Viessmann boilers are designed with advanced diagnostics to ensure safety and efficiency. When something goes wrong, they display specific Viessmann boiler error codes to help identify the issue. Understanding these fault codes is crucial for troubleshooting issues such as low water pressure, gas ignition failure, or frozen condensate pipes.

If your boiler displays an error code, this guide will help you interpret the message and determine whether it's a quick fix or a sign that you need a professional heating engineer. For fast assistance with any boiler fault, call Maintracts Services on 020 8682 2244 or email info@maintracts.co.uk.

 

At Maintracts Services, we combine over four decades of experience with expert knowledge to deliver reliable boiler repair and maintenance services across London.

 

Why Viessmann Boiler Faults Appear

Modern Viessmann boilers constantly monitor temperature, pressure, flame presence, and internal communication between components. When readings move outside safe operating limits, the control board generates a Viessmann fault code and, in many cases, safely shuts the boiler down. Common triggers include: 

  • Low system water pressure, often after bleeding radiators or small leaks. 
  • Gas supply interruption, ignition failure, or blocked flue gases. 
  • Boiler overheating is caused by restricted circulation or sensor error. 
  • Frozen condensate pipe during cold weather, resulting in a lockout. 
  • Electronic communication faults within the control or wiring harness. 

These alerts protect the appliance and the property. A quick check, a boiler reset, or a simple water pressure fix may restore operation. Repeated lockouts, unexplained error code diagnostics, or suspected component failure require a qualified heating engineer. 

Also Read: Glow-Worm Boiler Error Codes   

 

Viessmann Boiler Error Codes And What They Mean

Below are the Viessmann fault codes most often reported by UK households. Each includes plain‑English boiler error meanings and homeowner guidance. Only carry out steps you are comfortable with. If you are in doubt, please call us on 020 8682 2244. 

1. F1: Flue Temperature Or System Pressure

What it means: The boiler has detected a maximum flue gas temperature condition or low water pressure in the central heating system. Both can trigger protective shutdown. 

User checks: 

  • Look at the pressure gauge. Most domestic systems operate best at 1 to 1.5 bar when cold. 
  • If low, follow your user manual to top up using the filling loop, then bleed radiators and recheck. 
  • Ensure all radiator valves are open to maintain circulation. 

Call a professional if the Pressure drops again, the gauge will not rise, or the boiler overheats after reset, which may indicate an issue with the circulation pump or sensor. 

2. F2: Flame Loss During Heating

What it means: The boiler lit but lost its flame while running. Gas supply interruption, flue problems, or combustion issues are common causes. 

User checks: 

  • Confirm your gas supply is available. 
  • Inspect any visible external flue termination for blockages. 
  • Attempt one boiler reset following the manual. 

Call a professional if the flame keeps cutting out, you smell gas, or the fault returns after resetting. This could relate to ignition electrodes or the combustion setup.

3. F4: No Flame Detected At Start‑Up

What it means: The control tried to ignite but could not detect a flame. This may be caused by closed gas valves, a failed ignition, or a fault in the detection probe. 

User checks: 

  • Ensure the gas isolation tap supplying the boiler is open. 
  • Perform a single reset. 

Call a professional if the code reappears. Safe testing of ignition, burner, and flame sensing requires specialist equipment and a Gas Safe registered heating engineer. 

4. A4: Boiler Overheating Or Flow Issue

What it means: The boiler temperature has risen beyond safe limits, often because water is not circulating correctly through the system. Partially closed valves, air in radiators, or a blocked heat exchanger can contribute. 

User checks: 

  • Check that system valves are open and radiators are heating evenly. 
  • Bleed radiators to remove trapped air if some are cold at the top. 
  • Confirm system pressure is adequate. 

Call a professional if: The boiler locks out repeatedly, the code appears with loud boiling or kettling sounds, or there are signs of a pump or sensor fault. Persistent boiler overheating must be investigated. 

5. OC: Internal Communication Fault

What it means: An internal data or wiring fault is preventing components from communicating correctly with the control board. Power interruptions can also trigger this message. 

User checks: 

  • Switch the boiler off and on at the central control, then perform a boiler reset if advised in your manual. 

Call a professional if OC returns after reset. Wiring continuity, board integrity, and sensor connections need testing with proper instruments. 

6. D4: Frozen Or Blocked Condensate Pipe

What it means: The condensate leaving the boiler cannot drain, often because the external condensate pipe has frozen in cold weather. A blockage elsewhere in the condensate line can also cause a shutdown. 

User checks: 

  • If the weather is cold and the pipe runs outside, pour warm (not boiling) water along the pipe to thaw it. 
  • Ensure the pipe discharges freely into a drain with an adequate fall. 

Prevent repeat faults: Insulate the external section of the condensate pipe before winter. 

Call a professional if: The pipe is clear but the code persists, which could indicate an internal trap or sensor issue. 

Also Read: Ideal Logic Boiler Fault Codes   

 

Preventing Future Boiler Issues

Small preventative steps reduce central heating problems and help avoid repeated Viessmann boiler faults: 

  • Annual service: Schedule a comprehensive service each year to inspect combustion, clean components, and verify flue safety. 
  • Monitor pressure: Glance at the gauge monthly. Low readings increase the risk of lockout and can trigger F-series codes. 
  • Protect the condensate pipe: Insulate external runs to avoid a frozen condensate pipe in winter. 
  • Keep vents and flues clear: Outdoor terminals must not be blocked by leaves or debris, which can cause gas supply interruption or flame failure behaviour. 
  • Act early on noises: Tapping, gurgling, or circulation issues can lead to error code diagnostics later. 

Also Read: Baxi Boiler Error Codes   

FAQs

What does error E20 mean on a Viessmann boiler?

E20 most often relates to a central heating temperature sensor fault, often referred to as an NTC issue. The boiler may shut down to protect itself. Try one reset, then arrange professional testing in your London property by calling 020 8682 2244. 

How do I fix a Viessmann boiler error E110?

E110 indicates a safety overheat trip, commonly linked to low water pressure or restricted flow. Repressurise the system to around 1 to 1.5 bar when cold, bleed radiators, then reset the boiler. If the code returns, contact our heating engineer today. 

Why is my Viessmann boiler showing E119?

E119 signals low system pressure below the safe operating threshold. Top up via the filling loop, following the manual. Check for visible leaks around the pipework, then reset. Repeated pressure loss in a London home should be investigated professionally.

What causes code E133 on Viessmann boilers?

E133 points to a gas supply interruption or flame detection failure. Confirm the gas supply is on, ensure the condensate pipe is not frozen, and perform a single reset.

What should I do if my Viessmann boiler displays E168?

E168 is a general error message recorded after a detected fault or power disruption. Note any earlier codes before resetting, as these can help with fault-finding. If E168 keeps returning, book error code diagnostics with Maintracts Services on 020 8682 2244. Online resources, including Baxi customer support pages, show similar behaviour across some other boiler brands, so always confirm the boiler make before ordering parts. 

Keep Your Boiler Running Smoothly With Maintracts Services

Understanding Viessmann boiler error codes enables you to respond quickly, minimise downtime, safeguard and protect your heating system. When a simple boiler reset or water pressure fix does not resolve the issue, expert intervention prevents more serious faults. Maintracts Services provides complete diagnostics, repair, and annual servicing for Viessmann and other leading boiler brands across London. Qualified heating engineers perform safe testing, component replacement, and long-term system maintenance. 

If your display is showing a fault, or you want to prevent future boiler issues, call us on 020 8682 2244 or email info@maintracts.co.uk today.