5-5 installation and maintenance instructions (0020244997_07, 2021): appendix E, overview of fault codes Checked 2026-09-28.

F.27 Flame detected when there should be none

What it means
The flame sensor reports a flame while the gas should be off (“flame simulation”).
Likely cause
Vaillant’s checks: gas pressure, the monitoring electrode, the circuit board and the gas solenoid 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: every check Vaillant lists for this code is in the installation manual, inside the boiler.

Source: Vaillant, ecoTEC plus VU, VUW ..6/5-5 installation and maintenance instructions (0020244997_07, 2021): appendix E, overview of fault codes Checked 2026-09-28.

F.29 Flame lost while running

What it means
The flame went out during operation and the boiler could not relight it.
Likely cause
The gas supply was interrupted for a moment, flue gas being drawn back in, a blocked condensate route, earthing, or a loose lead to the gas valve or 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: every check Vaillant lists for this code is in the installation manual, inside the boiler.

Source: Vaillant, ecoTEC plus VU, VUW ..6/5-5 installation and maintenance instructions (0020244997_07, 2021): appendix E, overview of fault codes Checked 2026-09-28.

F.32 Fan fault

What it means
The fan is faulty or not connected, or is not running at the right speed.
Likely cause
The fan itself (blocked or failing), its plug and wiring, its speed sensor, the circuit board, or a blocked flue.
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: every check Vaillant lists for this code is in the installation manual, inside the boiler.

Source: Vaillant, ecoTEC plus VU, VUW ..6/5-5 installation and maintenance instructions (0020244997_07, 2021): appendix E, overview of fault codes Checked 2026-09-28.

F.61 F.62 Gas valve control fault

What it means
F.61: the boiler cannot operate the gas valve. F.62: the gas valve shut late after the flame went out.
Likely cause
The gas valve, its wiring or the circuit board; for F.62 also a dirty burner surface.
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: every check Vaillant lists for this code is in the installation manual, inside the boiler.

Source: Vaillant, ecoTEC plus VU, VUW ..6/5-5 installation and maintenance instructions (0020244997_07, 2021): appendix E, overview of fault codes Checked 2026-09-28.

F.63 F.64 Electronics fault

What it means
F.63: the boiler’s memory chip (EEPROM) is faulty — Vaillant’s measure is a new circuit board. F.64: the electronics, a safety sensor or its cable is faulty.
Likely cause
A faulty circuit board, sensor or cable.
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: every check Vaillant lists for this code is in the installation manual, inside the boiler.

Source: Vaillant, ecoTEC plus VU, VUW ..6/5-5 installation and maintenance instructions (0020244997_07, 2021): appendix E, overview of fault codes Checked 2026-09-28.

F.65 Electronics too hot

What it means
The electronics are faulty or have overheated from something around the boiler.
Likely cause
Vaillant’s measures: check the circuit board and, if needed, lower the temperature around the boiler.
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: every check Vaillant lists for this code is in the installation manual, inside the boiler.

Source: Vaillant, ecoTEC plus VU, VUW ..6/5-5 installation and maintenance instructions (0020244997_07, 2021): appendix E, overview of fault codes Checked 2026-09-28.

F.68 Unstable flame

What it means
The flame monitor sees an unstable flame.
Likely cause
The air–gas ratio, gas pressure, a blocked condensate route, the gas injector, the flame-sensing electrode or its cable, or flue gas being drawn back in.
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: every check Vaillant lists for this code is in the installation manual, inside the boiler.

Source: Vaillant, ecoTEC plus VU, VUW ..6/5-5 installation and maintenance instructions (0020244997_07, 2021): appendix E, overview of fault codes Checked 2026-09-28.

F.70 Device number missing

What it means
The boiler’s Device Specific Number is wrong or missing.
Likely cause
Usually after the display and the circuit board were both replaced: the engineer re-enters the number (setting D.93).
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: every check Vaillant lists for this code is in the installation manual, inside the boiler.

Source: Vaillant, ecoTEC plus VU, VUW ..6/5-5 installation and maintenance instructions (0020244997_07, 2021): appendix E, overview of fault codes Checked 2026-09-28.

F.73 F.74 Water pressure sensor signal

What it means
F.73: the water pressure sensor signal is too weak (it reads the pressure as too low). F.74: the signal is too strong (pressure too high).
Likely cause
The water pressure itself, or the sensor, its cable, plug or earth. For F.74 Vaillant’s first measure is to drain water.
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: every check Vaillant lists for this code is in the installation manual, inside the boiler.

Source: Vaillant, ecoTEC plus VU, VUW ..6/5-5 installation and maintenance instructions (0020244997_07, 2021): appendix E, overview of fault codes Checked 2026-09-28.

F.75 Pump or water shortage

What it means
When the pump started, the pressure did not rise as it should.
Likely cause
The water pressure sensor, a blocked pump, air or too little water in the heating circuit, the bypass, or an external expansion vessel.
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: every check Vaillant lists for this code is in the installation manual, inside the boiler.

Source: Vaillant, ecoTEC plus VU, VUW ..6/5-5 installation and maintenance instructions (0020244997_07, 2021): appendix E, overview of fault codes Checked 2026-09-28.

F.83 Flow and return barely warming

What it means
When the burner starts, the flow and return sensors see too small a temperature change.
Likely cause
Too little water in the boiler, or the sensors not sitting properly on the pipes.
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: every check Vaillant lists for this code is in the installation manual, inside the boiler.

Source: Vaillant, ecoTEC plus VU, VUW ..6/5-5 installation and maintenance instructions (0020244997_07, 2021): appendix E, overview of fault codes Checked 2026-09-28.

A code that is not listed here

Vaillant’s code table says that, because it covers several products, some codes may not appear on a given boiler; its 2023 operating instructions add that for any fault code or limp-home (N.) code not in the householder’s table, action by a competent person is required. An S. code (S.04, S.34 …) is a status, not a fault.

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 Vaillant 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.