Buderus Fault Code 6A: No Flame Detected — Causes, Fixes & Costs
Fault code 6A on a Buderus boiler means the boiler attempted to light the burner but failed to detect a flame. After four unsuccessful ignition attempts, the boiler escalates from a temporary blocking state into a full lockout and shuts down for safety. When the code is flashing on the display, this confirms a locking fault that requires a manual reset — but only after the underlying cause has been identified and resolved. Repeatedly pressing the reset button without addressing the root cause can damage the ignition transformer.
General guidance only — not a substitute for professional advice. 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.
What does the Buderus 6A fault code mean?
Fault code 6A on a Buderus boiler means the boiler attempted to light the burner but failed to detect a flame. After four unsuccessful ignition attempts, the boiler escalates from a temporary blocking state into a full lockout and shuts down for safety. When the code is flashing on the display, this confirms a locking fault that requires a manual reset — but only after the underlying cause has been identified and resolved. Repeatedly pressing the reset button without addressing the root cause can damage the ignition transformer.
Common causes
- Contaminated ionisation electrode Common
The ionisation rod measures a tiny electrical current through the flame to confirm it is burning. If the rod is coated in soot or scale, the current drops below the 1.5 mA threshold needed, and the boiler treats this as no flame — even when the burner has briefly lit. This is the single most frequent cause of repeated 6A lockouts on Buderus boilers.
- Dirty or misaligned ignition electrode Common
The ignition electrode produces the spark that lights the gas. If it is caked in carbon deposits or has shifted out of position, the spark either fails to form correctly or lands in the wrong place relative to the gas flow. The burner then never lights in the first place.
- Interrupted gas supply Common
If the gas cock serving the boiler is partially or fully closed, or if supply pressure has fallen below roughly 20 mbar (for natural gas), there simply is not enough fuel to sustain a flame. This can also happen if a gas meter fault or upstream supply issue affects the whole property.
- Blocked condensate trap or bottle Sometimes
Buderus condensing boilers collect acidic condensate in a removable bottle beneath the boiler. If this bottle or the connecting pipework becomes clogged, the resulting backpressure can interfere with burner operation and prevent ignition. Debris resembling cement particles is sometimes found inside the trap.
- Blocked or partially obstructed flue Sometimes
If the flue intake or exhaust terminal is restricted by a bird nest, ice, debris, or even recirculation of exhaust gases back into the fresh-air intake (common on GB142/162 installations with poorly designed terminal positions), the boiler cannot breathe properly and ignition fails.
- Faulty UBA (Universal Burner Automation) control board Sometimes
The UBA is the dedicated burner controller that coordinates gas valve opening, ignition timing, and flame sensing. If it is not commanding the ignition transformer to fire for the correct 4–5 second glow period, or is misreading ionisation signals, a 6A lockout will follow even if all other components are healthy.
- Faulty or seized gas valve Sometimes
If the gas valve fails to open on demand — due to a coil failure, mechanical seizure, or a wiring fault between the UBA and the valve — no gas reaches the burner and ignition cannot succeed.
- Scale or debris build-up on the heat exchanger Rare
Heavy limescale or debris deposits on the heat exchanger can disrupt combustion airflow and flame geometry to the point where the ionisation rod cannot maintain a stable reading. In some reported cases, cement-like material has coated the entire heat exchanger surface.
- Wiring or earthing fault Rare
The ionisation signal travels along a dedicated cable back to the UBA. A damaged cable, corroded connector, or poor earth connection (yellow/green wire) can cause the UBA to receive a false 'no flame' signal regardless of what the burner is actually doing.
How to fix it
- Check that the gas supply is turned on DIY safe
Locate the gas isolation valve on the pipe feeding the boiler — typically a lever or handle near the unit. Ensure it is fully open (lever parallel to the pipe). Also check that other gas appliances in the property, such as the hob or gas fire, are working normally. If no gas appliances are functioning, the issue lies upstream of the boiler and you should contact your gas supplier.
- Attempt a manual reset — no more than twice DIY safe
When fault code 6A is flashing, press the reset button on the boiler control panel and hold it for roughly three seconds. Allow the boiler to complete a full ignition attempt before pressing again. Do not reset more than two or three times in succession — repeated attempts without success can damage the ignition transformer. If the boiler locks out again immediately, stop resetting and move to the next step.
- Inspect and clear the condensate trap or bottle DIY safe
Locate the condensate collection bottle — usually a small transparent or white container at the base of the boiler, often with a push-fit or twist-lock connection. Place a towel underneath, remove the bottle carefully, and rinse it out with clean water. Check the short pipe connecting the boiler to the bottle for any visible blockage and clear it. Refit the bottle securely before attempting another reset.
- Check the flue terminal for obvious external blockages DIY safe
Go outside and visually inspect the flue outlet and air intake terminals, which are usually wall-mounted or roof-level pipes. Look for debris, bird nesting material, ice build-up, or anything physically blocking the opening. Do not attempt to dismantle any flue components — simply clear any accessible external obstruction. If the terminals appear fine, note whether they are positioned close together, as this can cause exhaust gas recirculation.
- Arrange for a Gas Safe engineer to inspect and clean the electrodes Gas Safe engineer
An engineer will remove the ionisation and ignition electrodes, inspect them for soot, scale, or physical damage, clean them carefully, check the electrode gap and positioning, and refit them. This is the most common fix for a 6A fault and typically resolves it when the gas supply is confirmed normal. Only a Gas Safe registered engineer should perform this work as it involves accessing the combustion chamber.
- Have the engineer verify the ignition transformer and UBA operation Gas Safe engineer
The engineer should confirm that the ignition transformer is firing for the correct 4–5 second period during each ignition attempt. If there is no audible spark when the ignition cable is disconnected and the unit is commanded to start, the transformer may need replacing. If sparking is confirmed but flame is still not detected, attention moves to the ionisation circuit and UBA.
- Have the engineer test the gas valve and supply pressure Gas Safe engineer
The engineer will measure gas supply pressure at the valve inlet and confirm it meets the minimum specification for natural gas. They will also verify that the gas valve coil is receiving the correct voltage signal from the UBA and that the valve is opening fully. A faulty valve coil or seized valve body will require replacement.
- Have the engineer check all wiring, connectors, and earth connections Gas Safe engineer
Damaged insulation on the ionisation cable, corroded plug connectors, or a poor earth on the chassis can all produce false 6A faults. The engineer will carry out a visual and continuity check of the relevant wiring, paying particular attention to the yellow/green earth cable and the ionisation lead.
- If required, arrange a heat exchanger inspection and clean Gas Safe engineer
If no other cause is found, the engineer may inspect the heat exchanger for heavy scale or debris deposits. Buderus recommends applying mineral oil to the fins to help break down deposits, leaving it to penetrate, then rinsing thoroughly. In severe cases the heat exchanger may need to be replaced.
- If the fault persists after all checks, have the UBA control board assessed Gas Safe engineer
If the ignition transformer, electrodes, gas valve, wiring, and flue are all confirmed to be in good condition but the boiler continues to lockout on 6A, the UBA board itself may be misreading signals or failing to sequence correctly. Board replacement is a significant cost and should only be confirmed after all other components have been ruled out.
Parts you may need
- Ionisation electrode · from £35
- Ignition electrode · from £30
- Ignition transformer · from £85
- Gas valve · from £175
- Condensate trap / bottle assembly · from £25
- 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 £100–£350, depending on the underlying cause.
Frequently asked questions
Can I fix a Buderus 6A fault myself?
The safe DIY checks are limited to confirming the gas supply is on, inspecting the flue terminal for obvious blockages, clearing a blocked condensate bottle, and resetting the boiler a maximum of two or three times. Any work involving the electrodes, gas valve, ignition transformer, or control board must be carried out by a Gas Safe registered engineer. Attempting gas-related repairs without registration is illegal in the UK and potentially dangerous.
How many times can I reset the boiler when it shows code 6A?
Buderus advises against resetting more than two or three times consecutively. Repeated reset attempts without a successful ignition can overheat and damage the ignition transformer, turning a relatively inexpensive repair into a more costly one. If the boiler locks out again immediately after one or two resets, stop and call an engineer.
Will an annual service prevent the 6A fault from coming back?
In many cases, yes. The most common cause of a 6A lockout is a contaminated ionisation or ignition electrode — something that a competent engineer will clean or replace as part of a standard annual service. A yearly service, typically costing between £80 and £150, also gives the engineer a chance to spot early signs of electrode wear, heat exchanger debris, or condensate trap blockage before they cause a breakdown.
Could a 6A fault mean I need a new boiler?
Not usually. Most 6A faults are resolved by cleaning or replacing the electrodes, clearing a blocked condensate trap, or — less commonly — replacing the gas valve or ignition transformer. These are all straightforward repairs well within the typical £100–£350 range. A heat exchanger replacement or UBA board failure at the higher end of costs (£400–£750 or more for a heat exchanger) might prompt a conversation about boiler age and value, particularly on older Buderus models where parts can be harder to source, but a 6A code alone is rarely a sign that a full replacement is necessary.