What Are Boiler Error Codes?
Modern boilers are equipped with sophisticated diagnostic systems that monitor every aspect of the appliance's operation. When something falls outside of safe or normal parameters, the boiler displays an error code (sometimes called a fault code) and locks out — shutting down to prevent damage or danger.
These codes are your boiler's way of telling you what's wrong, and understanding them can save you time, money, and stress. While some errors indicate simple issues you can resolve yourself, others require a Gas Safe registered engineer. This guide covers the most common error codes from the UK's four most popular boiler brands.
Worcester Bosch Error Codes
Worcester Bosch is the UK's best-selling boiler brand, and their Greenstar range is found in millions of homes across London. Here are the most common fault codes:
EA / EA 227 — Ignition Failure
The boiler has tried to ignite but failed. This could be caused by a faulty ignition electrode, a gas supply issue, or a blocked flue. Try resetting the boiler once. If the error returns, you'll need an engineer.
E9 / CE 207 — Low Pressure
System pressure has dropped below the minimum threshold. Check your pressure gauge and repressurise the system using the filling loop. If pressure drops repeatedly, there may be a leak.
D5 / C6 — Sensor Fault
A temperature sensor (NTC) has failed or is reading incorrectly. This requires an engineer to diagnose and replace the faulty sensor.
A1 / 2970 — Blocked Condensate
The condensate pipe is blocked, often because it's frozen in cold weather. You may be able to thaw an external condensate pipe by pouring warm (not boiling) water over it. If it's internal or the error persists, call an engineer.
E5 — Overheating / Overheat Thermostat
The boiler has overheated and the safety thermostat has activated. This can be caused by a pump failure, sludge buildup, or a blockage in the system. Do not reset — call a heating engineer.
Vaillant Error Codes
Vaillant's ecoTEC range is another hugely popular choice in London. Their error codes always start with "F.":
F.28 — Ignition Failure
The most common Vaillant fault. The boiler couldn't ignite after the maximum number of attempts. Causes include a faulty ignition lead, gas valve issue, frozen condensate pipe, or gas supply interruption. Reset once — if it returns, book an engineer.
F.22 — Low Water Pressure
The pressure sensor has detected insufficient water in the system. Repressurise via the filling loop. If this code appears frequently, investigate for leaks.
F.75 — Pump/Pressure Sensor Fault
The boiler has detected that the pump isn't creating sufficient pressure change when it starts. This usually means the pump is failing or the pressure sensor is faulty. An engineer is needed.
F.27 — False Flame Detection
The flame sensing electrode is detecting a flame when there shouldn't be one, or moisture is causing a false signal. This is a safety fault that requires professional attention.
F.29 — Flame Extinguishes During Operation
The boiler ignites but the flame goes out during operation. This could indicate a gas supply problem, a blocked flue, or recirculation of flue gases. Do not ignore this code — it needs investigation.
Ideal Error Codes
Ideal boilers (Logic, Vogue, and newer models) use letter-number combinations:
L1 / F1 — Low Pressure
System pressure is below the minimum. Top up via the filling loop as described in our pressure guide.
L2 / F2 — Ignition Failure
The boiler couldn't establish a flame. Check that gas is reaching the boiler (are other gas appliances working?) and try a single reset. If the fault persists, contact an engineer.
L5 — False Flame / Flame Detected Without Gas
A safety fault indicating the boiler's electronics are detecting a flame when the gas valve should be closed. This requires immediate professional attention.
L8 — Return Sensor Fault
The return temperature sensor has failed or is giving incorrect readings. An engineer will need to test and replace the sensor.
FD — Fan Fault
The fan isn't reaching the correct speed, or the pressure switch isn't confirming adequate airflow. The fan, pressure switch, or wiring may need attention.
Baxi Error Codes
Baxi boilers use "E" codes on most models:
E119 — Low Water Pressure
The most common Baxi fault. Repressurise the system to between 1.0 and 1.5 bar. If pressure keeps dropping, there's a leak somewhere in the system.
E133 — Ignition Fault
The boiler has failed to ignite. Check your gas supply, try resetting once, and if the problem persists, book an engineer.
E125 — Circulation Fault
Not enough water is circulating through the boiler, causing it to overheat locally. This is often caused by a pump failure, blockage, or closed valves. An engineer is required.
E110 — Overheat
The safety overheat thermostat has activated. The boiler has overheated, possibly due to a pump failure, system blockage, or failed thermostat. Do not keep resetting — call an engineer.
E168 — Internal Fault
An electrical or PCB fault has been detected. This usually requires a PCB replacement, which should only be done by a qualified engineer.
What You Can Safely Do Yourself
Some boiler faults have simple fixes you can try before calling an engineer:
- Low pressure errors (E9, F.22, L1, E119): Repressurise the boiler using the filling loop.
- Frozen condensate pipe: Pour warm water over the external section of the condensate pipe to thaw it.
- Single lockout after power interruption: Reset the boiler using the reset button (hold for 3–5 seconds).
- Gas supply interruption: Check your prepayment meter hasn't run out, or that the gas isolation valve on the meter hasn't been turned off.
When to Call an Engineer
You should always call a Gas Safe registered engineer if:
- The same error code keeps appearing after you've reset the boiler.
- The error relates to ignition, flame detection, or overheating.
- You can smell gas anywhere near the boiler.
- The boiler is making unusual noises alongside the error code.
- You're unsure what the error code means — it's always better to be safe.
At Heat-Plex, our engineers are experienced with all major boiler brands and carry common spare parts in their vans for first-visit fixes. We offer same-day and next-day appointments across London.
Preventing Error Codes
The best way to avoid unexpected boiler faults is regular maintenance:
- Annual servicing catches developing problems before they trigger error codes.
- A magnetic filter prevents sludge from damaging internal components.
- Insulating external condensate pipes prevents winter freeze-ups.
- Regular pressure checks prevent low-pressure lockouts.
Our service plans include annual servicing and priority repairs, giving you peace of mind that your boiler is always in good hands. Get in touch to learn more or book a repair.