Boiler fault codes

Vaillant boiler faults: fault codes, causes and parts

All 228 fault codes for Vaillant boilers, plus the faults that show no code. Type your code or choose your symptom.

In brief

A fault on a Vaillant boiler usually means that the appliance has locked out or is reporting a fault code. Note the code on the display and look it up below: for each code you will find its meaning, checks you can safely do yourself and the parts that are often checked or replaced. Frequently searched: F.22 (system water pressure too high, Defective expansion…), F.75 (no pressure jump is measured when the…), Emergency mode “Speed” (no speed signal from fan). No code? Check the water pressure (1.5 to 2 bar when cold), the power supply and the gas tap, and reset once. If the fault comes back, call in a certified gas engineer. If you smell gas, call your national gas emergency number or 112 immediately.

Most searched Vaillant faults

These are the codes that visitors to this site search for most often.

F.22System water pressure too high, Defective expansion…F.75No pressure jump is measured when the…Emergency mode “Speed”No speed signal from fan F.12No signal on user interface F.62Gas valve unit defective (gas solenoid valves…F.17Insufficient mains voltage < 170 V ~…F.25Wrong gas type F.27Incorrect ionisation measurement

See the 25 most searched fault codes of 2026, with the figures behind them →

Vaillant boiler faults without a code on the display

Not every fault shows a code. These are the symptoms we come across most often with Vaillant appliances, with what you can check yourself before calling someone out.

No hot water, but the heating works

The boiler heats the radiators but not the tap water. Check that the hot water function is switched on in the display (on some appliances it is called DHW mode or summer mode) and that the hot water valve under the boiler is open. If the water stays cold, this usually points to a fouled or leaking DHW heat exchanger, a faulty diverter valve or a DHW sensor giving incorrect readings. View the matching parts → Read the full explanation: no hot water, six causes →

Water pressure keeps dropping

Top up to 1.5–2 bar when cold and bleed the radiators. If the pressure drops again within a few days, the expansion vessel has often lost its pre-charge or there is a small leak in the system. A pressure relief valve that keeps dripping gives the same symptom. View the matching parts → Read the full explanation: water pressure keeps dropping – expansion vessel or leak? →

Boiler shuts down and locks out

The appliance shuts down and only starts again after a reset. Reset once; if the fault comes straight back, leave the reset button alone. Repeatedly resetting a locked-out boiler is not without risk: the safety device trips for a reason. View the matching parts → Read also: is the boiler cutting out because of a sensor or overheating? →

Radiators stay cold

Check that the thermostat is calling for heat, that the radiator valves are open and that the system has enough pressure. If everything stays cold while the boiler is firing, the circulation pump is often not running or the diverter valve is stuck in the hot water position. View the matching parts → Read the full explanation: radiators stay cold →

Ticking, humming or boiling noise

A banging noise at start-up usually points to air or limescale in the heat exchanger. A humming pump may have a worn bearing. A rattling fan is often rubbing or dirty. View the matching parts → Read also: replacing the fan – what to rule out first →

Water under the boiler

Check where it is coming from: the condensate drain (siphon blocked or cracked), the pressure relief valve (pressure too high or faulty expansion vessel) or a fitting. If in doubt, switch the boiler off and close the water supply. View the matching parts → Read the full explanation: water under the boiler – condensate or a leak? →

Vaillant boiler fault codes 1 to 150 of 228

The code list is split over 2 pages. Looking for a specific code? Use the search at the top of this page: it searches all 228 codes in one go.

Code	Cause
Err	Interface communication
F.00	The signal from the flow temperature sensor is open circuit.
F.00	Open circuit of flow NTC
F.00	Open circuit of flow temperature sensor (NTC)
F.00	Open circuit of central heating flow sensor
F.00	Open circuit of flow temperature sensor
F.00	No fault
F.01	Ignition fault (no flame detection), No gas/gas tap closed, No ignition
F.01	The signal from the return temperature sensor is open circuit.
F.01	Open circuit of return temperature sensor
F.01	Open circuit of return sensor
F.01	Open circuit of return temperature sensor (NTC)
F.02	The signal from the hot water connection temperature sensor is open circuit.
F.02	Open circuit of DHW outlet sensor
F.02	Temperature limiter > 105 °C
F.03	Dry-fire protection
F.03	The signal from the cylinder temperature sensor is open circuit.
F.03	Open circuit of cylinder temperature sensor/domestic hot water temperature sensor
F.04	Fan fault
F.04	See F 1
F.05	Flame signal invalid
F.05	Limit fault, flow temperature too high (pump not running, voltage present)
F.05	Open circuit of outside flue gas sensor
F.05	Open circuit in outdoor temperature sensor
F.06	Open circuit of inside flue gas sensor
F.06	Flow NTC fault (the limit thermostat must be reset manually), NTC is broken, or wiring not in order, Burner control unit or pump cable defective, Radiators are closed or air in boiler or system, Pump setting too low, Flow NTC deviates or is loose:
F.06	Electronics
F.07	DHW NTC fault, NTC is broken, or wiring not in order
F.07	Flow sensor
F.08	Return sensor
F.09	Domestic hot water sensor
F.09	Water pressure sensor broken or wiring faulty
F.09	Short circuit or open circuit of domestic hot water sensor
F.10	Short circuit of flow sensor
F.10	Short circuit in flow NTC
F.10	Short circuit of flow sensor
F.10	Short circuit of flow temperature sensor
F.10	Short circuit of flow temperature sensor
F.10	Return NTC broken, or wiring not in order
F.105	Gas valve offset in D.052 set incorrectly
F.11	Short circuit of return sensor
F.11	Short circuit of return sensor
F.11	Short circuit of return temperature sensor
F.11	Short circuit of return temperature sensor
F.11	No signal on the main PCB, Fan defective
F.12	No signal on user interface
F.12	Short circuit of DHW outlet sensor
F.12	Short circuit at DHW outlet temperature sensor (NTC)
F.13	Short circuit of cylinder temperature sensor
F.14	Flow temperature greater than 95 °C
F.14	Circulation problem
F.15	Short circuit in outdoor temperature sensor
F.15	Short circuit of outside flue gas sensor
F.16	Short circuit of inside flue gas sensor
F.16	Incorrect flame detection (flame visible for a short time, then lost)
F.16	Short circuit in room temperature sensor
F.17	Insufficient mains voltage < 170 V ~ Defective main PCB
F.18	User interface fault
F.19	CH NTC (flow and/or return) open circuit, Connector detached from NTC, Break in wiring, NTC defective
F.194	Mains adapter of the PCB defective
F.195	Variations (voltage too low) in the supply voltage
F.196	Supply voltage too high
F.20	Maximum limit protection (flow temperature too high)
F.20	Temperature limiter has tripped
F.20	Max. temperature limiter (flow temperature too high)
F.20	Software not compatible
F.20	Safety shutdown: safety temperature limiter
F.20	Temperature limiter has been activated
F.21	System water pressure too low, Leak in the system or Expansion vessel defective
F.22	System water pressure too high, Defective expansion vessel System filled to too high a pressure
F.22	Safety shut-down: water shortage
F.22	Dry-fire protection
F.22	Dry-fire protection (no water in appliance)
F.22	Boiling dry
F.22	Low water protection active dry-fire protection active
F.23	Low water (temperature difference between flow and return sensors too large)
F.23	Low water (temperature spread too large)
F.23	Low water; temperature spread between flow and return temperature sensors too large
F.23	Safety shutdown: temperature spread too large
F.24	Safety shutdown: temperature rise too rapid
F.24	Return temperature > 90 °C, Insufficient circulation or No circulation
F.24	Water shortage (temperature rising too quickly)
F.24	Low water (temperature rises too quickly)
F.24	Low water, temperature rise too rapid
F.24	Low water protection contact closed without the pump running
F.25	Wrong gas type
F.25	Open circuit in wiring harness of burner, fan and gas valve
F.25	Flow temperature rising too quickly. See F 05. System resistance too high, bypass insufficient
F.26	Too large a temperature difference between flow and return temperature in central heating mode
F.26	Gas valve stepper motor current implausible
F.26	Problem with gas valve unit
F.26	Modulation coil current unexplainably high
F.26	Gas valve fault
F.27	Incorrect ionisation measurement
F.27	Ionisation signal with closed gas valves
F.27	Incorrect ionisation signal
F.27	Incorrect ionisation signal
F.27	Safety shut-down: flame simulation
F.28	The flame signal was not detected during the ignition phase
F.28	Appliance does not start: ignition attempts failed during start-up
F.29	Lockout during operation: re-ignition unsuccessful
F.29	Ionisation protection (flame goes out during operation and subsequent ignition attempts failed)
F.29	Flame extinguished during operation
F.29	Flame goes out during operation and the subsequent ignition attempts failed
F.29	Flames extinguish during operation and subsequent ignition attempts fail
F.30	Open circuit of boiler temperature sensor
F.317	Volume sensor in hot water circuit not connected/loose/defective
F.318	Diverter valve motor fault
F.32	Air-side frost protection (fan speed deviation)
F.32	Air-side frost protection is active, no tacho signal from the fan
F.32	Frost protection active
F.32	Fan speed deviation
F.320	Pump blocked
F.321	Pump defective
F.322	Pump overheated
F.323	Pump running dry
F.324	Cable to pump is defective
F.325	Pump blocked and/or defective
F.326	Diverter valve motor fault
F.327	Hot water circuit clogged/blocked
F.33	Air pressure switch does not switch
F.33	Frost protection has responded three times and is active
F.33	Air-side frost protection is active, no signal from the pressure switch
F.33	Pressure switch does not switch
F.344	Ionisation/control electrode fault
F.346	Failed burner start
F.35	Flue gas outlet/air supply fault
F.35	Flue gas outlet/air supply fault
F.36	TTB fault
F.36	n/a
F.36	Flue gas outlet recognised
F.363	Display EEPROM defective
F.37	Incorrect fan speed during normal operation
F.37	Fan speed too high or too low during operation
F.37	Speed deviation during operation
F.37	Speed deviation during operation
F.38	Short circuit of fan
F.38	Short circuit in the fan
F.38	Short circuit of fan
F.390	Initialisation missing
F.40	The amount of air is too low
F.40	Fault in the flue gas path due to recirculation or flue gas blockage
F.41	No gas-side setting (new electronics)
F.42	Coding resistor error
F.43	No valid appliance variant
F.44	Ionisation too low
F.44	Fault in the flue gas path due to recirculation or flue gas blockage
F.46	Short circuit: temperature sensor at the water inlet of the solar circuit
F.47	Disconnection of the hot water sensor in the cylinder outlet
F.49	Voltage on EBUS too low

Order parts for your Vaillant boiler

Vaillant parts are available from Instalpunt; shipping to EU countries from €17.95 (€24.95 for more distant countries), usually 3–5 working days.

Parts by Vaillant series: ecoTEC plus · ecoTEC pro.

View Vaillant parts →

Frequently asked questions about Vaillant faults

What does a fault on a Vaillant boiler mean?

The appliance is reporting that it has shut itself down or cannot continue. The code on the display points to the group of causes: ignition, water pressure, a sensor, the fan or communication with the thermostat. The table above shows, for each code, what Vaillant means by it.

How do I reset my Vaillant boiler?

The reset button is on the control panel of your Vaillant boiler; on most appliances you hold it down for two to five seconds until the display responds. Reset once. If the same fault comes straight back, resetting will not solve it and a part or a setting needs attention. Always consult the manual for your model: the position of the button and how long to press it differ per series.

Can I fix a Vaillant fault myself?

You can top up the water pressure, bleed the system, replace filters and clean a siphon yourself. Anything behind the sealed section of a gas appliance is a different matter: work on the gas valve, burner, heat exchanger and flue must be carried out by a certified gas engineer.

How much does it cost to fix a fault?

That depends on the cause and whether a part is needed. With us you can see the part straight away with its price and delivery time, so you know where you stand in advance. If you call in an engineer, always ask about the call-out charge and hourly rate before they come.

Where can I find the right part for my Vaillant?

Check the rating plate for the exact type and use it to search our catalogue, or search directly by the manufacturer number of the faulty part. Each part lists the appliances it fits.