Worcester Bosch EC 256 Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch EC 256 fault code mean?
On Worcester Bosch boilers, 256 is a cause code — a three-digit secondary diagnostic number that appears alongside a primary locking or blocking code (such as EA, E9, or F0) to give engineers more precise information about why the boiler has shut down. You access it by pressing the return button when a blocking error is active, or it will flash on screen during a locking fault. Cause code 256 specifically points to an insufficient ionisation current during the burning phase. In plain terms: the boiler attempted to light the burner but the flame-sensing circuit could not confirm that a stable flame was present, so the boiler locked out as a safety precaution. The fault is rooted in the burner control unit (sometimes called the KIM — boiler identification module), its wiring and plug connections, or the electrodes it relies on to detect the flame. Related cause codes in the F0 2xx range (for example F0 242 through to F0 272) all point toward the burner control unit and its associated wiring, so if your display shows any code in that band alongside a primary fault, the diagnosis and checks described here apply equally.
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
- Worn, dirty or mispositioned flame-sensing electrode Common
The flame-sensing electrode sits inside the combustion chamber and passes a small ionisation current through the flame to confirm it is burning. If the electrode tip is coated in residue, cracked, or has shifted out of position, the signal back to the burner control unit drops below the required threshold and the boiler locks out. Electrodes are consumable components and are typically inspected and replaced at each annual service.
- Faulty or damaged burner control unit (KIM/BCU) or its wiring Common
The burner control unit manages the entire ignition sequence and processes the ionisation signal. Cause code 256 directly implicates this unit. Loose plug connections, chafed or pinched wiring in the harness, or an internally failed BCU can all prevent the ionisation reading from registering correctly, even when the electrodes themselves are fine.
- Ignition electrode failure Common
There are two distinct electrode types: the ignition electrode creates the spark that initiates combustion, while the flame-sensing electrode monitors the result. If the ignition electrode is cracked or worn, the spark may be too weak to light the gas reliably, meaning no flame is ever established for the sensor to detect.
- Disrupted or low gas supply Sometimes
If the gas pressure at the boiler is insufficient — due to a meter fault, a closed service valve, or a supply interruption from the network — the burner cannot light at all regardless of how well the ignition components are working. Checking whether other gas appliances in the property are functioning is a quick first step.
- Dirty or faulty gas valve Sometimes
The gas valve controls the flow of gas to the burner. If it is sticking, failing to open fully, or modulating incorrectly, there may be too little gas present for a stable flame to form and be detected by the ionisation circuit.
- Frozen or blocked condensate pipe Sometimes
In cold weather the condensate pipe — which carries acidic water from the boiler to a drain — can freeze solid, particularly where it runs outside the property. When this happens the boiler is prevented from operating safely and will lock out. This can sometimes present with secondary flame-detection faults.
- Flue or combustion air supply obstruction Rare
A partially blocked flue terminal or an obstruction restricting fresh air to the boiler can disrupt the air-to-gas mixture inside the combustion chamber, causing unstable or failed ignition and a resulting loss of ionisation signal.
- Faulty PCB (printed circuit board) Rare
The main PCB interprets signals from the burner control unit and coordinates boiler operation. In rare cases a failing PCB can misread or fail to process the ionisation signal correctly, triggering a 256 fault even when the electrodes and BCU are serviceable.
How to fix it
- Check that other gas appliances are working DIY safe
Try your gas hob or another gas appliance in the property. If nothing is working, the issue is likely with the gas supply rather than the boiler itself — contact your gas supplier or call the National Gas Emergency line on 0800 111 999 if you smell gas. If you have a prepayment meter, check it has sufficient credit.
- Thaw a frozen condensate pipe if the weather is cold DIY safe
If outside temperatures have been at or below freezing and your condensate pipe runs externally, look for visible ice around the pipe. Pour warm (not boiling) water over the frozen section or wrap it with a warm damp cloth until it clears. Once thawed, reset the boiler. This is a safe homeowner task.
- Top up system pressure if it is below 1 bar DIY safe
Check the pressure gauge on the boiler's front panel. If it reads below 1 bar, the system may need represurising via the filling loop — usually a flexible braided hose beneath the boiler with one or two valves. Consult your boiler manual for the correct procedure and aim for around 1.0–1.5 bar. Do not exceed 2.5 bar.
- Reset the boiler — no more than two or three times DIY safe
Hold the reset button (usually marked with a flame symbol or an arrow) for a few seconds and wait for the boiler to attempt a restart. If it locks out again and displays the same code, do not keep resetting it. Repeated resets on a genuine ionisation fault can cause further damage to the burner control unit and other components.
- Do not attempt to access internal components yourself Gas Safe engineer
Any work inside the boiler casing — including inspecting or replacing electrodes, checking the BCU or its wiring, testing the gas valve, or examining the PCB — must only be carried out by a Gas Safe registered engineer. Working on gas appliances without the correct registration is illegal and dangerous.
- Book a Gas Safe registered engineer to inspect the ignition and flame-detection circuit Gas Safe engineer
An engineer will use diagnostic tools to read the fault history, measure the ionisation current, and physically inspect the flame-sensing and ignition electrodes, their leads, and the plug connections to the burner control unit. They will clean or replace electrodes as needed and confirm whether the BCU itself needs replacing.
- Allow the engineer to test and if necessary replace the gas valve or PCB Gas Safe engineer
If electrode and BCU checks do not resolve the fault, the engineer will move on to testing gas valve operation and pressure, as well as examining the PCB for any signs of failure. These are specialist tasks requiring Gas Safe registration and appropriate test equipment.
Parts you may need
- Flame-sensing electrode · from £35
- Ignition electrode · from £30
- Electrode lead / ignition lead set · from £25
- Burner control unit (KIM/BCU) · from £180
- Gas valve · from £120
- PCB (main printed circuit board) · from £220
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–£380, depending on the underlying cause.
Frequently asked questions
What is cause code 256 on a Worcester Bosch boiler — is it different from a main fault code?
Yes. Worcester Bosch uses a two-layer code system. The primary code (such as EA, E9, or F0) tells you the type of fault and whether the boiler has locked out or is blocking. The three-digit cause code — in this case 256 — appears as a secondary code and pinpoints the specific component or circuit involved. You reach it by pressing the return button during a blocking fault, or it flashes on screen during a locking fault. Cause code 256 indicates insufficient ionisation current and implicates the burner control unit and flame-detection circuit.
Can I fix Worcester Bosch EC 256 myself?
There are a small number of safe checks a homeowner can carry out: confirming the gas supply is on, thawing a frozen condensate pipe in cold weather, topping up system pressure if it has dropped, and attempting a single reset. However, the underlying fault almost always requires a Gas Safe registered engineer. Replacing electrodes, inspecting the burner control unit wiring, testing the gas valve, and any other work inside the boiler casing is not legal or safe for an unregistered person to carry out.
How much does it cost to repair a Worcester Bosch EC 256 fault in the UK?
The most common repair — replacing the flame-sensing or ignition electrodes — typically costs between £90 and £160 including parts and labour, and usually takes less than an hour. If the burner control unit (KIM/BCU) needs replacing, expect to pay in the region of £250 to £380 for parts and labour. In the less common scenario where the PCB or gas valve is at fault, costs can rise further — PCBs can reach £400 to £500 fitted and gas valves around £300 — but these are not the typical outcome for a 256 code and are better discussed with your engineer after a proper diagnosis.
Why does my Worcester Bosch keep locking out with EC 256 after I reset it?
If the boiler relights and then faults again within a short time, the underlying cause has not been resolved. The boiler is shutting down correctly as a safety measure each time it fails to detect a stable flame. Continuing to reset it is not advisable — on ionisation faults especially, repeated restarts can cause additional wear to the burner control unit and electrodes. The correct approach is to have a Gas Safe engineer identify and fix the root cause before resetting the boiler.
Is EC 256 linked to other Worcester Bosch codes like F0 242 or EA faults?
Yes. Cause code 256 sits within the broader F0 2xx family of diagnostic codes (covering approximately F0 242 through F0 272), all of which relate to the burner control unit and its wiring. It is commonly seen alongside primary locking codes in the EA family, which indicate flame or ignition failure leading to lockout. If your display shows a combination such as EA with cause code 256, or any F0 2xx variant, the diagnosis points to the same area: the ionisation and flame-detection circuit centred on the BCU, electrodes, and their wiring.