This fault can involve incomplete combustion and carbon monoxide.
If anyone in the property feels sick, dizzy or drowsy, or you can smell gas: open the windows, leave the building, and do not switch anything on or off. Then call the National Gas Emergency Service.
0800 111 999Worcester Bosch E9 224 fault codewhat it means and how to fix it
One of the two safety thermostats has tripped: the flue overheat thermostat or the high-limit stat on the heat exchanger. Worcester's own check list includes the burner gasket for flue gas leakage.
Stop — this needs an engineer
Get a Gas Safe registered engineer to look at your Worcester Bosch E9 224
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
What happens when you reset it?
The right answer depends on what you can see. Pick the one that matches.
Choose the option on the left that matches what your appliance is doing, and the answer for that case appears here.
It runs for a while, then locks out on 224 again
A thermostat is genuinely reaching its limit each cycle. Either the water is not carrying heat away or the flue gases are too hot, and both need the case off to tell apart.
Leave the boiler off and book a Gas Safe registered engineer. Repeated cycles into an overheat are what damage the heat exchanger.
It locks out on 224 again straight away, stone cold
Nothing can be overheating from cold, so the safety circuit itself is open — a thermostat that has failed, or a harness connector that has come adrift.
Still an engineer visit, but tell them this: it points them straight at the thermostats and wiring rather than the pump and system.
There is a smell of fumes near the boiler
Combustion products may be escaping inside the case. That is the burner gasket scenario Worcester list, and it is the reason this code is treated as a danger code.
Switch the boiler off, open windows, and do not use it until an engineer has checked the flue and burner seal. If anyone feels unwell, leave the property and call 0800 111 999.
What E9 224 means
Worcester's Greenstar i table lists this under "Temperature fault" as E9 with cause code 224, described as "flue or high limit thermostat activated (safety circuit)". The Compact family manuals put it more bluntly: "max temperature thermostat activated — flue gas thermostat overheat". It is a different fault from E9 219, which is the electronic safety sensor reading too hot. Here a physical thermostat has opened. Two of them sit in one series loop: a flue overheat thermostat clipped to the flue outlet, and a high-limit stat on the heat exchanger body.
If either one opens, the control board loses the circuit and locks out with 224. The engineer's checks in Worcester's order are the thermostat harness and connector, the operation of both thermostats, circulation through the heating system, the burner gasket for leakage of flue gases, the heat exchanger for debris, and the air-gas ratio.
Two of those six are about combustion products escaping where they should not, which is why this page carries no steps for you to try. A flue thermostat that keeps tripping is telling you the flue gases are hotter than they should be, and the usual reasons are a heat exchanger fouled with debris on the combustion side or a burner running off-ratio. A high-limit stat that keeps tripping is a circulation problem that the pump, air locks or sludge will explain. Worcester describe a reset as a brief press of the reset button on the i and Compact ranges, but the fault will simply return until the thermostat that opened has been found and the reason for it dealt with.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Poor circulation through the heat exchanger | A failing pump, an air lock or a sludged system lets the exchanger get hot enough to open the high-limit stat even though the burner is modulating normally. | Common |
| Flue gases running too hot | Debris on the combustion side of the heat exchanger, or an air-gas ratio that has drifted, sends hotter products up the flue and trips the flue overheat thermostat. | Common |
| Burner gasket leaking | Worcester name this specifically. A deteriorated gasket lets hot combustion products escape past the burner and heat parts of the appliance the thermostats protect. | Occasional |
| Thermostat or harness fault | A corroded connector or a thermostat that has failed open gives 224 with nothing actually overheating. The engineer proves this with a continuity check before condemning anything else. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£90–£450
Time on site
1h 30m
Flue or high-limit thermostat £90–£180 fitted. Heat exchanger combustion-side clean £120–£250. Pump £280–£450.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Central heating circulating pump
CH-PUMP-15-60£90–£180Plumbing merchant
Price checked 28 September 2026
Worcester Bosch Greenstar high limit thermostat
87072061960£15–£45Authorised Worcester parts distributor
Price checked 28 September 2026
Worcester Bosch Greenstar flue overheat thermostat
87105062670£20–£50Authorised Worcester parts distributor
Price checked 28 September 2026
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Worcester Bosch fault codes
- E9 219Stop — call an engineerThe boiler has overheated and the safety thermostat has tripped. Water is not carrying heat away from the exchanger, and it should not be run until that is fixed.Related: overheat and circulation
- A1 229Stop — call an engineerThe boiler has detected a dry-fire condition — it is trying to heat water that is not moving. Turn it off and leave it off until an engineer has seen it.Related: overheat and circulation
- E5 218Get it checkedThe flow temperature sensor has measured water leaving the heat exchanger far hotter than it should be. The boiler shut down to protect itself, and heat is not being carried away fast enough.Related: overheat and circulation
- E2 222Get it checkedThe flow temperature sensor is giving a reading the appliance cannot trust. Usually the sensor rather than the temperature.Related: overheat and circulation
- D1 240Get it checkedThe return temperature sensor has short circuited. The boiler has lost its reading of how warm the water is when it comes back from the radiators.Related: overheat and circulation
- D4 341Get it checkedThe water temperature in the boiler is climbing faster than it should, which means very little of it is moving. Worcester's checks are pressure, air, circulation and the pump, in that order.Related: overheat and circulation
Own a Worcester Bosch boiler? Save it and its codes come first next time.
Related guides
Common questions about Worcester Bosch E9 224
Is 224 the same as E9 219?
No. Both are E9 temperature faults and both stop the boiler, but 219 is the electronic safety sensor on the heat exchanger reporting too high a temperature, and 224 is a mechanical thermostat in the flue or on the exchanger physically opening. They have overlapping causes but an engineer diagnoses them differently.
My display just says E9 with no number — is that this fault?
On a Greenstar CDi Classic the display only ever shows the two characters, and Worcester describe E9 there as the safety temperature limiter in the flow having tripped. Treat it as the same family of overheat fault. On a Greenstar i or Compact you can read the three-digit number in the info menu or, during a lockout, flashing under the letters.
Can I reset it and carry on?
You can press reset once — Worcester's user guide allows that — but if 224 comes back, stop. Each overheat cycle stresses the heat exchanger, and if the cause is flue gas leaking past the burner gasket, running the boiler is not safe.
What does the repair cost?
If it is a thermostat or connector, around £90 to £180 including the visit. A combustion-side clean of the heat exchanger at a service is £120 to £250. If the pump has failed, £280 to £450. A burner gasket is cheap as a part but is fitted as part of a strip-down and combustion check.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.