A Worcester Bosch oil boiler that won’t fire has almost always gone into lockout. That means the burner tried to light, didn’t detect a flame and shut itself down. The fault is usually in the oil supply rather than the boiler: a low tank, air in the pipe, a clogged filter or water in the oil. Burner parts such as the nozzle, electrodes and flame sensor are the next place to look.
Worcester’s Greenstar Danesmoor and Heatslave oil boilers are reliable machines, and most lockouts are cheap to put right once the cause is found. These are the questions we get asked most.
Which Worcester Bosch oil boiler do I have?
The model name is on the data badge, usually inside the front door. The current oil range is small and easy to identify.
| Model | Type | Outputs |
|---|---|---|
| Greenstar Danesmoor Utility | Regular (heat only), indoor | 12/18, 18/25, 25/32 kW |
| Greenstar Danesmoor System Utility | System, with built-in pump and expansion vessel | 12/18, 18/25, 25/32 kW |
| Greenstar Danesmoor External | Outdoor, regular or system | 12/18, 18/25, 25/32 kW |
| Greenstar Heatslave II | Combi, indoor | 12 to 32 kW |
| Greenstar Heatslave II External | Combi, outdoor | 12/18, 18/25, 25/32 kW |
Older homes may have an earlier Danesmoor, a Heatslave or a Camray. The faults below apply to them too, although parts for the oldest models can be harder to get.
Why has my Worcester oil boiler gone into lockout?
The causes fall into three groups. Work through them in this order and you will cover most cases.
1. Oil isn’t getting to the burner properly
- The tank has run dry or very low, so the pump is drawing air.
- There is air in the oil line after a run-out, a delivery or a loose fitting.
- The oil filter is blocked with sludge or debris.
- Water has got into the tank through a damaged lid or gauge, causing sludge and sometimes bacterial growth.
- The supply pipe is leaking, letting air in and oil out. You may smell oil or see a damp patch.
- A valve has been left closed or the tank vent is blocked.
2. The burner can’t make or keep a flame
- A worn or blocked nozzle gives a poor spray.
- Worn or sooty electrodes give a weak spark.
- A dirty or failed flame sensor shuts the burner down a few seconds after lighting.
- A failed ignition transformer means no spark at all.
- A seized motor coupling or failed capacitor leaves the motor humming without turning.
- A worn oil pump, sticking solenoid valve or failed control box.
3. A safety device or the controls are stopping it
- No power, or the thermostat and timer aren’t calling for heat.
- The overheat cut-out has tripped.
- The fire valve has closed.
- Pressure has dropped too low on a sealed system or combi.
- A zone valve has stuck shut, so the boiler is never asked to fire.
- The condensate pipe has frozen.
- The flue or air intake is blocked.
How do I reset a Worcester Bosch oil boiler?
Press the illuminated reset button on the burner control box once. The boiler should go through its start-up and light within a minute or two. If it locks out again, leave it. A second reset after a few minutes is the limit, because each failed start leaves a little unburnt oil behind and repeated attempts can cause a bang on ignition. Our guide to resetting a Worcester boiler covers the gas models too.
How can I tell if the problem is my oil and not the boiler?
Suspect the oil if any of these sound familiar:
- The boiler runs well for a few days after a repair, then locks out again.
- It failed shortly after an oil delivery, which can stir up sediment.
- The tank is old, steel, or has a cracked or missing lid.
- Your engineer found water or black sludge in the filter bowl.
Contaminated oil needs dealing with at the tank, not the boiler. That can mean removing the water, cleaning the tank and flushing the line, or replacing a tank that is past its best. OFTEC suggests tanks are assessed for replacement once they reach about 20 years old.
My Heatslave gives heating but no hot water. Why?
On a combi, hot water problems with working heating are usually down to the diverter valve or a scaled plate heat exchanger rather than the burner. In hard water areas scale builds up faster. If the hot water is lukewarm rather than absent, a partly scaled heat exchanger is the more likely culprit.
Why does my Worcester oil boiler keep losing pressure?
Sealed system and combi models need around 1 to 1.5 bar when cold. If the pressure keeps dropping there is a leak somewhere, often a weeping radiator valve, a failed pressure relief valve or a tired expansion vessel. You can top it up yourself, and our guide on repressurising a Worcester boiler shows how, but a pressure that falls again needs finding.
Is my Worcester oil boiler still under warranty?
Worcester’s standard guarantee on its current Danesmoor and Heatslave II boilers is two years. It rises to five years when a Worcester Accredited Installer fits a Greenstar System Filter, and to seven years with the filter plus a small fee. You will normally need yearly service records to claim, so keep the paperwork from every oil boiler service. If you are unsure, Worcester can confirm your cover from the serial number.
Should I repair or replace my Worcester oil boiler?
If the fault is a filter, nozzle, electrode or sensor, repair it. These are routine parts. If you are facing a control box, pump and motor on a boiler that is well over 15 years old, or the heat exchanger has started to corrode, a replacement usually makes better sense over the next decade. A new Danesmoor or Heatslave II is an easy like-for-like swap in most homes. See our oil boiler replacement prices, or read our Worcester Bosch boiler reviews before you decide.
If your boiler isn’t a Worcester, our guides for Firebird oil boilers and Grant oil boilers cover their specific lights and resets.
Worcester Bosch oil boiler FAQs
Is a lockout the same as a breakdown?
Not necessarily. A lockout is the boiler protecting itself, and sometimes the fix is as simple as ordering oil and bleeding the line. It is still worth finding out what caused it.
Can I run my Worcester on HVO?
Worcester sells HVO conversion kits for Danesmoor and Heatslave boilers made after September 2018. Conversion should be done by an OFTEC registered engineer.
Why did my boiler fail just after I had oil delivered?
The delivery can disturb sludge at the bottom of the tank or leave air in the line if the tank was nearly empty. A new filter and a bleed usually fix it.
How often should I check the oil tank?
Glance at the gauge every few weeks in winter and look over the tank for damage, leaks or a loose lid every few months. Your engineer should check it properly at each annual service.
With years of experience in boilers and home improvements in general, Myles Robinson is a passionate advocate for making home heating simpler, safer, and more affordable for households across the UK. With years of experience in the heating industry, Myles combines hands-on knowledge with a focus on customer-first service, helping homeowners choose the right boiler, understand their systems, and keep their heating running efficiently all year round. His articles break down complex topics into clear, practical advice you can trust.