Vaillant Boiler Problems & Fault Codes: Complete Troubleshooting Guide

If your Vaillant has stopped working, start with what you can actually observe.

Modern Vaillant boilers continuously monitor operating conditions including water pressure, temperatures, water circulation, fan operation, ignition, flame detection, gas-valve control, combustion, flue conditions, and electronic sensors and connections. When the boiler detects something outside the permitted operating conditions, it may stop normal operation and display an F-code. The code helps narrow the diagnostic area. However, it is important to understand what it does not necessarily tell you.

For example: F75 does not automatically mean the pump has failed. F62 does not automatically mean the gas valve needs replacing. F20 does not prove that the boiler physically overheated. F76 does not automatically mean the heat exchanger needs replacing. The code identifies the condition detected by the boiler. Professional diagnosis establishes why that condition occurred.

Vaillant displays more than one type of code. Codes beginning with F generally indicate a fault that requires attention - examples include F22, F28, F32, F61 and F75. Codes beginning with S are generally status codes rather than fault codes; they indicate what the boiler is doing or waiting for during normal operation. An S-code relating to fan operation should not automatically be interpreted as the equivalent F-code.

The following are the main Vaillant fault codes we currently cover in dedicated technical guides. These codes fall into several useful diagnostic families, and understanding those relationships is often more useful than looking at each fault in isolation.

F22 - Low Water Pressure

F22 is one of the most common Vaillant pressure-related faults. Vaillant uses F22 when the boiler detects insufficient water pressure or a related low-water condition. The first homeowner check is the pressure display - Vaillant's general guidance uses approximately 1-1.5 bar as the normal cold system-pressure range. If pressure is below 1 bar, a competent homeowner may be able to repressurise the boiler using the correct filling arrangement and the instructions for that specific model; do not guess which valves to operate. If pressure is already above 1 bar, do not continue adding water - the problem may involve the water-pressure sensor, sensor or pump wiring, pump operation, the expansion vessel, or another pressure-management fault. Read our full Vaillant F22 Fault Code guide for the complete diagnostic explanation.

F75 - Pressure Change Too Low When the Pump Starts

F75 is frequently confused with F22. F22 asks: is there enough system pressure? F75 asks: did the expected pressure change occur when the heating pump started? Possible areas include low system pressure, air within the boiler or heating system, the water-pressure sensor, the heating pump, wiring, or circulation/hydraulic problems. F75 therefore does not automatically mean the pump has failed. See our detailed F75 Vaillant Boiler Fault guide.

F23 - Flow/Return Temperature Difference Too Great

F23 means the temperature difference between the heating flow and return has become too large, typically directing diagnosis towards the circulation side of the boiler and heating system. Depending on the Vaillant model, relevant areas can include air within the system, insufficient circulation, the heating pump, valves, restrictions or blockages, flow/return temperature sensors, and the expansion vessel. F23 is generally an engineer-level diagnostic fault rather than a homeowner repair.

F24 - Temperature Rising Too Quickly

F24 means the boiler has detected that temperature is increasing faster than expected, which can indicate that heat is being generated but is not being carried away through the heating circuit effectively. Possible diagnostic areas can include air in the system, low water pressure, pump performance, circulation restrictions, valves, temperature sensors and the expansion vessel. A useful distinction: F23 means flow and return temperatures are too far apart; F24 means the temperature is increasing too quickly.

This is one of the most useful Vaillant fault-code families because F27, F28, F29, F61 and F62 are frequently confused online. They describe very different events.

F27 - False or Unexpected Flame Signal

F27 means the boiler detects a flame signal while the burner should be off - almost the opposite of F28. Current manufacturer-listed causes include a gas solenoid valve not sealing correctly or a defective PCB. Professional model-specific diagnostics can additionally involve flame-monitoring electronics, moisture and the monitoring electrode. F27 is not simply a failed ignition code. Read our complete Vaillant F27 Fault Code guide.

F28 - Flame Not Detected During Ignition

F28 means the boiler tried to ignite but did not successfully detect the required flame signal. Possible causes can include insufficient gas supply, gas-meter/prepayment issues, a blocked or frozen condensate pipe, ignition-electrode problems, flue issues, electronic/PCB problems, or other ignition-system faults. Safe external checks include whether other gas appliances work, whether a pre-payment meter has credit, and whether an external condensate pipe is visibly frozen during freezing weather. Anything involving the internal gas or ignition system requires a Gas Safe registered engineer. See the full Vaillant F28 Fault Code guide.

F29 - Flame Lost During Operation

F29 is different from F28: the boiler successfully establishes a flame, the flame is then lost during operation, the boiler attempts to reignite, and successful re-ignition does not occur. Possible causes include temporary gas-supply interruption, insufficient gas pressure, flue-gas recirculation or blockage, blocked condensate discharge, poor electrical earthing, or ignition/electrode problems. F28 means it did not light successfully; F29 means it lit, went out and could not successfully relight. Read our Vaillant F29 Fault Code guide.

F54 - Gas Supply Fault With F28/F29

F54 can appear in combination with F28 or F29 and points more specifically towards the gas-supply side. Vaillant associates F54 with insufficient gas supply, a closed gas isolation valve, low gas flow pressure, or a defective gas valve. If F54 appears alongside F28 or F29, tell the engineer, since that combination provides additional diagnostic information.

F61 - Gas-Valve Control Fault

F61 relates to control of the gas-valve system. On established ecoTEC generations, Vaillant associates F61 with gas-valve wiring or cable-harness faults, a defective gas-valve assembly, or an electronic/PCB fault. F61 does not automatically mean the gas valve itself has failed - the wiring and electronics can produce the same fault. See our dedicated Vaillant F61 Fault Code guide.

F62 - Delayed Flame Switch-Off

F62 means the boiler detected that the flame did not disappear within the expected time during shutdown. Current Vaillant guidance identifies a defective gas valve, PCB, or ignition electrode. Older Vaillant documentation describes the same condition as a gas-valve shut-off delay. F61 means a gas-valve control problem; F62 means flame/gas-valve shut-off occurs too slowly. Read our full Vaillant F62 Fault Code guide.

F32 - Fan Fault

Vaillant currently defines F32 as a fan fault. Possible areas include the fan, fan electrical connection, cable harness and PCB. Professional model-specific diagnostics can additionally involve fan blockage, incorrect fan speed, the Hall sensor and the flue-gas route. F32 should not automatically be described as a blocked-flue fault - that is more directly associated with F35. Read our Vaillant F32 Fault Code guide.

F35 - Air/Flue Fault

F35 concerns the boiler's air/flue operating conditions. Vaillant's homeowner guidance describes F35 as the air/flue pipe being blocked. Depending on the boiler generation, professional diagnosis may include physical flue restriction, flue-gas recirculation, flue damage, incorrect flue arrangement, insufficient combustion air, blocked condensate drainage, low gas flow pressure, and model-specific combustion monitoring. F35 is therefore not simply "the fan is broken," nor does it automatically prove that carbon monoxide is entering the property. It is a safety-critical air/flue fault requiring proper investigation. Read our full Vaillant F35 Fault Code guide.

F25 - Flue-Gas Temperature Too High

F25 concerns excessive flue-gas temperature rather than an air/flue blockage. Vaillant's current guidance can associate it with a circulation fault, pump/valve/blockage, the wiring harness, or flue-gas temperature monitoring.

F20 - Safety Thermal Cut-Out

F20 means the boiler's temperature safety protection has operated. Vaillant currently describes this as the safety thermal cut-out shutting the boiler down. Possible causes depend partly on the boiler generation but can include flow or return temperature sensors, sensor connections/wiring, low system pressure, circulation problems, the pump, valves, restrictions, trapped air, or pressure-sensor problems. An important distinction: F20 does not necessarily prove the boiler physically overheated. A defective temperature sensor or electrical connection can also cause the boiler to detect an overtemperature condition. Read our complete Vaillant F20 Fault Code guide.

F76 - Primary Heat-Exchanger Overheating Protection

F76 is another overheating-related code but it is not the same as F20. F76 concerns the overheating protection associated specifically with the primary heat exchanger. Vaillant documentation identifies immediate diagnostic areas including the fuse cable, the fuse connection, and the primary heat exchanger. This means F76 does not automatically mean the boiler genuinely overheated, the heat exchanger has cracked, or the entire heat exchanger needs replacing - a fault in the protection circuit can also cause F76. Read our detailed Vaillant F76 Fault Code guide.

Vaillant boilers use numerous temperature, pressure and combustion sensors. Many less common fault codes point specifically towards these electronic monitoring systems and do not currently have a dedicated guide on our site, but are worth recognising if they appear on your display.

Not every Vaillant boiler uses exactly the same diagnostic system. The meaning and diagnostic causes of a code can vary slightly between ecoTEC plus, ecoTEC pro, ecoFIT, open-vent boilers, system boilers, combination boilers, and older and newer product generations. The basic fault family usually remains recognisable, but the exact diagnostic tree can change. For example, older manuals may use different terminology, newer models may use additional combustion sensors, one generation may list a Hall sensor where another does not, and open-vent and sealed systems can require different system checks. That is why a good engineer should identify the exact Vaillant model before deciding which component needs testing. A fault-code website that states one universal cause list for every Vaillant generation risks over-diagnosing the boiler.

Sometimes the boiler stops working without displaying an obvious F-code. There are several safe external checks you can make.

Repeated lockout means the boiler is detecting a condition it cannot safely ignore. The most useful thing you can do is record the code before resetting. If you repeatedly reset the appliance without noting the fault, valuable diagnostic information may be lost.

Record the F-code, boiler model, pressure, whether heating or hot water was running, any unusual noise, weather conditions where relevant, whether another code appeared first, and how long the boiler works after reset. A lockout that keeps returning requires diagnosis.

Sometimes. A reset can clear the boiler's fault state and may restore operation after a temporary condition has been corrected. However, resetting is not the same as repairing the fault. A reset cannot repair a defective gas valve, failed PCB, damaged wiring, faulty fan, blocked flue, faulty pressure sensor, failed pump, heat-exchanger fault, or defective temperature sensor. Follow the instructions for your exact Vaillant boiler model. If the same fault immediately or repeatedly returns, stop resetting and arrange diagnosis.

Most boiler faults should not become DIY repair jobs. However, there are several safe observations a homeowner can make, and depending on the specific fault and manufacturer guidance, a few limited actions too.

A boiler fault code does not automatically mean there is an immediate gas emergency. However, some symptoms require a different response regardless of the displayed code.

Professional diagnosis should follow the code rather than replacing the component most commonly blamed online.

How Much Does a Vaillant Boiler Repair Cost?

There is no meaningful single price for "a Vaillant fault code." Consider the difference between topping up low pressure, repairing a sensor connection, replacing a temperature sensor or pressure sensor, repairing a pump, replacing a fan, replacing a gas valve, replacing a PCB, correcting a flue problem, or replacing a major heat exchanger. The costs are completely different. This is why we avoid telling homeowners "F75 costs £X" or "F62 always costs £Y" - F75, F62 and F20 can all have several different causes. The accurate process is to diagnose first, identify the actual repair, then provide the appropriate quotation.

Does a Vaillant Fault Mean I Need a New Boiler?

Usually, no. A fault code is not a boiler-condemnation code. Many Vaillant faults involve repairable components or system conditions. Replacement becomes worth discussing where several factors come together, such as repeated major breakdowns, multiple ageing components, significant corrosion or water damage, poor overall condition, difficult parts availability, a major heat-exchanger or electronic repair, or repair costs becoming disproportionate to the appliance's condition. Avoid rules such as "any boiler over ten years old should be replaced" - age matters, but it should be considered alongside the actual condition and repair economics. The correct sequence is: diagnose, quote, then compare repair and replacement if necessary.

Can Annual Servicing Prevent Vaillant Boiler Problems?

Servicing cannot guarantee that your boiler will never develop a fault. Electronic components can fail. Pumps can fail. Gas valves can fail. External condensate pipes can freeze. Heating systems can develop leaks. However, regular servicing allows the appliance to be inspected and can identify developing problems before they result in complete breakdown. A proper service can also provide an opportunity to assess boiler condition, combustion, flue, seals, visible deterioration, system pressure, previous fault history and other manufacturer-required service checks.

A quick reference across the fault codes covered in this guide.