There’s nothing worse than switching on your Vaillant boiler as the weather cools, only to see a fault code appear and no hot water or heating.
These Vaillant boiler fault codes are designed to help both homeowners and Gas Safe engineers diagnose issues quickly. While they give a strong indication of the problem, they aren’t always definitive – further checks may be needed to confirm and fix the fault.
To save you time and money, we’ve put together the most common Vaillant error codes, what they mean, the likely causes, and when you should call an engineer.
What are Vaillant Boiler Fault Codes?
Vaillant boilers are programmed to display a fault code whenever something inside the system isn’t working as it should. These codes consist of letters and numbers (for example, F22 or F28) that correspond to a specific issue.
By showing these codes, the boiler makes it easier for an engineer (or sometimes the homeowner) to identify and fix the problem.
Why Do Fault Codes Appear?
Fault codes typically appear when:
- Water pressure is too low or too high
- There’s a gas supply problem
- A component (like a pump, fan, or sensor) has failed
- There’s an ignition fault
- The condensate pipe is blocked
- Internal wiring or electronics have developed an error
When to Call a Gas Safe Engineer
Some Vaillant fault codes are simple and safe for you to address – for example, topping up system pressure to clear F22 (low pressure).
However, if the error relates to ignition, gas supply, or any internal components, you should always contact a Gas Safe registered engineer. They have the tools, training, and legal certification to carry out repairs safely.
Vaillant Boiler Fault Code List (Full Table)
| Fault Code | Description | Common Causes | Suggested Actions |
| F1 | Ignition Fault | No gas supply, faulty ignition electrode, blocked burner | Check gas supply, call engineer |
| F20 | Overheating | Blocked heat exchanger, pump failure | Switch off boiler, call engineer |
| F22 | Low water pressure | Leak in system, insufficient pressure | Re-pressurise boiler, check for leaks |
| F23 | Flow/return temperature spread too large | Pump or system blockage | Call engineer |
| F24 | Dry fire/low water | Circulation issue | Top up system pressure |
| F25 | Flue gas temperature too high | Blocked flue, faulty sensor | Top up system pressure |
| F27 | Flame detection fault | Faulty flame sensor, gas supply issue | Call engineer |
| F28 | Ignition failed | Ignition failed | Check supply/pipe, reset once, call engineer |
| F29 | Flame goes out during operation | Gas pressure issues, flue problem | Call engineer |
| F32 | Fan fault | Fan failure or wiring fault | Engineer required |
| F49 | eBUS fault | Control unit or wiring issue | Call engineer |
| F61 | Gas valve fault | Gas valve defective or wiring fault | Engineer only |
| F62 | Gas valve doesn’t close properly | Gas safety risk | Switch off boiler, call engineer |
| F64 | Internal NTC sensor fault | Faulty sensor | Engineer required |
| F67 | Faulty flame signal | Electronics fault | Engineer required |
| F72 | Flow/return sensor error | Incorrect valves | Call engineer |
| F75 | No pressure rise detected | Pump failure or pressure sensor fault | Call engineer |
| F76 | Safety switch-off; heat exchanger | Overheating | Engineer required |
| F83 | Flow sensor temperature rise too quick | Pump or air lock | Call engineer |
| F84 | Return sensor temperature rise too quick | Same as F83 | Engineer required |
| F85 | Flow/return sensors reversed | Incorrect installation | Engineer required |
| F91 | Faulty air/flue system | Flue blocked or sensor fault | Call engineer |
| F92 | Combustion fault | Incorrect gas/air mixture | Engineer required |
| F93 | Combustion regulation fault | Gas/air imbalance | Engineer required |
| F94 | Internal safety lockout | Multiple causes | Engineer required |
| F95 | Safety device triggered | Overheat or system protection | Engineer required |
| F96 | Flame signal outside range | Faulty sensor | Engineer required |
| F97 | Control board error | PCB fault | Engineer required |
| F98 | Gas valve regulation fault | Faulty gas valve | Engineer required |
| F99 | Flame detection during shutdown | Gas valve issue | Engineer required |
Read more: How to reset your boiler in three easy steps
Top Vaillant Fault Codes Explained
F22 – Low Water Pressure
What the code means
The Vaillant F22 fault code will show on your boiler when your system water pressure is too low.
Possible causes
Leak in system, bleeding radiators, pressure loss.
- Leak in system
- Bleeding radiators
- Pressure loss
What you can check safely
- Top up your boilers pressure to 1/1.5 bar
When to call a gas safe engineer
If the F22 fault stays after you topped up the pressure, or returns shortly after the pressure has been topped up to the recommended amount, it’s time to call a gas safe engineer. They can check the boiler and heating system for any leaks, and advise on the right course of action.
F28 – Ignition Fault
What this code means
The F28 fault means your boiler is trying to ignite but can’t. Without ignition, the burner won’t light, so no heating or hot water. The boiler locks out as a safety measure.
Possible causes
- Gas supply turned off or interrupted
- Frozen condensate pipe preventing ignition
- Faulty ignition leads, spark electrode, or gas valve
- Low gas pressure
What you can safely check
- Ensure your gas supply is on and working elsewhere in the property (e.g. hob).
- Check that the condensate pipe outside isn’t frozen or blocked.
When to call a Gas Safe engineer
If the fault persists after basic checks, you’ll need an engineer to inspect ignition components and the gas valve.
F29 – Flame Goes Out
What this code means
The F29 code means your boiler ignites successfully but the flame keeps going out during operation.
Possible causes
- Low or fluctuating gas pressure
- Blocked burner or flue issue
- Faulty flame sensor or PCB
What you can safely check
There are no safe homeowner checks for this fault.
When to call a Gas Safe engineer
Call an engineer to test gas pressure, inspect the burner, and check the flue and sensors.
F20 – Overheating
What this code means
The F20 fault shows the boiler temperature has risen too high and the system has shut down to prevent damage.
Possible causes
- Pump failure or circulation problem
- Blocked heat exchanger or sludge buildup
- Thermistor or sensor fault
What you can safely check
- Turn off the boiler and allow it to cool.
- Check radiator valves are open and system pressure is normal.
When to call a Gas Safe engineer
An engineer should check the pump, heat exchanger, and sensors before restarting the system.
F61 – Gas Valve Fault
What this code means
The F61 code points to a problem with the gas valve or its electrical connections.
Possible causes
- Faulty gas valve
- Damaged or loose wiring
- PCB communication error
What you can safely check
There are no safe checks for homeowners.
When to call a Gas Safe engineer
You’ll need an engineer to test and, if necessary, replace the gas valve or control board.
F62 – Gas Valve Doesn’t Close
What this code means
This is a serious fault — the gas valve isn’t closing properly, which is a safety risk. The boiler locks out immediately.
Possible causes
- Stuck or damaged gas valve
- Fault in valve control electronics
What you can safely check
- Switch off the boiler and do not attempt to restart it.
When to call a Gas Safe engineer
Contact an engineer immediately to check and replace the valve if needed.
F75 – No Pressure Rise
What this code means
The boiler detects no increase in system pressure when the pump starts, indicating a problem with circulation or pressure sensing.
Possible causes
- Failed or seized pump
- Pressure sensor fault
- Blockage in the system
What you can safely check
There are no safe homeowner checks for this fault.
When to call a Gas Safe engineer
An engineer will test the pump, pressure sensor, and system flow to locate the issue.
F27 – Flame Detection Fault
What this code means
The boiler has detected a false flame signal — meaning it’s sensing a flame when there shouldn’t be one.
Possible causes
- Faulty gas valve leaking slightly open
- Defective PCB (main control board)
- Sensor malfunction
What you can safely check
There are no safe checks for this issue.
When to call a Gas Safe engineer
Call an engineer to inspect the gas valve, PCB, and flame sensing components.
Need Help With Your Vaillant Boiler?
If your Vaillant boiler shows a fault code that you can’t easily resolve with a reset or topping up the pressure, don’t hesitate to call in a Gas Safe registered professional.
Many error codes relate to ignition, gas, or safety issues – which should only ever be handled by a qualified engineer.
Contact us today for quick, reliable Vaillant boiler repairs and servicing.
