Buderus 6C 306 Fault Code: Causes, Fixes & Repair Costs
What does the Buderus 6C 306 fault code mean?
The 6C 306 fault code on Buderus boilers (including the Logamax Plus GB062 range) is a safety lockout that triggers when the boiler's flame-detection circuit reports a live flame after the gas supply has been cut. In normal operation, once the burner shuts down the ionisation signal should drop to zero almost immediately. When the control board sees a signal persisting beyond that point, it interprets the situation as potentially dangerous — the burner should be off but apparently isn't — and locks the boiler out to prevent any risk. A reset is required to restart, but the underlying cause must be identified and resolved or the code will return. Sub-code 228, which may appear alongside 6C on some displays (shown as '6C 228'), carries the same core meaning: flame detected while the burner is commanded off. Note that the related code 6A covers flame loss during normal running, whereas 6C specifically flags a flame signal that persists or appears when it should be absent.
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
- Faulty or leaking gas valve Common
The gas valve is the first component the official Buderus service procedure points to when this code appears. If the valve fails to close completely when commanded, a residual flame can persist and the ionisation circuit will continue to detect it. Internal wear, contamination, or a seized solenoid can all prevent a clean shut-off. This is the most common root cause found by engineers attending this fault.
- Contaminated or degraded ionisation electrode Common
Carbon deposits build up on the ceramic base of the ionisation rod over time. This carbon layer is conductive and can create a leakage path from the rod to earth, generating a small electrical signal that the PCB misreads as a flame signal — even when the burner is completely cold. Cleaning the electrode base removes the carbon bridge in the majority of cases; if the ceramic is cracked or the rod itself is corroded, replacement is needed.
- Defective or damaged cable harness Sometimes
The wiring loom connecting the ionisation electrode to the control board can develop breaks, chafed insulation, or poor connections that produce a spurious signal. Buderus lists this as the second diagnostic step once the gas valve has been addressed. Any flex point along the harness where cables are routed close to heat sources is particularly prone to insulation degradation.
- Moisture or humidity on the electronic control board (PCB/UBA) Sometimes
Condensation or a small water ingress event can cause tracking across the PCB surface, creating false signals in the flame-detection circuit. The board should be inspected for signs of moisture, corrosion, or burn marks. Drying the board may resolve the issue temporarily, but a board showing visible damage will need replacement.
- Defective PCB or UBA (Universal Burner Adapter) Rare
If gas valve, electrode, and harness checks all come back clear, the control board itself may be generating or misinterpreting the ionisation signal. The PCB and UBA are the final steps in the official Buderus diagnostic tree and represent the least common but most expensive resolution.
- Loose gas connections at the valve Sometimes
A poorly tightened gas supply connection at the inlet to the gas valve can allow a small amount of gas to bypass the valve seat, sustaining a weak flame after shut-down. Engineers have resolved this fault on site simply by correctly torquing the gas inlet fitting and confirming there are no leaks — a straightforward fix once identified.
How to fix it
- Do not repeatedly reset the boiler DIY safe
A single reset attempt is reasonable — press and hold the reset button on the boiler's control panel until the lockout clears. If the boiler locks out again within a short period, do not reset it more than two or three times in total. Repeated resets on a fault like this, which involves a potential gas-side issue, are not advised.
- Check that your gas supply is active DIY safe
Confirm that other gas appliances in the property (hob, fire) are working normally. If there is no gas at any appliance, contact your gas supplier rather than the boiler engineer. If gas supply is normal, the issue is within the boiler itself.
- Note any sub-code displayed alongside 6C DIY safe
Some Buderus control panels show a two-part code such as '6C 228' or '6C 306'. Write down the full code as displayed and pass it to your engineer — it helps narrow down the diagnosis before they arrive.
- Contact a Gas Safe registered engineer to inspect the gas valve Gas Safe engineer
The engineer's first task is to check whether the gas valve is closing fully when commanded. This involves electrically isolating the boiler, removing the valve, inspecting the solenoid and valve seat for wear or contamination, and replacing the unit if it cannot achieve a reliable seal. O₂ combustion analysis should be carried out after any gas valve work to confirm correct combustion.
- Have the ionisation electrode inspected and cleaned or replaced Gas Safe engineer
The engineer will remove the ionisation rod and examine the ceramic insulator base for carbon tracking. Carefully cleaning away carbon deposits with a fine abrasive or appropriate solvent removes the conductive path in most cases. If the ceramic is cracked or the rod tip is heavily corroded, a replacement electrode is fitted. This is a relatively quick and inexpensive repair.
- Ask the engineer to test the cable harness Gas Safe engineer
If the gas valve and electrode are both found to be in order, the wiring loom from the electrode to the PCB should be tested for continuity, insulation resistance, and secure connections at both ends. A faulty harness is replaced as a complete loom assembly.
- Have the PCB or UBA inspected and replaced if necessary Gas Safe engineer
As the final step in the diagnostic sequence, the engineer will inspect the control board for signs of moisture ingress, tracking, or component failure. If the board is the source of the false flame signal, it will need replacing. Given the cost of a new PCB, it is worth asking your engineer to also assess the overall age and condition of the boiler — on an older unit, a new boiler may offer better value.
Parts you may need
- Gas valve (Buderus GB062 compatible) · from £95
- Ionisation/flame sensing electrode · from £18
- Cable harness / wiring loom · from £55
- PCB / UBA control board · from £320
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–£380, depending on the underlying cause.
Frequently asked questions
Is the Buderus 6C 306 fault dangerous?
The boiler has detected what it considers an unsafe condition — a flame signal when the burner should be off — and has locked itself out as a precaution. The lockout itself means the boiler is not operating, so the immediate risk is low. However, the underlying cause (most often a gas valve that isn't closing cleanly) is a gas-side issue and must be assessed by a Gas Safe registered engineer before the boiler is returned to normal service. Do not keep resetting the boiler and hoping the fault clears on its own.
What does sub-code 228 mean alongside 6C?
On Buderus boilers the display sometimes shows the fault as '6C 228' rather than just '6C 306'. Both point to the same condition: flame detected after the burner has been commanded off. The sub-code is part of the internal diagnostic reference and does not change the repair approach — the engineer will follow the same sequence of checking the gas valve, ionisation electrode, cable harness, and PCB in that order.
How much does it typically cost to fix a 6C 306 fault?
For the two most common causes — ionisation electrode cleaning or replacement, and gas valve replacement — most homeowners pay between £120 and £380 including parts and a Gas Safe engineer's labour. Electrode work tends to sit at the lower end (roughly £100–£200), while a gas valve replacement typically costs £180–£300. If the PCB or UBA turns out to be at fault, the cost rises considerably — board replacement can reach £500–£700 or more once parts and labour are included — which is why engineers work through the cheaper components first.
Can I fix a Buderus 6C 306 fault myself?
The homeowner checks you can safely carry out are limited: confirming your gas supply is working, noting the exact code displayed, and attempting a single reset. Everything beyond that — inspecting or replacing the gas valve, electrode, wiring loom, or PCB — must be done by a Gas Safe registered engineer. Working on gas components without registration is illegal in the UK and could void your home insurance.