Worcester Bosch 2927 Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch 2927 fault code mean?
Fault code 2927 (sometimes displayed as 2927 B, where the 'B' denotes a blocking error rather than a full lockout) means the boiler successfully ignited the burner but then lost the flame while running. The ionisation electrode, whose sole job is to confirm a live flame inside the combustion chamber, detected that the flame had gone out mid-cycle. The boiler's PCB responded by cutting the gas supply as a safety precaution and halting operation. The boiler will not restart until the underlying cause is resolved and the unit is manually reset.
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.
Common causes
- Faulty or degraded ionisation electrode Common
The ionisation electrode is a consumable component that gradually wears or becomes coated with carbon deposits over time. When it can no longer produce a reliable signal confirming the presence of a flame, the PCB assumes the burner has extinguished and shuts the boiler down. A damaged connecting lead can produce the same symptom.
- Gas supply disruption Common
If the gas pressure at the boiler drops unexpectedly — because of a partially closed meter valve, work on the gas main, or high demand across the property — the burner flame can weaken and go out mid-cycle. Checking that other gas appliances in the home are working normally is a quick first indicator.
- Flue system blockage or integrity failure Common
A partially blocked, damaged, or incorrectly sealed flue restricts combustion air and prevents exhaust gases from escaping properly. This can starve the burner of the air it needs to hold a stable flame, triggering flame-loss detection. Birds' nests, debris at the flue terminal, or a loose flue joint are typical culprits.
- Blocked or frozen condensate discharge pipe Sometimes
Particularly in cold weather, the condensate pipe can freeze solid, causing a build-up of flue gases that disrupts combustion airflow. This can trip the flue gas thermostat and contribute to mid-cycle shutdowns showing as a flame-loss fault.
- Dirty or scaled heat exchanger (heating block) Sometimes
A heat exchanger heavily fouled with limescale or combustion residue on the flue-gas side can overheat rapidly, interfering with stable combustion and causing the flame to extinguish. This tends to occur in areas with hard water or on boilers that have missed annual servicing.
- Faulty burner control unit (BCU) Rare
The burner control unit manages the ignition sequence and monitors the flame signal. If it has developed an internal fault, it may misread or fail to read a valid flame signal even when the flame is present, or it may fail to maintain gas delivery correctly. This is usually only confirmed after other causes have been ruled out.
- Inadequate room ventilation Rare
Boilers installed in plant rooms or cupboards need a minimum free air opening. If this has been inadvertently blocked — by stored items, a door seal, or building alterations — the burner can be starved of combustion air and drop the flame during operation.
How to fix it
- Check that the gas supply to the property is live DIY safe
Turn on a gas hob or check another gas appliance to confirm gas is flowing to the property. If no gas appliances are working, contact your gas supplier — do not attempt to investigate the meter or gas pipework yourself.
- Attempt a single boiler reset DIY safe
Press and hold the reset button on the boiler (usually marked with a flame or rotating arrow icon) for approximately three seconds, then wait for the boiler to complete its ignition sequence. If the fault clears and does not return within the next few cycles, monitor the boiler over the next 24 hours. Do not reset the boiler more than two or three times — repeated resets without resolving the root cause can cause damage and may mask a serious underlying problem.
- Check for a frozen or blocked condensate pipe DIY safe
In cold weather, locate the condensate discharge pipe — typically a white plastic pipe (often 21.5 mm or 32 mm diameter) running from the boiler to an external drain or internal soil stack. If the external section feels solid or the boiler is making a gurgling noise, the pipe may be frozen. You can carefully pour warm (not boiling) water over the frozen section or wrap it with a warm cloth to thaw it. Once thawed, reset the boiler.
- Check system pressure is correct DIY safe
Look at the pressure gauge on the boiler. It should read between 1.0 and 1.5 bar when the system is cold. If it reads below 0.8 bar, top it up using the filling loop (consult your boiler's user guide for the location). While low pressure alone rarely causes a flame-loss fault, it can contribute to overall poor performance.
- Visually inspect the flue terminal DIY safe
From outside the property, check that the flue outlet (usually a white grille or plastic terminal on an external wall or roof) is free from obvious obstructions such as leaves, debris, or a bird's nest. Do not attempt to probe or dismantle the flue — simply note what you can see from ground level and report it to the engineer.
- Have a Gas Safe engineer inspect and test the ionisation electrode and connecting lead Gas Safe engineer
An engineer will remove the electrode assembly, check for carbon fouling, cracking, or wear, and measure the ionisation signal with the appropriate test equipment. In most cases the electrode and its lead can be replaced quickly and at relatively low cost, often resolving the fault in a single visit.
- Have the engineer inspect and clean the heat exchanger Gas Safe engineer
If the electrode is in good condition, the engineer will examine the heat exchanger on the flue-gas side for limescale or combustion deposits. A clean may be sufficient, or if the heat exchanger is heavily fouled it may need to be replaced.
- Have the engineer carry out a full flue integrity and combustion analysis check Gas Safe engineer
Using flue gas analyser equipment, the engineer will verify that combustion is stable, the air-to-gas ratio is correct, and that the flue is sealed and drawing correctly. They will also confirm the ventilation opening in the boiler's installation space meets minimum requirements.
- If all other checks pass, have the burner control unit assessed and replaced if necessary Gas Safe engineer
The BCU is typically the last component to be investigated. An engineer will test its outputs and, if it is found to be faulty, replace it. This is a more involved repair but should resolve persistent flame-loss faults that have not responded to other remedies.
- Call a Gas Safe registered engineer if the fault persists or returns regularly Gas Safe engineer
If the boiler resets and runs for a short period before showing 2927 again, or if you have already tried the homeowner checks above without success, contact a Gas Safe registered engineer. You can verify an engineer's registration at gassaferegister.co.uk. Repeated flame-loss faults should never be ignored — continuing to reset without investigation is both unsafe and likely to cause further damage to the boiler.
Parts you may need
- Ionisation / flame-detection electrode (Worcester Bosch compatible) · from £55
- Ignition electrode connecting lead · from £18
- Burner control unit (BCU) · from £120
- Condensate trap (if blocked internally) · 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 £90–£300, depending on the underlying cause.
Frequently asked questions
Is it safe to keep resetting the boiler when fault code 2927 appears?
You can try one or two resets to see whether the fault clears on its own, but repeatedly resetting a boiler that keeps dropping its flame is not safe practice. Flame-loss faults mean the boiler is shutting down as a safety precaution. If the fault returns after one or two resets, stop resetting and call a Gas Safe registered engineer to investigate. Masking the fault with repeated resets can cause further damage and, in a worst-case scenario, creates a risk of unburnt gas entering the combustion chamber.
How much does it typically cost to repair a Worcester Bosch 2927 fault in the UK?
For most homeowners, the repair comes down to a new ionisation electrode and connecting lead, which — including a Gas Safe engineer's labour — typically costs between £90 and £160. If the heat exchanger needs cleaning the job may run to around £200–£300 including the call-out. If the burner control unit (BCU) turns out to be faulty, parts and labour can reach £300–£400. A gas valve replacement is a less common cause of this code but would cost in the region of £220–£350 fitted. These are the realistic ranges for the repairs most people actually need — not worst-case figures.
What does the 'B' suffix in fault code 2927 B mean on a Worcester Bosch boiler?
Worcester Bosch uses letter suffixes to indicate the type of shutdown. A 'B' denotes a blocking error, meaning the boiler has temporarily halted operation but has not entered a hard lockout — it will attempt to restart, or can be manually reset. A locking fault (typically shown with an 'L' suffix or a padlock symbol) requires a manual reset before any restart can be attempted. Either way, the fault code itself tells you why the shutdown occurred; the suffix tells you how the boiler responded.
Could a frozen condensate pipe really cause a 2927 flame-loss fault?
Yes, indirectly. When the condensate discharge pipe freezes in cold weather, condensate backs up inside the boiler. This can interfere with flue-gas flow and trigger the flue-gas thermostat, which in turn disrupts the combustion process enough for the ionisation electrode to lose its flame signal. Thawing the condensate pipe (by applying warm water to the frozen external section) and then resetting the boiler often resolves the fault completely during cold snaps. If the fault persists after the pipe is thawed, there is likely a separate underlying issue.