Buderus 6C 228 Fault Code: Causes, Fixes & Repair Costs
What does the Buderus 6C 228 fault code mean?
The 6C 228 fault code on a Buderus boiler means the control system has shut off the gas supply and switched the burner off, but the ionisation sensor is still reporting the presence of a flame. Because a flame signal when the burner should be off is a potentially dangerous condition, the boiler locks out as a safety precaution. The display will typically show '6C 228' alongside a flashing 'H' symbol to indicate a lockout state. A manual reset is required to restart the boiler, but unless the underlying cause is fixed, the code will return. Note that the 6C code on its own (without sub-code 228) can also appear to indicate a flame-detection failure at startup — a slightly different presentation, but rooted in the same sensing and gas-valve circuit. Sub-code 228 specifically points to a ghost or residual flame signal being present after a deliberate shutdown.
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
- Contaminated or damaged ionisation electrode Common
Carbon deposits can build up on the ionisation electrode rod, creating an unwanted electrical path across the ceramic insulator to earth. This makes the control board believe a flame is still present even after the burner has shut down. A cracked ceramic body or an eroded electrode tip produces the same false signal. This is by far the most frequent cause of 6C 228 and the cheapest to resolve.
- Moisture or condensation on the PCB Sometimes
If water or condensation reaches the boiler's printed circuit board — often from a nearby condensate pipe, a roof leak, or general boiler room damp — it can create stray electrical paths that mimic a genuine ionisation signal. The board may need drying or full replacement depending on the extent of the moisture damage.
- Faulty gas valve failing to fully close Sometimes
If the gas valve does not seat properly or close completely when the burner is commanded off, a trickle of gas can sustain a small flame after shutdown. The ionisation electrode correctly detects this residual flame and the control locks out. Wiring faults to the gas valve can produce a similar effect.
- Faulty UBA control board Sometimes
The Universal Burner Adapter (UBA) is the dedicated burner-management module on Buderus boilers. If the UBA develops an internal fault, it can misinterpret or falsely generate ionisation signals, triggering 6C 228 even when the electrode and gas valve are in good condition. Loose or corroded wiring to the UBA can cause the same symptom.
- Blocked condensate trap or flue system obstruction Rare
A blocked condensate trap or a restricted flue can alter the combustion environment enough to produce unusual ionisation readings during and after shutdown. While not the primary cause, these are part of the full diagnostic check recommended by Buderus service guidance.
- Fan mixer non-return valve cracked or dirty Rare
A cracked or fouled non-return valve in the fan air-mixing assembly can disrupt air-to-gas ratios, leading to combustion irregularities that confuse the flame-sensing circuit. This is an infrequently seen cause but is included in the manufacturer's diagnostic sequence.
How to fix it
- Attempt a single boiler reset DIY safe
Press and hold the reset button for approximately 2–3 seconds until the boiler attempts to restart. If the 6C 228 code clears and the boiler runs normally, monitor it over the next few hours. If the fault returns, do not reset more than two or three times in total — repeated reset attempts when the burner fails to stay lit can damage the ignition transformer.
- Check the gas supply to the property DIY safe
Confirm that your gas meter is on and that other gas appliances in the home (hob, fire, etc.) are working normally. If there is no gas at other appliances, contact your gas supplier rather than an engineer. If gas supply is confirmed, the fault lies within the boiler itself.
- Check the system water pressure DIY safe
Look at the pressure gauge on the boiler — for most Buderus models it should read between 1.0 and 1.5 bar when cold. While low pressure alone does not cause 6C 228, it is worth ruling out before calling an engineer. If pressure is below 1 bar, top up via the filling loop according to your boiler's user guide.
- Inspect and clean or replace the ionisation electrode — engineer required Gas Safe engineer
A Gas Safe engineer will isolate the boiler, remove the electrode assembly, and examine the ionisation rod and ceramic insulator for carbon fouling, cracks, or erosion. Light carbon contamination can sometimes be carefully cleaned away; a cracked or badly eroded electrode must be replaced. This is the single most likely fix for 6C 228.
- Inspect the PCB for moisture damage — engineer required Gas Safe engineer
If the electrode is found to be in good condition, the engineer will remove the PCB and check it carefully for signs of water ingress, corrosion, or condensation tracks. A mildly damp board may be dried and reinstalled; one that shows signs of corrosion or burn marks will need to be replaced.
- Test and inspect the gas valve and its wiring — engineer required Gas Safe engineer
The engineer will check the wiring harness to the gas valve for continuity and signs of damage, then test whether the valve closes fully when commanded off. A valve that allows gas to pass when it should be closed must be replaced. This work falls entirely within Gas Safe registered scope.
- Check the UBA wiring and connections — engineer required Gas Safe engineer
The engineer will inspect all wiring connectors to the UBA burner-management board for corrosion, looseness, or heat damage. If connections are sound but the UBA is identified as faulty through diagnostic testing, it will need to be replaced.
- Check the condensate trap, flue system, and fan mixer valve — engineer required Gas Safe engineer
As part of a thorough service visit, the engineer should clean the condensate trap outlet, inspect the flue for blockage or leakage, and check the non-return valve in the fan mixing assembly for cracks or dirt. These secondary checks rule out the less common causes of 6C 228.
- Call a Gas Safe registered engineer if the fault persists or you have not already done so Gas Safe engineer
Because 6C 228 involves a flame-detection failure in an active gas circuit, all meaningful diagnosis and repair must be carried out by a Gas Safe registered engineer. You can verify an engineer's registration at the Gas Safe Register website (gassaferegister.co.uk). Do not attempt further resets or any disassembly of the boiler yourself.
Parts you may need
- Ionisation / flame-sensing electrode · from £35
- Electrode gasket and sealing kit · from £12
- Gas valve assembly · from £120
- PCB / control board · from £280
- UBA burner management module · from £220
- Fan mixer non-return valve · from £25
- Condensate trap assembly · from £20
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 £120–£400, depending on the underlying cause.
Frequently asked questions
Is Buderus 6C 228 dangerous — can I keep using the boiler?
The boiler has already made the safe decision for you by locking out. The 6C 228 code triggers specifically because the control system has detected what it believes to be a flame when the burner should be off — a condition that could indicate a gas valve that is not closing fully. Do not attempt to force the boiler to keep running by repeatedly resetting it. Switch the boiler off at the isolator and arrange for a Gas Safe registered engineer to visit. In the meantime, avoid using any gas appliances if you suspect a gas smell, and call the National Gas Emergency Service on 0800 111 999.
What is the difference between fault code 6C and 6C 228 on a Buderus boiler?
On Buderus boilers the main code (6C) describes the general fault category — in this case a flame-detection anomaly — while the three-digit sub-code (228) pinpoints the specific condition within that category. Sub-code 228 means the ionisation sensor is reporting a flame signal after the burner has been commanded off. The 6C code without a sub-code, or with a different sub-code, can indicate a related but distinct problem such as a failure to detect a flame at startup rather than a ghost flame after shutdown. Always tell your engineer the full code including the sub-code.
How much does it cost to fix a Buderus 6C 228 fault in the UK?
For the most common cause — cleaning or replacing the ionisation electrode — you should expect to pay roughly £120–£180 including the engineer's labour. A gas valve replacement typically costs £200–£350 all-in. PCB or UBA replacement sits at the higher end, generally £300–£450 for parts and labour combined, though in rare cases a specialist PCB repair service can reduce this to around £150–£200. Emergency call-out rates will add to any of these figures if you need an out-of-hours visit.
How many times can I reset a Buderus boiler showing 6C 228?
Buderus guidance is clear that you should not press the reset button more than two or three times in succession when the burner is failing to stay lit. Repeated reset attempts put stress on the ignition transformer and can cause additional damage. If the 6C 228 code returns immediately after one or two resets, this confirms a component fault that needs an engineer's attention — further resets will not fix it and may make things worse.