Buderus 4C 224 Fault Code: Causes, Fixes & Repair Costs
What does the Buderus 4C 224 fault code mean?
The 4C 224 fault code on a Buderus boiler signals that one or both of the boiler's safety temperature limiters (STBs) — one monitoring the primary heat exchanger block and one monitoring flue gas temperature — have detected dangerous overheating and have tripped to protect the appliance. When the code is shown with a flashing display, the boiler is in full lockout: heat demand is suspended and the circulation pump is shut down approximately three minutes after the trip. A manual reset is required before the boiler will attempt to fire again. This is a serious safety condition; if the root cause is not identified and resolved, the lockout will recur immediately or shortly after reset.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Blocked or seized circulation pump Common
If the circulation pump is jammed, running too slowly, or has seized, hot water cannot move freely through the heat exchanger. Heat builds up rapidly, causing the STB to trip. This is one of the most frequent triggers for 4C 224, particularly in systems that have been idle over summer.
- Low system pressure Common
When system pressure drops below roughly 1.0 bar, the volume of water circulating through the heat exchanger is insufficient to absorb combustion heat effectively. The boiler overheats and the limiter trips. Pressure should normally sit between 1.0 and 1.5 bar when cold.
- Air trapped in the system or heat exchanger Common
Air locks prevent proper water flow through the heat exchanger, creating hot spots. The boiler may fire normally but the trapped air stops adequate circulation, causing localised overheating that trips the STB.
- Flue or flue gas pathway blockage Sometimes
A blocked, kinked, or incorrectly terminated flue prevents combustion gases from escaping efficiently. Retained flue gas heat can trigger the flue gas temperature limiter. Bird nests, debris, or a collapsed flue terminal are common culprits.
- Dirty or scaled primary heat exchanger Sometimes
Limescale, sludge, or debris build-up on the water side of the heat exchanger significantly reduces heat transfer efficiency. The water cannot absorb heat fast enough, temperatures climb, and the STB trips.
- Damaged safety temperature limiter (STB) or wiring Sometimes
The STB itself, or the cable connecting it to the PCB, may have developed a break, corrosion, or a poor connection. A damaged limiter can trip at normal operating temperatures or fail to reset properly even when the underlying overheat condition is resolved.
- Missing or incorrectly fitted heat exchanger splitter Rare
On certain Buderus models the heat exchanger contains a flow-directing splitter. If this is absent or incorrectly seated following a service, water flow distribution is disrupted, leading to localised overheating and an STB trip.
- Faulty PCB or electrical fault Rare
A failing main printed circuit board can misread sensor signals or send incorrect pump commands, producing apparent overheat conditions. Look for visible signs such as burn marks, deformation, or condensation on the board. This is usually a diagnosis of exclusion after other causes have been ruled out.
How to fix it
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the boiler front panel. The reading should be between 1.0 and 1.5 bar when the system is cold. If it is below 1.0 bar, top up via the filling loop until it reaches approximately 1.2–1.5 bar, then close the filling loop valves fully. If pressure drops again within a few days, there is likely a leak or a faulty expansion vessel that needs professional attention.
- Allow the boiler to cool, then attempt a reset DIY safe
With a lockout fault the boiler must be manually reset. Wait at least 10–15 minutes to allow temperatures to fall, then press and hold the Reset button on the control panel for 2–3 seconds. If the boiler fires and runs normally, monitor it closely over the next hour. Do not attempt more than two or three resets in total — repeated resetting without resolving the underlying cause can damage the appliance or mask a serious fault.
- Check the gas supply to the property DIY safe
Confirm that other gas appliances (hob, fire) are working normally. If there is no gas elsewhere, contact your gas supplier. Buderus boilers can display overheat-related codes after an interrupted ignition sequence, and restoring gas supply before resetting may resolve the fault.
- Check the flue terminal externally for obvious blockages DIY safe
From outside the property, visually inspect the flue outlet (usually on an external wall or roof). Look for an obvious obstruction such as a bird nest, debris, or a damaged terminal cowl. Do not attempt to dismantle any flue component — if a blockage is suspected inside the flue, this must be investigated by a Gas Safe engineer.
- Bleed radiators and check for air in the system DIY safe
Turn the heating off and let the system cool. Use a radiator bleed key to release air from the highest and furthest radiators first, then work back towards the boiler. Re-check system pressure afterwards and top up if needed. Purging trapped air can restore adequate flow through the heat exchanger.
- Engineer inspection of the circulation pump Gas Safe engineer
A Gas Safe engineer will check pump operation using the boiler's built-in service functions (typically t03 on Buderus Logamax Plus models), verify the pump head is developing sufficient pressure, and confirm it is running at the correct speed setting. A seized pump impeller can sometimes be freed; otherwise the pump head or complete pump assembly will need replacing.
- Engineer check and bleed of the heat exchanger water circuit Gas Safe engineer
Using the boiler's service menu (function 2.2C on applicable models), the engineer will bleed the heat exchanger to remove any air locks. They will also inspect the water side of the heat exchanger for scale or sludge and assess whether a chemical clean or replacement is required.
- Engineer inspection of the STB limiters and connecting wiring Gas Safe engineer
The engineer will test continuity and resistance of both the heat block STB and the flue gas temperature limiter, along with their connecting cables. A limiter that has failed open-circuit will need replacing. On some Buderus Logamax Plus units the STB has a manual reset button located beneath a cap to the left of the water regulator; the engineer will check whether this needs pressing or whether the limiter itself has failed permanently.
- Engineer inspection of the flue gas pathway and heat exchanger splitter Gas Safe engineer
The engineer will verify the entire flue run is clear and correctly assembled, and — on applicable models — confirm the heat exchanger splitter is correctly installed and seated. An incorrectly positioned splitter will cause persistent overheating and repeated 4C 224 faults regardless of other repairs.
- Engineer assessment of the PCB if all other causes are excluded Gas Safe engineer
If the STBs, pump, heat exchanger, and flue all test satisfactory and the fault persists, the engineer will evaluate the main PCB for signs of electrical damage or sensor misreading. PCB replacement is typically a last resort given its cost, but a competent engineer can confirm this diagnosis before committing to the part.
- Call a Gas Safe registered engineer if the fault persists or recurs Gas Safe engineer
If the boiler locks out again after the homeowner checks above, or if you are at all unsure, contact a Gas Safe registered engineer. Provide them with the exact fault code (4C 224) and your boiler model. Persistent overheat lockouts should never be ignored — continued resetting without repair can damage the heat exchanger or, in the worst case, compromise boiler safety.
Parts you may need
- Circulation pump assembly (e.g. Grundfos UPS/Wilo compatible for Buderus Logamax Plus) · from £95
- Safety temperature limiter (STB) — heat block · from £35
- Flue gas temperature limiter (STB) · from £40
- Primary heat exchanger · from £280
- Main PCB (control board) · from £220
- Heat exchanger splitter/baffle · from £25
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £150–£420, depending on the underlying cause.
Frequently asked questions
Can I reset a Buderus 4C 224 fault myself?
Yes, a single reset attempt is safe for a homeowner to try. Press and hold the Reset button for 2–3 seconds after the boiler has had time to cool. However, limit yourself to two or three attempts maximum. If the lockout returns shortly after reset, the boiler is telling you there is an underlying fault that needs professional diagnosis — repeated resetting without fixing the root cause risks damaging the heat exchanger or other components.
Why does my Buderus boiler keep locking out on 4C 224?
Recurring lockouts almost always mean the root cause has not been fixed. The most common repeat-offenders are a worn or partially seized circulation pump (flow is marginal and the boiler overheats under load), progressive limescale build-up inside the heat exchanger reducing heat transfer, or a safety temperature limiter that has weakened and trips at lower-than-intended temperatures. A Gas Safe engineer should carry out a full diagnostic rather than simply resetting the unit each time.
How much does it cost to repair a Buderus 4C 224 fault in the UK?
For the most common causes — pump replacement or an STB swap — most homeowners pay in the region of £150 to £420 including parts and labour. Pump head replacement sits towards the lower end (roughly £150–£280 all-in), while a new STB limiter typically costs £160–£250. If the primary heat exchanger needs replacing, costs can rise significantly — typically £500–£750 or more — and at that level it is worth comparing against the cost of a new boiler, particularly if the appliance is over ten years old.
What is the STB limiter on a Buderus boiler and can it be manually reset?
STB stands for Safety Temperature Begrenzer (safety temperature limiter). It is a one-shot or auto-reset thermal fuse that cuts power to the burner or pump circuit if a dangerously high temperature is reached. On some Buderus Logamax Plus models there is a manual reset button on the STB, accessed by removing a cap on the component. However, locating and pressing this should be done by an engineer — if the limiter has actually failed rather than just tripped, pressing the reset button will have no effect and the part will need replacing. More importantly, the engineer must establish why overheating occurred before resetting the STB, otherwise the fault will return immediately.