Worcester Bosch E9 318 Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch E9 318 fault code mean?
The E9 fault code appears on Worcester Bosch boilers when the central heating flow temperature has exceeded its safe operating limit — typically around 105°C — causing the built-in safety temperature limiter to trip and shut the boiler down. The boiler will remain locked out until the underlying cause is identified and resolved. On newer Greenstar models, a three-digit cause code is shown alongside E9 to help pinpoint the problem more precisely: cause code 318 indicates a general safety limiter trip due to overheating; 219 points specifically to a heat exchanger issue, commonly linked to limescale or sludge restricting heat transfer; 220 indicates a short circuit on the safety temperature sensor itself; and 221 indicates an open circuit on the same sensor, usually caused by a wiring fault or a sensor that is no longer being detected. Whichever sub-code is displayed, the boiler must not be repeatedly reset without diagnosing the root cause, as persistent overheating can damage internal 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 seized circulation pump Common
If the pump is not moving water around the system efficiently — whether due to a seized impeller, air lock, or internal wear — heat builds up inside the boiler and trips the limiter. This is one of the most frequent practical causes of E9 318, particularly if radiators are heating unevenly or the boiler locks out shortly after firing up.
- Sludge or debris restricting circulation Common
Accumulated magnetite sludge, scale, or debris in the pipework and radiators reduces water flow through the boiler. This is especially common in older systems that have never been powerflushed or that lack a magnetic filter. Restricted flow means heat cannot be carried away quickly enough, causing the temperature limiter to trip.
- Low system water pressure Common
If the boiler pressure has dropped below around 1 bar, there is insufficient water volume circulating to absorb and carry away heat from the heat exchanger. The boiler can then overheat and trigger E9. Check the pressure gauge on the boiler front panel — it should sit between 1 and 1.5 bar when the system is cold.
- Scaled or blocked heat exchanger (cause code 219) Sometimes
Limescale deposits on the primary heat exchanger reduce its ability to transfer heat into the water efficiently. The burner continues to fire but the water temperature rises rapidly, triggering E9. This is more common in hard-water areas and in boilers that have not been regularly serviced. If E9 appears mainly when hot water is being drawn off, a scaled plate heat exchanger is a strong suspect.
- Faulty safety temperature sensor — short or open circuit (cause codes 220 and 221) Sometimes
Cause code 220 means the safety temperature sensor has developed a short circuit, sending an incorrect signal to the PCB. Cause code 221 means the sensor circuit is open — the PCB cannot read it at all, likely due to a loose connector or a failed sensor. In both cases the boiler may not actually be overheating; instead, it is responding to a false or absent reading. A Gas Safe engineer will test the sensor and wiring to confirm whether replacement is needed.
- Faulty PCB (printed circuit board) Rare
A damaged or malfunctioning PCB may misinterpret sensor data, fail to control the pump or burner correctly, or trigger spurious lockouts. Water ingress and loose connections are common PCB failure modes. A faulty PCB can also display incorrect or misleading cause codes, complicating diagnosis.
How to fix it
- Check the boiler pressure gauge DIY safe
Look at the pressure gauge on the front of the boiler. It should read between 1 and 1.5 bar when the system is cold. If it is below 1 bar, the low water volume may be causing the boiler to overheat. Top up slowly using the filling loop valves beneath the boiler until the gauge reads around 1.2 bar, then close the valves firmly. If you are unsure how to do this, your boiler manual will have a step-by-step guide specific to your model.
- Bleed the radiators to remove trapped air DIY safe
Air trapped in radiators reduces water circulation, which can contribute to overheating. Use a radiator bleed key to open the bleed valve at the top of each radiator until water (not air) starts to escape, then close it. Check the boiler pressure afterwards and top up if it has dropped. This does not require any gas work and is safe for a homeowner to carry out.
- Check that your gas supply is active DIY safe
Verify that other gas appliances in your home (such as a gas hob) are working normally. If there is no gas supply to the property, contact your gas supplier rather than an engineer. If gas is reaching other appliances but not the boiler, this points to a boiler-specific fault requiring professional attention.
- Attempt a single boiler reset DIY safe
Once you have checked the pressure and bled the radiators, press the reset button on the boiler (usually marked with a flame and a line, or labelled 'Reset'). Allow the boiler to go through its start-up sequence. If it fires and runs normally, monitor it closely over the next hour. If E9 returns, do not keep resetting — repeated resets without fixing the root cause can cause further damage. Move on to calling an engineer.
- A Gas Safe engineer should diagnose the specific cause code Gas Safe engineer
An engineer will read the three-digit cause code displayed alongside E9 — 318, 219, 220, or 221 — and use this to guide their inspection. They will check the circulation pump for seized or sluggish operation, inspect the heat exchanger for scale or sludge, and test the safety temperature sensor and its wiring harness. A logical diagnosis at this stage avoids unnecessary part replacement.
- Engineer to inspect, free, or replace the circulation pump Gas Safe engineer
If the pump is found to be seized, the engineer may be able to free it manually. If it is worn or failing, a replacement pump will be fitted. This involves draining part of the system, disconnecting electrical connections, and refilling — all of which require competent heating engineer skills.
- Engineer to carry out a powerflush if sludge or scale is present Gas Safe engineer
If poor circulation is due to system-wide sludge or limescale, a powerflush using a specialist machine will be recommended. This circulates high-velocity water and cleaning chemicals through the entire system, flushing out debris. The engineer should also fit a magnetic system filter if one is not already present, to prevent future build-up.
- Engineer to replace the safety temperature sensor if cause code 220 or 221 is shown Gas Safe engineer
A faulty sensor with a short or open circuit will need to be replaced. The engineer will confirm the fault by testing resistance values with a multimeter before ordering the correct part. Wiring connections will also be inspected and reseated as needed.
- Engineer to assess the heat exchanger if cause code 219 is shown Gas Safe engineer
Depending on the extent of scaling, the engineer may attempt to descale the heat exchanger chemically, or recommend replacement if it is beyond recovery. On older boilers, this repair cost should be weighed against the cost of a new boiler.
- Engineer to test or replace the PCB if all other components check out Gas Safe engineer
If no mechanical or sensor fault can be found, the PCB is the remaining suspect. The engineer will check for signs of water damage, burnt components, or failed connections before deciding whether a repair or full replacement is required. PCB replacement is the most expensive common repair for this fault code.
Parts you may need
- Circulation pump (e.g. Grundfos UPS2 or Worcester-compatible replacement) · from £85
- Safety temperature limiter / STL sensor · from £35
- Primary heat exchanger (model-specific) · from £180
- Plate heat exchanger (DHW side) · from £75
- PCB (model-specific) · from £220
- Magnetic system filter (e.g. Magnaclean Pro2) · from £55
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–£400, depending on the underlying cause.
Frequently asked questions
Can I keep resetting my boiler when E9 318 appears?
No — a single reset is reasonable once you have checked the pressure and bled the radiators, but if the fault returns you should stop resetting and call a Gas Safe engineer. Repeatedly restarting a boiler that is genuinely overheating risks damaging the heat exchanger, pump seals, or PCB, potentially turning a straightforward repair into a far more expensive one.
What is the difference between cause codes 318, 219, 220, and 221 on a Worcester Bosch E9 fault?
These three-digit numbers appear alongside E9 on newer Greenstar models and give the engineer a more specific starting point. Code 318 is the general overheating / safety limiter trip — the boiler got too hot. Code 219 suggests the problem is at the heat exchanger, often because of limescale or sludge reducing heat transfer. Code 220 means the safety temperature sensor has short-circuited and is sending incorrect data to the PCB. Code 221 means the sensor circuit is open — the PCB cannot detect it at all, pointing to a wiring fault or a dead sensor. In the case of 220 and 221, the boiler may not actually be overheating; it is responding to a faulty sensor reading.
How much does it cost to fix a Worcester Bosch E9 fault?
For the most common repairs — replacing a faulty temperature limiter or freeing a seized pump — costs typically fall between £120 and £250 including labour. A full pump replacement usually runs to around £200–£350. A powerflush to clear sludge tends to cost £300–£500 depending on the size of your system. More involved repairs such as a heat exchanger replacement or a new PCB can cost £400–£780, and if your boiler is over ten years old it may be worth getting a replacement quote at the same time.
What can I do to prevent E9 faults coming back?
The most effective steps are: book an annual Gas Safe service so a professional can spot early signs of pump wear or sludge; fit a magnetic inline filter (such as a Magnaclean) if you do not already have one — this captures iron oxide particles before they accumulate in the heat exchanger; and consider adding a central heating inhibitor to the system water. If you live in a hard-water area, a scale reducer on the cold mains supply to the boiler is also worth discussing with your engineer.