Vaillant Boiler Fault Codes
Gas Safe registered fault diagnosis for Vaillant ecoTEC, ecoFIT, and combiMAGIC models. F.28, F.75, F.22, F.29, F.62 — all explained with repair costs and safe homeowner checks.
Understanding Vaillant boiler fault codes
Vaillant boilers display fault codes as F-numbers on the digital display when the boiler locks out or detects a problem. Each code corresponds to a specific fault that the boiler's diagnostic system has detected. Some codes indicate simple problems a homeowner can safely resolve; most require a Gas Safe registered engineer.
This guide covers the five most commonly searched Vaillant fault codes in the UK, with technical explanations of what each fault means, what causes it, and what a Gas Safe engineer will do to fix it. Reion Miller (Gas Safe 919881) carries Vaillant parts and handles Vaillant repairs across London.
Vaillant fault codes explained
Ignition failure
The boiler is trying to light but cannot establish a flame. Usually a blocked burner, failed ignition electrode, or gas supply issue.
Pressure sensor fault
The pump is not creating sufficient pressure, or the pressure sensor itself has failed. Can also show when system pressure is too low.
Low water pressure
System pressure has dropped below the minimum threshold. Often needs a simple repressurise, but check for a leak first.
Flame loss
The boiler lit successfully but the flame went out during operation. Common causes include a faulty flame sensor or intermittent gas supply.
Gas valve fault
The gas valve is taking too long to close or open. Usually requires gas valve replacement by a Gas Safe engineer.
Vaillant fault code table
Extended fault code reference for common Vaillant models.
| Code | Meaning | Common cause | Action |
|---|---|---|---|
| F.22 | Low water pressure | System pressure below 0.5 bar | Repressurise to 1.2–1.5 bar (homeowner safe if no leak) |
| F.28 | Ignition fault | No gas, blocked burner, failed electrode | Gas Safe engineer required |
| F.29 | Flame loss | Flame sensor, gas supply intermittent | Gas Safe engineer required |
| F.62 | Gas valve delay | Gas valve slow to close/open | Gas Safe engineer — likely gas valve replacement |
| F.73 | Water pressure sensor | Sensor or pressure fault | Gas Safe engineer |
| F.75 | Pump/pressure sensor | Pump not running or pressure sensor | Gas Safe engineer — pump or sensor |
| F.76 | Overheating (primary) | Limescale, blocked plate heat exchanger | Gas Safe engineer — clean or replace HX |
| F.83 | Flue temperature fault | Blocked flue, condensate pipe frozen | Check condensate in winter; else Gas Safe engineer |
| S.04 | Anti-cycling mode | Normal — short cycle protection | No action needed; wait for cycle to complete |
| S.54 | Waiting for demand | Normal standby state | No action needed |
Why Vaillant boilers fault more often in London
London's water is among the hardest in England. Thames Water — which supplies Wandsworth, Merton, Lewisham, Lambeth, Southwark, Greenwich, and Bromley — draws from chalk aquifers that carry high concentrations of calcium and magnesium.
Inside a Vaillant boiler, this hard water deposits as limescale on the heat exchanger. Over time the limescale restricts water flow, causes overheating, and eventually cracks the heat exchanger. The symptoms appear as repeated F.28 and F.29 ignition faults, F.75 pressure sensor faults from reduced pump output, and ultimately F.76 overheating lockouts.
The solution: an annual boiler service to check for limescale, combined with a Fernox TF1 or Adey MagnaClean magnetic filter. Reion fits these on every new boiler installation in South London as standard.
Vaillant boiler fault code questions
How do I reset a Vaillant boiler after a fault code?
Why does my Vaillant boiler keep showing error codes in London?
Can I fix a Vaillant fault code myself?
How much does a Vaillant boiler repair cost in London?
Which Vaillant boiler models are most common in South London?
Vaillant fault showing on your boiler?
Reion is Gas Safe registered (919881), carries Vaillant parts, and serves all of South London. Fixed price after the £80 diagnostic — no obligation to proceed.
