Worcester Bosch 9F 856 fault codewhat it means and how to fix it
Four 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.
Take care
Get an OFTEC registered technician to look at your Worcester Bosch 9F 856
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 has happened since 856 appeared?
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.
You switched the boiler off and on and it now shows F0 263
That is the documented sequence: the power-down clears the count and leaves 263, appliance powered off with a locking fault present.
Press reset once to clear 263. If the burner then locks out on 9F 855 again, do not use the new attempts — book an OFTEC registered technician and tell them about the four.
After the power-down it lit and has run since
The fault was intermittent, most often an oil supply that had just been restored or air working its way out of the line.
Watch the tank level and mention the episode at the next service. Four lock-outs in one demand is a reason to look at the filters and nozzle even though it is running.
It still shows 9F 856
The controls have not been power-cycled — the fascia reset does not clear 856 — or the power was off too briefly.
Switch off at the boiler's mains isolator, wait thirty seconds and switch on, which is also the user guide's remedy for a display that has stopped responding. Then leave it for the technician.
What 9F 856 means
9F 856 is not a new fault. It is the Heatslave II telling you that the reset button has been used on the same burner lock-out four times in one heating or hot water demand, and that it will not accept a fifth. Worcester's 2022+ manual describes it in the burner diagnostics section: if a burner lock-out is reset four times in one demand period using the fascia reset button, the controls lock out with the new code 9F 856, "to protect the burner components against over use of the reset", and clearing it requires a power-down of the appliance.
Table 12 adds the sequence that follows: after switching the boiler off and on, cause code F0 263 is displayed — the code for a locking fault present when power was lost — and pressing the reset button clears that, after which another four burner lock-out resets can be attempted. The operating instructions put the same thing to the householder: if you try more than four times to reset and fail, no more resets can be attempted, 9F 856 is displayed, and you should call Worcester quoting the codes. The Riello control box on the burner has the same discipline built in, and if its own button is pressed four times it will not reset until the power to the burner has been interrupted.
The count is the point. Four failures to prove a flame in one demand are not a nuisance trip; whatever stopped the burner lighting the first time — no oil, dirty oil, a nozzle that will not atomise, a photocell that cannot see — has been there through all four, and each attempt has asked the same burner to try again against it. The power-down is a way of letting a technician work, not a way of getting the heating back. One detail differs between the documents: the 2016 External ErP+ manual says five lock-outs rather than four. The meaning and the remedy are the same.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| The 9F 855 fault was never dealt with | 856 is a count of resets, not a diagnosis. The oil supply, filters, nozzle, photocell or control box problem that produced the first lock-out has produced all four, and it is still in the boiler after the power-down. | Common |
| Resetting through an empty or aerated oil line | A tank run low draws air and sediment into the supply, and each reset only runs the pump against it. Worcester's user guide puts the oil-level check before the second reset, not after the fourth. | Common |
| A burner component failing on every attempt | A motor or oil pump that starts and stalls, or an oil valve driver fault, locks the burner out on each try. The Riello box's LED code says which, and it is read behind the case by the technician. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£400
Time on site
1h 30m
The visit is the same as for a single 9F 855 lock-out — call-out with diagnosis typically £80–£150 — but the cause is more often a part than a nuisance: photocell £40–£90, oil pump £150–£300, control box £150–£350, all fitted. If the tank had run dry, a filter clean and a bleed of the line may be the whole job.
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 nozzle
£8–£25Authorised 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
- 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 856
Is it four lock-outs or five?
Worcester's 2022+ manual and operating instructions say four: after four 855 lock-outs in one demand period, 856 is displayed. The 2016 manual for the External ErP+ says five. The mechanism, the power-down and the F0 263 that follows are identical, so the difference does not change what to do.
Why has the reset button stopped working?
By design. Worcester's stated reason for the limit is to protect the burner components against over-use of the reset, and the burner control box has the same rule of its own. Once 856 is showing, the fascia button does nothing until the appliance has been powered off and on.
Can I keep switching it off and on to get the heating back?
Each power-down buys four more attempts, and using them to force the boiler to run is exactly what the limit exists to prevent. A burner that has locked out eight times has told you everything it can. Oil work is not legally restricted in the way gas work is, but an OFTEC registered technician is the recognised standard, and this is the code that says to call one.
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.