Vaillant boilers are amongst the most reliable heating systems in the UK, but like all mechanical equipment, they occasionally display fault codes that can leave homeowners puzzled. These error codes serve as your boiler's way of communicating specific problems, ranging from simple fixes to complex issues requiring professional attention.
Understanding the meaning of these Vaillant boiler fault codes can help you avoid unnecessary stress, reduce repair costs, and resolve issues more quickly. While some faults can be resolved with basic DIY skills, most require the expertise of a Gas Safe registered engineer for safety and legal compliance.
This comprehensive guide examines the most common Vaillant boiler problems, explains what each fault code signifies, and helps you determine when professional intervention is necessary.
- What We Discuss Here
- » Understanding Vaillant Boiler Error Codes
- » Most Common Vaillant Boiler Fault Codes
- - F13: Cylinder Temperature Sensor Short Circuit
- - F22: Low Water Pressure
- - F25: Flue Gas Temperature Too High
- - F28: Ignition Failure
- - F29: Flame Loss During Operation
- - F61: Control Unit Timing Error
- - F62: Flame Switch-Off Detected After Delay
- - F75: Pressure Sensor Fault
- » Vaillant Combi Boiler Fault Codes
- - ecoTEC Plus and ecoTEC Pro Combi Boilers
- - ecoFIT Pure Combi Boilers
- - System and Regular Boiler Fault Codes
- » When Can You Fix Vaillant Boiler Faults Yourself?
- » Professional Boiler Repair Costs
- » Preventing Vaillant Boiler Faults
- » Taking Action on Vaillant Boiler Problems
- » Frequently Asked Questions
Understanding Vaillant Boiler Error Codes
Vaillant boilers use alphanumeric fault codes to identify specific malfunctions within the system. These codes appear on your boiler's digital display and correspond to particular components or operational issues.
Each Vaillant boiler model, whether a combi, system, or heat-only, has its own set of fault codes, although many codes are consistent across different models. The codes typically begin with the letter "F" followed by two digits, making them relatively easy to identify.
Your boiler manual contains the complete list of fault codes specific to your model. However, understanding the most frequent codes can help you assess the urgency of the situation and determine whether immediate help is required for a professional boiler repair.
Most Common Vaillant Boiler Fault Codes
F13: Cylinder Temperature Sensor Short Circuit
The F13 error indicates that the cylinder temperature sensor has short-circuited, preventing the system from accurately reading the temperature.
Possible causes:
- Fault in the electrical connection of the sensor
- Defective cylinder temperature sensor
How to fix it:
This is not a user-fixable issue. A professional heating engineer is required to test, repair or replace the sensor safely.
F22: Low Water Pressure
The F22 fault code is arguably the most common Vaillant boiler problem. It indicates that your boiler's water pressure has dropped below the minimum operating threshold, typically below 1 bar.
Possible causes include:
- Natural pressure loss over time
- Small leaks in the heating system
- Recently bled radiators
- Faulty pressure relief valve
This is one of the few fault codes that homeowners can often resolve themselves by repressurising the boiler using the filling loop. Check your boiler pressure gauge, if it reads below 1 bar, you'll need to top up the system.
F25: Flue Gas Temperature Too High
The F25 boiler fault appears when the boiler detects that the flue gas temperature has exceeded safe operating limits.
Possible causes:
- Faulty or damaged wiring harness
- The flue gas temperature monitor has triggered (if installed)
How to fix it:
A qualified engineer must inspect this fault. High flue temperatures can be dangerous and should not be handled as a DIY repair.
F28: Ignition Failure
The F28 fault code appears when your boiler fails to ignite during startup. This fault prevents your boiler from producing heat or hot water.
Common causes include:
- Gas supply issues (closed isolation valve or low gas pressure)
- Faulty ignition electrode
- Blocked flue or air intake
- Defective gas valve
- PCB malfunction
F28 faults typically require professional diagnosis, as they involve gas components and electrical systems that should only be handled by qualified engineers.
F29: Flame Loss During Operation
Similar to F28, the F29 code indicates that your boiler successfully ignited but then lost the flame during operation.
This can result from:
- Interrupted gas supply
- Blocked flue causing poor combustion
- Faulty flame detection electrode
- Problems with the burner assembly
Both F28 and F29 codes require immediate attention from a Gas Safe engineer, as they involve gas safety components.
F61: Control Unit Timing Error
The F61 fault code appears when the boiler's internal control board (ASIC or microcontroller) fails to operate within the expected timing sequence. This stops the boiler from working safely.
Possible causes include:
- There is a fault in the electrical wiring to the gas valve
- Defective or damaged PCB (Printed Circuit Board)
- Loose or corroded connections within the control unit
How to fix it:
F61 is not a DIY-safe fault. It involves gas valve control and electronic components, so only a Gas Safe registered engineer should inspect and repair it.
F62: Flame Switch-Off Detected After Delay
This F62 fault indicates that the burner flame did not shut off immediately as expected, which can affect safe boiler operation.
Possible Causes:
- Defective gas valve
- Faulty PCB (Printed Circuit Board)
- Damaged or incorrectly positioned ignition electrode
How to fix it:
This is not a DIY repair. The issue involves gas and electrical components, so it must be inspected by a qualified heating engineer. Trying to repair it yourself can be unsafe and may also void your warranty.
F75: Pressure Sensor Fault
The F75 fault code relates to issues with the boiler's pressure sensor or pump operation. It typically appears when the system detects insufficient pressure change during pump startup.
Potential causes include:
- Defective water pressure sensor
- Failed internal heating pump
- Low system pressure
- Faulty expansion vessel
- Air trapped in the system
This fault requires professional diagnosis to determine whether the issue lies with the sensor, pump, or broader system problems.
Vaillant Combi Boiler Fault Codes
Vaillant's combi boiler ranges share many common fault codes, though specific models may have unique variations.
ecoTEC Plus and ecoTEC Pro Combi Boilers
These popular models share identical fault codes:
- F10: Flow temperature sensor short circuit
- F13: Cylinder temperature sensor fault
- F23: Temperature spread too large (poor circulation)
- F24: Temperature rising too quickly
- F25: Flue gas temperature too high
- F32: Fan fault or disconnection
- F64: Electronics or sensor defect
- F67: Implausible flame signal
- F73/F74: Water pressure sensor faults
ecoFIT Pure Combi Boilers
The ecoFIT Pure range uses a simplified fault code system:
- F10: Flow sensor short circuit
- F22: Low water pressure safety switch-off
- F28/F29: Ignition and flame loss faults
- F49: eBus voltage too low
- F65: Electronics overheating
- F71: Flow/return sensor fault
- F87-F89: Component faults (electrodes, gas valve, pump)
System and Regular Boiler Fault Codes
Vaillant's system and regular boilers have similar fault code patterns, with some variations based on their different operational requirements.
System boilers, which work with separate hot water cylinders, may display additional codes related to cylinder sensors and diverter valve operations.
Regular boilers, which work with traditional systems that use an individual hot water cylinder and cold-water storage tank, are mostly affected by faults related to the heating circuit.
When Can You Fix Vaillant Boiler Faults Yourself?
Very few Vaillant boiler faults are suitable for DIY repair. Gas appliance work is heavily regulated, and most repairs must be carried out by a professional Gas Safe registered engineer for safety and legal reasons.
The primary exception is the F22 low-pressure fault, which you can often resolve by repressurising your boiler through the filling loop.
This process involves:
- Turning off your boiler
- Locating the filling loop (usually beneath the boiler)
- Opening both filling loop valves slowly
- Watching the pressure gauge rise to between 1 and 1.5 bar
- Closing both valves and restarting the boiler
However, if your boiler repeatedly loses pressure, this suggests an underlying leak that requires professional attention. Continuing to repressurise a leaking system can cause further damage and safety issues.
Professional Boiler Repair Costs
The cost of professional Vaillant boiler repairs varies significantly depending on the fault's complexity and required parts.
| Boiler repressurising | £30–£60 |
| PCB replacement | £360–£790 |
| Fan replacement | £160–£490 |
| Heat exchanger replacement | £460–£840 |
| Gas valve repairs | £200–£400 |
| Pump replacement | £150–£300 |
Preventing Vaillant Boiler Faults
Regular maintenance significantly reduces the likelihood of fault codes appearing. Annual boiler servicing by a qualified engineer helps identify potential issues before they cause complete breakdowns.
Key maintenance practices include:
- Annual boiler servicing
- Regular pressure checks
- Bleeding radiators when needed
- Keeping vents and flues clear
- Monitoring for leaks or unusual noises
Taking Action on Vaillant Boiler Problems
Understanding Vaillant boiler fault codes empowers you to make informed decisions about repairs and boiler maintenance. While most faults require professional attention, knowing what each code means helps you communicate effectively with engineers and avoid unnecessary panic.
Remember that attempting complex repairs yourself can void your warranty, create safety hazards, and potentially cause more expensive damage. When in doubt, always consult our gas heating engineer who specialises in Vaillant boilers.
If you're experiencing persistent fault codes or your boiler is beyond economical repair, consider upgrading to a new, more efficient model.
To learn more about common boiler faults, visit our ‘Boiler Error Codes’ blog section for detailed guidance and troubleshooting insights.
Frequently Asked Questions
How to diagnose boiler faults?Strange sounds are often the first warning sign. If your boiler starts vibrating, banging, gurgling, or whistling, it usually means something isn’t right. Trapped air in the system is a common cause, though low water pressure or a weak pump can create similar noises. Any unusual sound coming from your boiler or heating system should be treated seriously. Noise is rarely accidental—it’s the boiler’s way of signalling a fault that needs attention.
How to fix F28 boiler fault?The F28 error code points to an ignition failure, meaning the boiler has tried three times to ignite and couldn't start. This often happens due to a gas supply issue, a malfunctioning gas valve, or a faulty ignition electrode. Before calling an engineer, check that the gas supply is on, try resetting the boiler, and ensure the flue isn’t blocked.
What is the correct pressure for a Vaillant boiler?The ideal pressure for a Vaillant boiler is usually between 1 and 1.5 bar. You can check this on the pressure gauge. If the reading is below this range, the system likely needs repressurising. A sudden drop in pressure can also indicate a possible leak somewhere in the heating system.
