Vaillant F10, F11, and F23 Fault Codes: What They Mean and How to Respond

If your Vaillant boiler is displaying an F10, F11 or F23 fault code, you’re likely dealing with an issue related to the temperature sensors or water flow inside your heating system.

These errors are less dramatic than a full breakdown – but left unresolved, they can cause your boiler to work inefficiently, overheat, or eventually stop working altogether.

I’m Tony from Advanced Heating and Plumbing, and I’ve repaired countless boilers with these faults across Watford, London, Hertfordshire and Middlesex. Here’s what these codes actually mean and how to deal with them.

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I offer quick, clear diagnostics for sensor and flow issues – often on the same day you call.

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Understanding the Fault Codes

Each of these codes relates to problems detecting temperature correctly or safely:

• F10 – Flow temperature sensor fault
  
  
• F11 – Return temperature sensor fault
  
  
• F23 – Flow and return temperatures are too far apart
  
  

These sensors (called NTC sensors) are essential for telling the boiler how hot the water is. If one gives a false reading or can’t be read at all, the boiler may overheat or shut down as a precaution.

What Causes These Faults?

🔹 Sensor Failure or Wiring Fault

The most common issue is a faulty NTC sensor. These are small components clipped onto the pipework inside your boiler. Over time, they can fail – or the wiring connecting them may become loose, damaged, or corroded.

🔹 Blocked or Sludged Pipework

If your system is dirty inside (which is very common in older heating systems), water doesn’t flow properly. This creates inconsistent temperatures and triggers the F23 error.

🔹 Faulty Pump

If the boiler’s pump isn’t circulating water effectively, you’ll often get a large difference between the flow and return temperatures – resulting in F23.

Can You Check Anything Yourself?

• While most of this needs testing with proper equipment, you can do a couple of safe checks:

  • Check system pressure – Should be around 1.2–1.5 bar. Low pressure can cause flow issues.
    
    
  • Feel your radiators – If some are stone cold while others are hot, it could indicate a circulation or balancing issue.
    
    
  Reset the boiler – If the code disappears and returns later, the issue is likely intermittent – still worth checking.
  

When to Call a Heating Engineer

These fault codes don’t always cause immediate failure – but they do indicate a system that’s struggling. That can affect efficiency, running costs, and eventually lead to more serious faults like overheating.

When I’m called out to these issues, I:

• Test the flow and return NTC sensors

   

• Check the wiring to the PCB

   

• Inspect the pump and system flow

   

• Clean or replace faulty components as needed

   

I carry genuine parts for many Vaillant models and can usually get your boiler running smoothly again in a single visit.

Need Help with a Sensor or Flow Fault?

If your Vaillant boiler is showing an F10, F11 or F23 error, give me a call. I’m Tony – and I’ve been helping homeowners across the region fix heating faults quickly, affordably and with minimal disruption.

✅ Clear, upfront pricing
✅ Fast, local callouts
✅ Trusted independent engineer – not a call center

📞 Call me now on 0800 328 0449

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