Worcester Bosch 9F 855 fault codewhat it means and how to fix it
The 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.
Take care
What happens when you press reset?
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 lights and runs normally
A one-off lock-out — a momentary interruption in the oil supply or a draught at the wrong moment — which Worcester's own instructions allow you to clear.
Nothing more now. Keep an eye on the tank level, and if it happens again within a few days book a service rather than resetting through it.
It tries, then locks out on 9F 855 again
The burner is either not being fed or cannot prove a flame from what it is fed. Pump, filters, nozzle and photocell are the technician's checks, in Riello's order.
Confirm the tank has oil, then stop. Book an OFTEC registered technician and quote the code.
The display now shows 9F 856
Four lock-outs have been reset in one demand and the controls have locked the reset out to protect the burner. There is a separate page for 856.
Stop here. Switching the boiler off and on clears the count, but the fault that caused four lock-outs is still there and needs a visit.
There is a smell of oil or smoke around the boiler
Either kerosene is escaping or combustion products are, and neither is reached by pressing reset again.
Switch the boiler off at its mains switch, open a window, and do not reset it. Book an OFTEC registered technician as urgent. If anyone feels unwell, leave the building and call 999 or NHS 111.
What 9F 855 means
The Greenstar Heatslave II is Worcester's floor-standing oil-fired combi, and 9F 855 is the code it shows most: a burner lock-out. Table 12 of the 2022+ installation and servicing manual gives it in one line — "burner lock out, reset type reset, check the burner and fuel supply, ensure the oil supply is free of contaminants" — and the operating instructions give the householder a little more: press Reset to restart the burner, and if 9F 855 comes straight back, check the oil level in the tank. The burner is a Riello RDB with a digital control box, and a lock-out is that box refusing to run. It starts the motor and pump, purges, fires the ignition and then waits a fixed safety time for the photocell to see a flame; if it sees none, or sees one when it should not, or loses it too many times, it locks out and the boiler's controls cut power to the burner and flash the blue light with 9F and 855.
Behind the case the box's own LED says which kind: a steady red is no flame after the safety time, a slow red flash a false flame signal, a fast red flash too many flame-loss relights, and red with orange or green a fan motor, oil valve or memory fault. None of that is visible from the front, which is why the page can only tell you what the householder text does.
What it cannot do is make the reset a repair. Worcester count the attempts: after four 855 lock-outs are reset in one demand period the controls lock into 9F 856 and only switching the boiler off and on clears the count. The technician's chart runs through the oil level, the pump and its filter, the oil pressure, the nozzle, the photocell voltages and the box itself in order, and the pre-installation pages add the reason the table mentions contaminants: water or sludge drawn from the bottom of a tank blocks filters and nozzle, and Worcester say failing to keep the supply clean may invalidate the warranty.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Oil supply interrupted or run low | An empty or nearly empty tank, a closed outlet valve, or air drawn into a single-pipe line leaves the pump nothing to deliver, and the box locks out for no flame after its safety time. The user guide's only check is the oil level. | Common |
| Contaminated fuel blocking the filters or nozzle | Water and sludge from the bottom of the tank block the pump filter and the nozzle. Worcester's table says to make sure the supply is free of contaminants, and their pre-installation section warns it can invalidate the warranty. | Common |
| Nozzle or photocell not doing its job | A worn nozzle no longer atomises the fuel into something the electrodes can light, and a photocell reading outside Riello's figures of about 3.8 volts in the dark and 1.2 in the light cannot prove the flame it is looking at. | Occasional |
| Motor, pump or control box fault | The Riello box's LED distinguishes a seized fan motor or oil pump, an oil valve driver failure and a failure of its own memory from a plain no-flame lock-out. Each is a replace-the-component visit rather than a reset. | Occasional |
What to try yourself
Press the Reset button on the fascia once. Reset and a tick appear briefly, and the burner will try to light — give it a couple of minutes before deciding whether it worked.
If 9F 855 comes back, look at the oil level in the tank on its gauge or sight tube, and check the tank's outlet valve is open. Do not touch the fire valve, the pump or anything inside the boiler case.
Do not keep resetting. Worcester count the attempts, and after four in one demand the controls lock into 9F 856 — a code that only a power-down clears, and a burner that has still not lit.Write down how many times it locked out and whether the tank had run low. If there is oil and the burner still will not light, stop there and book an OFTEC registered technician, quoting 9F 855 and the count.
Get an OFTEC registered technician to look at your Worcester Bosch 9F 855
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 it costs to fix
Typical repair cost
£80–£350
Time on site
1h 30m
An OFTEC registered technician's call-out with diagnosis is typically £80–£150, and a nozzle and filter clean is usually done within it. Photocell £40–£90 fitted, oil pump £150–£300, burner control box £150–£350. Riello's chart works through them in order, so ask which line the fault stopped at.
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.
Worcester Bosch Heatslave II burner control box (Riello digital)
£90–£220Authorised Worcester parts distributor
Price checked 28 September 2026
Worcester Bosch Heatslave II burner photocell (flame sensor)
£20–£60Authorised Worcester parts distributor
Price checked 28 September 2026
Worcester Bosch Heatslave II burner oil pump
£80–£180Authorised Worcester parts distributor
Price checked 28 September 2026
Worcester Bosch Heatslave II burner nozzle
£8–£25Authorised Worcester parts distributor
Price checked 28 September 2026
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Related Worcester Bosch fault codes
- 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
- E9 210Stop — call an engineerOne 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.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 9F 855
Is a burner lock-out dangerous?
The lock-out itself is the burner control box refusing to run because it could not confirm a flame within its safety time, which is the safe outcome. The risk is in overriding it: repeated resets keep asking a burner that is not lighting to try again, which is what Worcester's four-attempt limit exists to stop. Oil work is not restricted by law the way gas work is, but Worcester's own user guide says the boiler must be serviced by a competent, qualified OFTEC registered engineer, and a lock-out that will not clear is exactly that visit.
Can I press the reset button on the burner itself?
Not from outside the case. The Riello control box has its own lock-out reset behind the casing, and Worcester's manual puts it inside a burner diagnostics mode for service engineers, entered by holding the service button while pressing the controls reset. The box counts too: after four presses it will not reset until the power to the burner has been interrupted.
The power went off during the lock-out and now it says F0 263
That is what Worcester's protection diagram says will happen. A power cut during any lock-out leaves F0 263 — appliance powered off with a locking fault present — on the display when the power returns. Press reset once to clear it. If the original lock-out was a burner lock-out, the controls initialise and lock out again on 9F 855, because the fault is still there.
Why does the table talk about contaminants in the oil?
Because that is where a great many lock-outs come from. Worcester's pre-installation section says failing to ensure the oil supply is clear of contaminants may invalidate the warranty. Water and sludge collect at the bottom of the tank, get drawn into the line when the level is low or just after a delivery has stirred it up, and block the pump filter and the nozzle. A lock-out that follows a low tank often needs the filters cleaned rather than a part replaced.
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.