The F28 fault code on a Vaillant boiler indicates an ignition lockout. The boiler has attempted to fire - opening the gas valve and triggering the ignition electrodes - but has not detected a flame within the permitted number of attempts. As a safety measure, it shuts down and displays F28 on the control panel.
In many cases, a single reset resolves the fault if it was triggered by a temporary gas supply interruption or a brief system glitch. However, a recurring or persistent F28 means something has failed and requires diagnosis by a Gas Safe registered engineer. This guide explains each cause clearly, what you can check yourself, and precisely where the line is between a homeowner reset and a professional repair.
What Does the F28 Fault Code Mean?
F28 is Vaillant's ignition failure fault code. It appears on ecoTEC Plus, ecoTEC Pro, ecoFIT Pure, and older turboTEC models when the boiler's control board has attempted to establish a flame and received no confirmation from the flame sensor that ignition was successful.
The sequence the boiler follows every time it tries to fire is as follows: the control board signals the gas valve to open, allowing a metered amount of gas to flow to the burner. Simultaneously, the ignition electrodes produce a spark. The flame sensor - an ionisation probe - then detects whether a flame has been established by measuring a small electrical current through the flame. If no current is detected within a fixed time window, the gas valve closes, and the board logs an ignition failure. After a set number of consecutive failures, the boiler enters lockout and displays F28.
The Four Most Common Causes of an F28 Fault
Understanding the likely cause determines the correct course of action. These four faults account for the vast majority of F28 occurrences on Vaillant boilers.
Gas Supply Interruption
The gas supply to the boiler is insufficient or absent. This may be a supply disruption in the local area, a pre-payment meter that has run out, or a gas valve within the boiler that is not opening correctly.
Faulty Ignition Electrodes
The electrodes that produce the ignition spark are worn, cracked, incorrectly gapped, or coated in carbon deposits. They are producing a weak or absent spark that is insufficient to ignite the gas at the burner.
Frozen or Blocked Condensate Pipe
The plastic condensate pipe, which routes acidic wastewater from the boiler to the drain, has frozen solid during cold weather or become blocked. The resulting back-pressure causes the boiler to fault and lock out.
PCB or Wiring Fault
The printed circuit board - the boiler's control unit - has developed a fault, or a wiring connection between the PCB, gas valve, or electrodes has failed. The PCB is unable to correctly sequence or monitor the ignition process.
| Cause | Typical Signs | Who Can Fix It |
|---|---|---|
| Gas supply interruption | Other gas appliances (hob, gas fire) also not working; pre-payment meter balance empty; F28 appears suddenly with no prior warning | CHECK YOURSELF Verify other appliances; top up meter or contact supplier |
| Faulty ignition electrodes | F28 appears repeatedly; boiler makes clicking sound but does not fire; fault returns after each reset | ENGINEER REQUIRED Electrode inspection, cleaning, or replacement |
| Frozen condensate pipe | F28 appears during or after a cold spell; gurgling sound from the boiler; condensate pipe visibly iced over | CHECK YOURSELF Thaw with warm water - see Section 4 |
| PCB or wiring fault | Multiple fault codes appearing; boiler behaves erratically; F28 persists after electrodes and gas supply have been confirmed serviceable | ENGINEER REQUIRED Electrical testing, PCB diagnosis, repair or replacement |
How to Reset a Vaillant Boiler Showing F28
A reset is always the correct first step when F28 appears, provided there is no smell of gas. The reset procedure varies slightly between Vaillant models, but the following steps cover the most common ecoTEC and ecoFIT range.
Note the Fault Code
Before resetting, confirm that the display shows F28 (or F.28 on some models). Take a note of any additional information displayed. If the boiler is alternating between two fault codes, both are relevant and should be mentioned when calling an engineer.
Check Other Gas Appliances First
Before pressing reset, go to your gas hob or gas fire and check whether it lights normally. If no gas appliances are working, the issue is with your gas supply, not the boiler. Contact your gas supplier or check your pre-payment meter before proceeding.
Locate and Press the Reset Button
On most Vaillant ecoTEC Plus and Pro models, the reset button is on the control panel - typically a button marked with a flame symbol and a cross, or labelled "Reset". Press and hold it for approximately 3 seconds until the display changes and the boiler begins its ignition sequence.
On older turboTEC models, the reset may be a red button visible through the casing fascia. Refer to your boiler's instruction manual if you are unsure of its location.
Listen and Observe
A healthy restart sounds like this: the fan starts, gas valve opens, electrodes click, the burner ignites within a few seconds, and the fan settles into a steady operating speed. The display should return to the normal operating screen. If the boiler fires and runs without returning to F28 within the next few minutes, the reset has been successful.
Stop After Two or Three Attempts
If the boiler returns to F28 after restarting, wait a few minutes and attempt one further reset. Do not reset more than two or three times in total. Repeated resets without resolving the underlying fault accelerate wear on the electrodes, the gas valve, and the pump. At this point, the fault requires a professional diagnosis.
Thawing a Frozen Vaillant Condensate Pipe
Condensing boilers - which includes all modern Vaillant ecoTEC models - produce acidic wastewater as a by-product of high-efficiency combustion. This condensate drains out through a plastic pipe, usually 21.5mm or 32mm diameter, routed through an external wall to the nearest drain. In cold weather, the section of pipe outside the building can freeze, causing a back-pressure fault that triggers F28 lockout.
A gurgling or bubbling sound coming from inside the boiler when it tries to fire is a strong indicator of a blocked condensate pipe. Visual inspection of the external pipe for ice is usually conclusive.
How to Thaw the Pipe
- Locate the external condensate pipe - typically white or grey plastic, exiting through the wall near the boiler
- Pour warm (not boiling) water over the frozen section - a kettle that has sat for 10 minutes is ideal
- Alternatively, apply a warm water bottle or a proprietary pipe heat wrap to the affected area
- Work along the pipe from the boiler outward until water begins to flow freely
- Once cleared, return indoors and reset the boiler
Preventing Future Freezes
- Lag the external section of condensate pipe with foam pipe insulation before each winter
- Ensure the pipe terminates at or below the level of the drain grate - a raised open end is more vulnerable to freezing
- Consider rerouting the condensate to an internal drain if the pipe run is long or particularly exposed
- Ask your engineer about a Vaillant-compatible condensate syphon trap which can reduce freeze risk
When the F28 Fault Requires a Gas Safe Engineer
Several F28 causes cannot be safely addressed by anyone other than a Gas Safe registered engineer. Attempting to inspect, clean, or replace gas components without the appropriate registration is illegal under Gas Safety (Installation and Use) Regulations 1998 and creates serious safety risks.
Faulty Ignition Electrodes
Ignition electrodes are a wear part. They develop hairline cracks in the ceramic insulator over time, and the electrode tip erodes with repeated use. When the ceramic cracks, high-voltage spark energy follows the path of least resistance through the crack rather than bridging across to the burner, resulting in no usable spark. Carbon deposits from incomplete combustion can also coat the tip, preventing a clean arc. An engineer can inspect the electrodes visually, test the ignition lead continuity, and clean or replace the assembly. Electrode replacement on a Vaillant ecoTEC is a straightforward task for an experienced engineer and is one of the more cost-effective F28 repairs.
Gas Valve Problems
If the gas valve is not opening sufficiently during the ignition sequence - whether due to low gas pressure, a sticky valve mechanism, or a failing valve actuator - insufficient gas reaches the burner for reliable ignition. An engineer will check the gas inlet pressure with a manometer, verify the valve is receiving the correct control signal from the PCB, and assess whether the valve requires cleaning, adjustment, or replacement.
PCB Failure
The printed circuit board controls every aspect of the ignition sequence - it sends the open signal to the gas valve, triggers the electrodes, monitors the flame sensor, and interprets the result. A PCB fault can mimic almost any other boiler fault because it sits at the centre of every control loop. Engineers typically diagnose a PCB fault by a process of elimination - once gas supply, electrodes, flame sensor, and wiring have all been confirmed serviceable and F28 persists, the PCB becomes the primary suspect. PCB replacement is the most significant repair in terms of cost but is straightforward in terms of labour on most Vaillant models.
Do's & Don'ts with an F28 Fault
Do
- Check for a gas smell before doing anything else
- Check that other gas appliances in the property are working
- Attempt one or two resets if no gas smell is present
- Inspect the condensate pipe during cold weather before calling an engineer
- Note the exact fault code and any other codes displayed
- Contact a Gas Safe registered engineer if F28 returns after resetting
- Book an annual service to reduce the likelihood of repeat F28 faults
Don't
- Attempt to reset if you can smell gas at any point
- Reset the boiler more than two or three times without investigating
- Attempt to inspect, clean, or replace electrodes or the gas valve yourself
- Pour boiling water directly onto a frozen condensate pipe
- Assume the fault has resolved itself if F28 clears but later returns
- Ignore a recurring F28 - intermittent faults worsen without intervention
- Use any gas appliance if you can smell gas anywhere in the property
Prevention Checklist: Reducing the Risk of F28
Annual Maintenance
- Boiler serviced annually by a Gas Safe registered engineer
- Ignition electrodes inspected and cleaned or replaced as part of service
- Flame sensor and ignition lead continuity checked during service visit
- Gas inlet pressure verified to Vaillant's minimum specification
- Condensate trap cleaned and syphon operation confirmed
Before Winter
- External condensate pipe lagged with foam pipe insulation
- Condensate pipe termination point confirmed clear and unobstructed
- Pre-payment meter balance checked and credit maintained during cold spells
- Boiler fired and tested before the heating season begins
- Carbon monoxide detector installed and tested - battery replaced if over 12 months old
