This fault can involve incomplete combustion and carbon monoxide.
If anyone in the property feels sick, dizzy or drowsy, or a carbon monoxide alarm is sounding: switch the boiler off, open the windows and leave the building. Do not go back in until it is safe, and do not use the boiler again until it has been checked. Call 999 if anyone is struggling to breathe, suddenly confused, weak, in chest pain or has collapsed; otherwise call NHS 111 for advice.
NHS 111Worcester Bosch E9 210 fault codewhat it means and how to fix it
One of three manual-reset safety devices has opened: the flue gas overheat thermostat, the air pressure switch or the tank overheat thermostat. Overheated flue gases or a restricted flue are the usual reasons.
Stop — this needs an engineer
Get an OFTEC registered technician to look at your Worcester Bosch E9 210
We do not pass enquiries to a repair firm, so there is no form here. Find an OFTEC registered technician through the OFTEC, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
What else have you noticed?
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.
Soot, smoke or a smell of fumes near the boiler or the flue terminal
Combustion products are escaping or the burner is running dirty. That is a carbon monoxide risk, not a nuisance trip.
Switch the boiler off at its mains switch, open the windows and do not reset anything. If anyone feels unwell, leave the building; call 999 if someone is struggling to breathe, suddenly confused, weak or in chest pain, NHS 111 otherwise. Then book an OFTEC registered technician as urgent.
The hot water had been much hotter than usual
Points at the tank overheat thermostat: the thermal store has passed its limit, which is a controls or circulation fault rather than a flue one.
Leave the boiler off. It is still a technician's visit — the reset is under a screwed cover and the reason it tripped is inside the case.
It started after a service or a nozzle change
Burner set-up — a nozzle of the wrong size or angle, or the air setting off — or a sensing tube left off the air pressure switch.
Call back the technician who did the work. It is a commissioning check rather than a new fault, and they should have the analyser readings.
What E9 210 means
On the Heatslave II, E9 210 is the safety circuit opening. Table 12 of the 2022+ manual lists it as "maximum temperature thermostat or air pressure switch activated — flue gas or tank thermostat overheat; reset the overheat thermostat or air pressure switch before resetting appliance", and section 8.13 spells out the three devices behind it. If the flue gas temperature exceeds its cut-off point, or the flue is restricted, the flue gas thermostat trips and the appliance locks out on E9 210.
If the pressure difference between the flue inlet and the exhaust exceeds a set value, the air pressure switch trips, and the code is the same. And if the thermal store passes its set temperature, the tank overheat thermostat trips, again with the same code. Each has its own manual reset — two buttons on the underside of the cross brace and a black button under a screwed cover — and all three are inside the appliance case, in a section of the manual written for the service engineer. That is why this page carries no steps. Two of the three triggers are about combustion products: flue gases hotter than they should be mean the burner is running dirty or the heat exchanger is fouled on the fire side, and a restricted flue means those gases are not being cleared as designed.
On an oil boiler, as on a gas one, that is the situation that produces carbon monoxide, and pressing three buttons to clear it would be pressing them for a reason nobody has found. The tank thermostat is the exception in kind — a controls or circulation fault rather than a combustion one — but the display does not tell you which device opened, and the reset that would tell you is behind the case. The gas Greenstar Compact ranges use 210 for the same flue thermostat trip as a blocking stage before E9 224; on the Heatslave II it is a locking fault, cleared with the fascia reset only after the device itself has been reset.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Flue gases running too hot | Soot on the fire side of the heat exchanger, a worn nozzle or a burner air setting that has drifted send hotter products up the flue until the flue gas thermostat opens at its cut-off point. | Common |
| Restricted or blocked flue | Worcester name a restricted flue as a trigger for the flue gas thermostat, and the air pressure switch watches the pressure difference between flue inlet and exhaust for the same reason. A blocked terminal, a failed seal or debris in the flue does it. | Common |
| Thermal store overheating | The tank overheat thermostat trips when the store passes its limit — a diverter valve or pump that is not moving heat away, or controls that did not stop the burn. Its black reset button is under a screwed cover. | Occasional |
| Air pressure switch tube or switch fault | A kinked or disconnected sensing tube — the commissioning checklist asks specifically whether the hoses are routed and free of kinks — or a failed switch opens the circuit with nothing wrong in the flue at all. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£90–£400
Time on site
1h 30m
Call-out with the tripped device found and the cause diagnosed £80–£150. Flue gas thermostat or air pressure switch £60–£150 fitted. A fire-side clean of the heat exchanger with a new nozzle, burner set-up and flue gas analysis £120–£250. Flue repairs depend entirely on what is wrong with the flue.
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 Heatslave II flue gas overheat thermostat
£30–£80Authorised Worcester parts distributor
Price checked 28 September 2026
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Related Worcester Bosch fault codes
- 9F 855Get it checkedThe oil burner could not establish or keep a flame and its control box has locked out. Worcester allow a reset and an oil-level check; four lockouts in one demand become 9F 856.Related: oil burner, flue and fuel supply
- 9F 856Get it checkedFour 9F 855 burner lock-outs have been reset in one demand, so the controls have locked the reset out to protect the burner. A power-down clears the count; the cause is still there.Related: oil burner, flue and fuel supply
- EA 227Stop — call an engineerThe boiler tried to light and never detected a flame, so it has shut the gas off and locked out. This is the most common Worcester fault code.Related: same brand
- CE 207Get it checkedSystem water pressure has dropped too low for the boiler to run safely. Topping the system back up to 1.2 bar usually clears it in five minutes.Related: same brand
- 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: same brand
- C6 224Get it checkedThe fan is not reaching the speed the boiler asked for, so combustion cannot be confirmed as safe and the burner has been prevented from firing.Related: same brand
Own a Worcester Bosch boiler? Save it and its codes come first next time.
Related guides
Common questions about Worcester Bosch E9 210
Can I press the reset buttons myself?
They are inside the case, and Worcester put the procedure in the fault-finding section written for the service engineer. More to the point, the device tripped for a reason, and resetting it without finding the reason puts the boiler back into the condition that tripped it. Oil work is not restricted by law in the way gas work is, but this is the fault where switch off, ventilate and get an OFTEC registered technician is the honest advice.
Is E9 210 the same as E9 224 on a gas Worcester?
Same idea, different boiler. On the gas Greenstar i the flue or high-limit thermostat opening is E9 224, and on the Compact ranges 210 is the blocking stage before it. On the Heatslave II, E9 210 is the locking fault itself, and it covers the air pressure switch and the tank overheat thermostat as well as the flue gas thermostat.
Why does the code not say which device tripped?
Because the display reports the safety circuit opening rather than which of the three devices opened it. The technician finds out by looking at which reset has tripped: the flue gas thermostat and air pressure switch buttons on the underside of the cross brace, or the black tank thermostat button under its cover.
The boiler keeps stopping with no code and restarting after half an hour — is that this?
No. That is a blocking overheat — 276, 277 or 359 in the info menu — which the user guide says usually clears itself once the boiler has cooled, and to wait thirty minutes. E9 210 is a locking fault: the blue light flashes with the code and nothing restarts until the tripped device and then the boiler have both been reset.
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
- Trade body guidance
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.