Vaillant fault codes
Check our list of Vaillant error codes to find out what the fault indicates and what steps you can take next.
Not sure what your Vaillant boiler error means?
Check our list of Vaillant fault codes to find out what the fault indicates and what steps you can take next.
Vaillant fault codes list
If your Vaillant boiler is showing an error or fault code, don’t panic — these codes are designed to help identify what’s wrong. Below, you’ll find a full list of Vaillant fault codes, what they mean, and what you can do next.
Vaillant F13 fault code
The cylinder temperature sensor has short-circuited.
Possible cause:
- Fault in the electrical connection for the cylinder temperature sensor
- Cylinder temperature sensor defective
How to fix it:
This is a case for an expert. Don’t try to solve it yourself as it can be dangerous.
To solve it just call us on 01275 878000 or click here to contact us. We can solve your issue.
Vaillant F20 fault code
The safety cut-out (SCO) interrupts the gas valve control. The gas valve was closed because the temperature of the flow or return temperature sensor has exceeded the maximum limit.
Possible cause:
- Insufficient/no water in the product.
- Fault in the electrical connection for the flow temperature sensor
- Fault in the electrical connection for the return temperature sensor
- Flow temperature sensor defective
- Return temperature sensor defective
- The pump is defective.
- 3-port diverter valve defective or blocked
- Water pressure sensor defective Volume flow sensor defective
- Black discharge via the ignition cable, ignition plug or ignition electrode
How to fix it:
This is a case for an expert. Don’t try to solve it yourself as it can be dangerous.
To solve it just call us on 01275 878000 or click here to contact us. We can solve your issue.
Vaillant F22 fault code
Low water pressure in the system
This is one of the most common Vaillant fault codes.
Possible cause:
- Insufficient/no water in the product.
- Fault in the electrical connection for the water pressure sensor
- Cable for the pump/for the water pressure sensor is loose/not connected/defective
- Water pressure sensor defective
- Pump operation interrupted
- Solenoid valve for the automatic filling device defective
- Internal expansion vessel defective
How to fix it:
This is a case for an expert. Don’t try to solve it yourself as it can be dangerous.
To solve it just call us on 01275 878000 or click here to contact us. We can solve your issue.
Vaillant F23 fault code
The temperature spread between the flow/return is too great.
Possible cause:
- Air in the product
- Fault in the electrical connection for the flow temperature sensor
- Fault in the electrical connection for the return temperature sensor
- Flow temperature sensor defective
- Return temperature sensor defective
- Pump blocked
- Pump runs with insufficient output
- 3-port diverter valve defective or blocked
- Internal expansion vessel defective
How to fix it:
This is a case for an expert. Don’t try to solve it yourself as it can be dangerous.
To solve it just call us on 01275 878000 or click here to contact us. We can solve your issue.
Vaillant F25 fault code
The flue gas temperature is too high.
Possible cause:
- Wiring harness defective
- If present: flue gas temperature monitor has triggered
How to fix it:
This is a case for an expert. Don’t try to solve it yourself as it can be dangerous.
To solve it just call us on 01275 878000 or click here to contact us. We can solve your issue.
Vaillant F27 fault code
A flame signal has been detected while the burner is off.
Possible cause:
- Gas solenoid valve not leaktight
- PCB defective
How to fix it:
This is a case for an expert. Don’t try to solve it yourself as it can be dangerous.
To solve it just call us on 01275 878000 or click here to contact us. We can solve your issue.
Vaillant F28 fault code
The flame signal was not detected during the ignition phase.
This is one of the most common Vaillant fault codes.
Possible cause:
- Hard ignition takes place
- Gas isolator cock closed
- Gas connection pressure too low
- Air in the gas supply (e.g. during initial start-up)
- Incorrect gas type set
- Gas valve assembly offset incorrectly stored in D.052 and in. D.182
- Air intake pipe blocked
- Ignition failed
- Gas flow pressure too low
- Control electrode comes into contact with the burner
- Ignition electrode defective
- Burner defective
- Condensate pump (if fitted) defective
- Gas valve assembly defective/incorrect spare part gas valve assembly
- Interruption in the wiring harness
- Earthing defective
- Ionisation flow interrupted
- Fault in the flue gas route caused by recirculation or flue gas blockage
- Ignition transformer not connected
- Defective ignition transformer
- PCB defective
- Condensate in the combustion chamber due to a blocked condensate discharge
How to fix it:
This is a case for an expert. Don’t try to solve it yourself as it can be dangerous.
To solve it just call us on 01275 878000 or click here to contact us. We can solve your issue.
Vaillant F29 fault code
The ignition following a flame loss during operation was unsuccessful.
Possible cause:
- The gas supply is interrupted
- Fault in the flue gas route caused by recirculation or flue gas blockage
- Earthing defective
- Air intake pipe blocked
- Ignition misfire
- Condensate pump (if fitted) defective
- Control electrode comes into contact with the burner
- Burner defective
- Interruption in the wiring harness
- Ignition electrode defective
- Gas valve assembly defective/incorrect spare part gas valve assembly
- Condensate in the combustion chamber due to a blocked condensate discharge
How to fix it:
This is a case for an expert. Don’t try to solve it yourself as it can be dangerous.
To solve it just call us on 01275 878000 or click here to contact us. We can solve your issue.
Vaillant F61 fault code
The ASIC or µControl does not run in the defined times.
Possible cause:
- Fault in the electrical connection for the gas valve assembly
- PCB defective
How to fix it:
This is a case for an expert. Don’t try to solve it yourself as it can be dangerous.
To solve it just call us on 01275 878000 or click here to contact us. We can solve your issue.
Vaillant F62 fault code
The flame switch-off was detected after a delay.
Possible cause:
- Gas valve defective
- PCB defective
- Ignition electrode defective
How to fix it:
This is a case for an expert. Don’t try to solve it yourself as it can be dangerous.
To solve it just call us on 01275 878000 or click here to contact us. We can solve your issue.
Vaillant F75 fault code
The pressure change is too low when the heat pump is started up.
Possible cause:
- Water pressure sensor defective
- Internal heating pump defective
- System pressure too low Internal expansion vessel defective
- Insufficient/no water in the product.
- Air in the product
- Open circuit in wiring harness (LIN cable)
- Interruption in the wiring harness
How to fix it:
This is a case for an expert. Don’t try to solve it yourself as it can be dangerous.
To solve it just call us on 01275 878000 or click here to contact us. We can solve your issue.
What to Do If You See a Vaillant Fault Code
Before calling an engineer, you can try simple checks like topping up the boiler pressure or resetting the boiler. However, if the Vaillant fault code reappears, it’s best to call a Vaillant MasterTEC installer to avoid further damage.
Vaillant Guarantee
In many cases, yes — but it depends on the cause of the fault and whether your boiler is still within its Vaillant manufacturer guarantee period.
- Covered:
If the fault is due to a manufacturing defect or component failure that occurs under normal use, Vaillant will usually repair or replace the part free of charge. - Not Covered:
Issues caused by poor installation, lack of servicing, limescale build-up, system sludge, frozen pipes, or accidental damage are not covered under the guarantee. - Important:
To keep your guarantee valid, your boiler must be installed and serviced annually by a Gas Safe Registered or Vaillant-accredited engineer (such as a Vaillant MasterTEC installer).
Call MasterTech Installers
When your Vaillant boiler shows a fault code, it can be tempting to try a quick fix — but calling a Vaillant MasterTEC installer ensures your system is diagnosed and repaired safely and efficiently. Here’s why:
- Expert Diagnosis
MasterTEC installers are specially trained by Vaillant to interpret fault codes accurately and identify the root cause — not just the symptom. - Genuine Vaillant Parts
They use only approved Vaillant components, maintaining your boiler’s warranty and ensuring long-lasting reliability. - Safe, Efficient Repairs
All work is carried out to Gas Safe standards, restoring your heating system’s performance without unnecessary risks or delays. - Priority Support & Aftercare
MasterTEC installers have direct access to Vaillant technical support and can provide ongoing maintenance to prevent future issues.
Serving Homes Across North Somerset
From coastal towns like Weston-super-Mare, Portishead, and Burnham-on-Sea to thriving communities such as Nailsea, Yatton, and Backwell, we diagnose, service, and fix Vaillant fault codes right across North Somerset.
With over a decade of local experience, we know the region’s homes inside out — from modern developments to traditional countryside cottages. Our Vaillant MasterTech Installers and Gas Safe Registered Heating engineers deliver trusted, high-quality workmanship tailored to each property’s unique heating needs.
Contact us today for friendly advice or a free quote from our North Somerset boiler experts.