Worcester Bosch E9 220 Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch E9 220 fault code mean?
The E9 code means the boiler's safety temperature limiter has tripped after detecting that water in the central heating flow reached or exceeded 105°C, causing the boiler to shut down to protect itself from heat damage. The 220 sub-code narrows this down to a safety sensor short circuit — the heat exchanger temperature sensor has failed and is sending an incorrect signal to the control board, which then shuts the boiler down as a precaution. In this scenario the boiler may not actually be overheating at all; the sensor itself is the problem. Other sub-codes you may see alongside E9 on Worcester Bosch boilers include 219 (overheating detected at the heat exchanger, often related to poor circulation or limescale) and 221 (safety limiter not recognised or open circuit), so if your display shows E9 219 or E9 221 rather than E9 220, the root cause may differ slightly — 219 often points to a genuine overheating issue such as a blocked heat exchanger or failing pump, while 221 suggests an open-circuit or disconnected limiter rather than a short circuit.
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
- Failed or short-circuit safety temperature sensor Common
The 220 sub-code points directly here. The sensor monitoring the heat exchanger temperature has short-circuited, feeding the PCB a false reading that looks like a dangerously high temperature. The boiler shuts down even though the water may be at a completely normal temperature. Replacing the sensor is typically a straightforward and relatively affordable repair for a Gas Safe engineer.
- Limescale build-up in the heat exchanger Common
In hard-water areas, limescale accumulates on the internal surfaces of the heat exchanger over time, acting as an insulator and preventing heat from transferring efficiently into the circulating water. Hot spots develop and the safety limiter trips. This is a very common cause of genuine E9 overheating faults and can eventually crack the heat exchanger if left unaddressed. A powerflush or descale is the usual remedy, and fitting a magnetic filter afterwards helps prevent recurrence.
- Circulation pump failure or seizure Sometimes
The pump is responsible for moving heated water continuously around the system. If it seizes, loses efficiency through wear, or develops an airlock, water stagnates inside the heat exchanger and its temperature climbs rapidly until the limiter trips. Signs include cold radiators, uneven heating, or an unusually quiet boiler during a heating call.
- Low system pressure Sometimes
When pressure drops below around 1 bar, the reduced volume of water circulating through the heat exchanger means heat builds up more quickly than normal, which can contribute to overheating faults. Check the pressure gauge on the boiler — it should read between 1 and 1.5 bar when the system is cold.
- Faulty PCB (control board) Rare
A damaged or deteriorating PCB can misinterpret signals from sensors and incorrectly trigger a shutdown. This is only likely if all other causes have been ruled out by an engineer, as PCB faults tend to produce a wider range of symptoms. It is also the most expensive component to replace.
How to fix it
- Do not repeatedly reset the boiler DIY safe
With any E9 fault, resist the urge to keep pressing the reset button. Each reset while an overheating condition exists risks causing real heat damage to components that were previously unaffected. Reset the boiler once, allow it to attempt a restart, and observe what happens. If it locks out again immediately, stop resetting and move on to the checks below.
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the front of the boiler — most Worcester Bosch models have a visible gauge or a digital pressure reading. It should sit between 1 and 1.5 bar when the system is cold. If it reads below 1 bar, top up the pressure using the filling loop (the braided hose under or near the boiler) until the gauge reaches approximately 1.2 bar, then reset the boiler once. If pressure is already in the correct range, this is not the cause.
- Check for cold radiators or uneven heating DIY safe
Before calling an engineer, note whether any radiators are cold, whether some rooms heat up much faster than others, or whether you can hear unusual noises from the boiler or pipework. These observations help the engineer narrow down whether poor circulation is involved and are worth mentioning when you call.
- Check whether the boiler returns an E9 220 or a different sub-code DIY safe
Confirm the exact code on the display. E9 220 indicates a sensor short circuit; E9 219 suggests genuine overheating often linked to circulation or limescale; E9 221 points to an open-circuit or disconnected limiter. Passing this information to the engineer saves diagnostic time and may reduce call-out time.
- Arrange for a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
Because E9 220 specifically indicates a sensor short circuit, the engineer will test the safety temperature sensor and its wiring for continuity and resistance. If the sensor has failed, replacing it is generally one of the more affordable E9 repairs. The engineer will also check for underlying overheating causes such as limescale or a tiring pump to ensure the new sensor is not simply masking a deeper problem. Do not attempt to access, test, or replace the sensor yourself — it sits on the heat exchanger and working in this area requires a Gas Safe registered professional.
Parts you may need
- Safety temperature limiter / overheat thermostat · from £25
- Heat exchanger NTC temperature sensor · from £20
- Central heating circulation pump · from £85
- Printed circuit board (PCB) · 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 £120–£380, depending on the underlying cause.
Frequently asked questions
Is E9 220 dangerous — do I need to evacuate?
No, you do not need to evacuate. E9 220 means the boiler has already shut itself down safely. Because the 220 sub-code points to a sensor short circuit rather than a confirmed overheating event, there is no immediate risk of fire or explosion. Simply leave the boiler off and arrange for a Gas Safe engineer to inspect it. There is no gas leak associated with this fault.
Why does my boiler show E9 220 straight away after a reset, without even running?
This strongly suggests the sensor itself has failed rather than the boiler actually overheating. If the safety temperature sensor has short-circuited, it sends a continuous fault signal to the PCB, which responds by locking out before the burner can even fire. An engineer can confirm this quickly with a resistance check on the sensor.
Can the repair cost be much higher than your typical range?
Yes, in some cases. The £120–£380 range covers the most common outcomes — sensor replacement plus a call-out, or pump work. If the engineer finds the heat exchanger is cracked from repeated overheating, replacement typically costs £330–£620 or more including labour. A PCB replacement, only needed if everything else has been ruled out, is usually £430–£780. If your boiler is over ten years old and needs either of these major components, it may be worth getting a new boiler quote alongside the repair quote.
How can I prevent E9 faults coming back in the future?
The best preventive steps are: booking an annual Gas Safe service (around £80–£120), which lets an engineer spot sludge build-up and a weakening pump before they cause a lockout; fitting a magnetic inline filter if you do not already have one, to capture iron oxide and debris; and, in hard-water areas, considering a scale inhibitor or water softener to slow limescale accumulation in the heat exchanger. If the engineer performed a powerflush to clear existing sludge, keeping the filter clean afterwards is especially important.