Worcester Bosch EF Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch EF fault code mean?
The EF fault code — typically displayed alongside cause code 349 — indicates that your Worcester Bosch boiler has detected a 'central heating boil' condition. This means the temperature difference between the flow pipe (water leaving the boiler) and the return pipe (water coming back) has exceeded 18°C while the burner is running at its lowest output. This gap should be far smaller during normal operation; when it is this large, the boiler interprets it as dangerously poor circulation and locks out to protect itself. Because it is a locking fault, the boiler shuts down completely, displays a flashing EF code alongside a triangle warning symbol, and will not restart until it is manually reset. The root cause is almost always a circulation problem somewhere in the heating system rather than a fault with the boiler's combustion components.
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 underperforming circulation pump Common
The central heating pump is responsible for keeping hot water moving through the system. If it has seized, is running too slowly, or is not responding correctly to signals from the PCB, water sits in the pipes far longer than it should — cooling excessively before returning to the boiler. This creates exactly the large flow-to-return temperature gap that triggers the EF code, making a pump fault the single most likely culprit.
- Sludge, limescale or system blockage Common
Over time, corrosion inside radiators and pipework produces a dark, gritty deposit known as system sludge, while hard water areas accumulate limescale on heat exchanger surfaces. Either of these can partially block the circuit, severely restricting how quickly water moves through the system. The boiler continues to add heat, but the sluggish flow means the return temperature lags far behind the flow temperature, tripping the EF lockout.
- Low system pressure Common
Central heating systems need to be pressurised — typically between 1 and 1.5 bar — to circulate water effectively. When pressure drops below this range, the volume of water in the circuit is reduced and the pump cannot maintain adequate flow. This can cause the boiler to heat a small volume of water very quickly, pushing the flow temperature well above the return and triggering the overtemperature differential fault.
- Blocked or partially restricted pipework Sometimes
Even without widespread sludge, a single partially closed valve, a seized thermostatic radiator valve (TRV), or a blockage in the heat exchanger itself can create a localised restriction. If the flow pipe feels very hot but the return pipe is noticeably cooler to the touch, a restriction somewhere in that circuit is likely. This prevents the normal heat exchange process and causes the temperature differential to rise above the 18°C threshold.
- Faulty flow or return temperature sensor Sometimes
The boiler's PCB relies on accurate readings from the NTC thermistors fitted to the flow and return pipes to calculate the temperature differential. If one of these sensors is giving an incorrect reading — due to deterioration, a poor connection, or physical damage — the PCB may calculate a false temperature gap and trigger the EF code even when circulation is perfectly adequate.
- PCB fault affecting pump control Rare
The printed circuit board governs the pump's speed and operation. If the PCB is sending incorrect signals or failing to communicate with the pump at all, the pump may run at the wrong speed or not at all, leading to the same poor-circulation symptoms as a mechanically faulty pump. This is a less common cause but worth investigating if the pump itself tests as mechanically sound.
How to fix it
- Check and note down the cause code shown alongside EF DIY safe
Worcester Bosch boilers typically display a secondary cause code next to the EF fault — most commonly 349 for central heating boil detected. Make a note of any numbers displayed alongside EF, as this will help an engineer diagnose the fault more quickly when they attend.
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the front of your boiler. It should read between 1 and 1.5 bar when the system is cold. If it is reading below 1 bar, you can top up the pressure yourself using the filling loop — a small braided hose or lever valve typically located beneath the boiler. Add water slowly until the gauge reaches 1.2–1.5 bar, then close the filling loop fully. If the pressure is already in the correct range, a low-pressure issue is not the cause.
- Reset the boiler once DIY safe
Once you have checked the pressure, attempt a single reset. On most Worcester Bosch models this involves holding the reset button (often marked with a flame symbol or labelled 'Reset') for a few seconds until the fault clears. Consult your model's user guide if you are unsure of the exact method. If the boiler fires up and runs normally, monitor it closely over the next hour. If the EF code returns, do not keep resetting — repeated resets on a locking fault can stress components and mask the underlying problem.
- Check whether radiators are heating evenly DIY safe
While the boiler is running, feel each radiator in the house. If some are cold at the bottom but warm at the top, or if certain radiators are not heating at all, this points to circulation restrictions or sludge. Also feel the flow and return pipes close to the boiler — if the flow is very hot but the return is noticeably cooler than expected, poor circulation is confirmed. Report these observations to your engineer.
- Do not attempt to inspect or replace the pump, sensors, or PCB yourself Gas Safe engineer
The circulation pump, NTC temperature sensors, heat exchanger, and PCB must only be inspected and replaced by a qualified professional. Tampering with these components can void your boiler warranty, may be unsafe, and in the case of any gas-side components is illegal without Gas Safe registration.
- Call a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
An engineer will measure the actual flow and return temperatures, check the pump's operation and speed, inspect the heat exchanger for blockages, and test the temperature sensors with a multimeter. Based on findings they may replace the pump, carry out a powerflush to clear sludge and debris from the system, replace a faulty sensor, or investigate the PCB. Provide them with the full fault code including any secondary cause code you noted in step one.
Parts you may need
- Central heating circulation pump (e.g. Grundfos UPS2 or equivalent) · from £85
- NTC flow/return temperature sensor · from £25
- Powerflush chemicals and inhibitor · from £40
- Magnetic system filter (e.g. Magnaclean) · from £75
- PCB (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 £150–£450, depending on the underlying cause.
Frequently asked questions
What does EF 349 mean on a Worcester Bosch boiler?
The number 349 is the cause code that most commonly appears alongside the EF fault on Worcester Bosch boilers. It specifically identifies the condition as 'central heating boil detected' — meaning the PCB has measured a temperature differential greater than 18°C between the flow and return pipes while the burner is at minimum output. In practice, EF and EF 349 refer to the same underlying circulation problem, so the diagnosis and repair steps are the same whether you see just 'EF' or 'EF 349' on your display.
Can I fix the EF fault myself by resetting the boiler?
A reset may clear the fault temporarily, but the EF code is a locking fault triggered by a real circulation problem — it will almost certainly return unless the root cause is fixed. You can safely check your system pressure and top it up via the filling loop if it is below 1 bar, and you can attempt one reset. However, if the code reappears, a Gas Safe registered engineer needs to investigate. Repeatedly resetting a locking fault is not recommended as it can put additional strain on components.
How much does it cost to fix an EF fault on a Worcester Bosch boiler?
Most people with an EF fault end up paying somewhere between £150 and £450 all in, depending on what the engineer finds. A straightforward pump replacement including labour is typically at the lower end of this range. A full powerflush on a larger system will cost more, often £300–£500 depending on the number of radiators. In rare cases where the PCB needs replacing, costs can rise beyond £500 including parts and labour — but this is not the most common outcome. If your boiler is over 10–12 years old and has been faulting repeatedly, it may be worth getting a new boiler quote alongside the repair quote.
Why does my Worcester Bosch boiler keep showing the EF code after being reset?
If the EF fault keeps returning after a reset, it means the underlying circulation problem has not been resolved. The most likely reasons are a circulation pump that is gradually failing, a significant sludge or limescale buildup slowing the flow of water around the system, or a faulty temperature sensor giving the PCB inaccurate readings. Each reset is only clearing the symptom, not the cause. A Gas Safe engineer should carry out a proper diagnosis — checking pump performance, system flow rates, and sensor accuracy — to identify and fix the actual fault.