Worcester Bosch Greenstar 4000 Problems — Faults & Fixes
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The Worcester Bosch Greenstar 4000 is a reliable workhorse found in thousands of UK homes, but like any combi boiler, it can develop faults over time. Most problems are straightforward to diagnose thanks to the clear digital display and fault codes, though some require a qualified Gas Safe engineer to put right. Whether you're dealing with no heating, no hot water, low pressure, or a cryptic error code on the screen, this guide walks you through the most common issues affecting this model and explains what you can safely tackle yourself versus when to pick up the phone. Many faults can be resolved in minutes with basic troubleshooting, but if you're ever in doubt—or smell gas—a Gas Safe registered engineer should always be your first port of call.
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Most likely cause & what to check
The Worcester Bosch Greenstar 4000 (24kW and 28kW combi variants) replaced the popular Si range and is now one of the most widely installed boilers in new-build properties. It has a clear digital display that shows fault codes directly. All fault code meanings are listed in the installation manual at worcester-bosch.co.uk/support.
Fault code 227 (low system pressure): this is the most common fault on the Greenstar 4000. The pressure gauge on the front should read between 1 and 1.5 bar when cold. Use the filling loop (two small valves underneath the boiler — usually a flexible silver hose) to top up to 1.2 bar, then close both valves firmly. Press reset. If pressure drops repeatedly over a few days, there is a leak in the system — call a Gas Safe engineer.
Fault code EA (ignition failure): the boiler has failed to light. Press reset and hold for 3 seconds. Check gas appliances elsewhere in the property to confirm gas is reaching the house. The Greenstar 4000 has a known sensitivity to gas supply interruptions — if a neighbour is having gas work done, a momentary interruption can trigger EA. If it returns without explanation, call a Gas Safe engineer to check the ignition electrode and gas valve.
Diverter valve issues: the Greenstar 4000 has a three-way diverter valve that directs hot water either to the radiators or to the hot water outlet. If you have heating but no hot water (or vice versa), the diverter valve may be stuck or failing. This is one of the most common repairs on this model — a Gas Safe engineer can replace the diverter valve cartridge, typically in under an hour.
Fault code E5 (flue temperature too high / fan fault): do not reset repeatedly. The flue fan is responsible for safely removing combustion gases and this fault indicates it may be failing. Call a Gas Safe engineer.
Warranty: the Greenstar 4000 carries a 5-year parts and labour warranty when registered within 30 days of installation at worcester-bosch.co.uk/warranty — this extends to 8 years with an annual Worcester Bosch service. Always keep your service records.
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Frequently asked questions
What should the boiler pressure gauge read on a Greenstar 4000?
When the system is cold, the pressure should sit between 1 and 1.5 bar—aim for 1.2 bar as your target. If it drops below 1 bar, the boiler will lock out on fault code 227 and you'll need to top it up using the filling loop beneath the unit.
How do I know if my Greenstar 4000 has a diverter valve fault?
You'll notice you have heating but no hot water coming from the taps, or hot water but the radiators won't heat up. This is a classic sign the three-way diverter valve is stuck—a Gas Safe engineer can replace the valve cartridge, usually within an hour.
Why does my Greenstar 4000 keep showing fault code EA?
Fault code EA means ignition failure—the boiler can't light. First, press reset and hold for 3 seconds, then check if gas is reaching other appliances in your home. The Greenstar 4000 is sensitive to gas supply dips, sometimes triggered by work at a neighbour's property, but persistent EA codes need a Gas Safe engineer to inspect the ignition electrode and gas valve.
Can I reset my Greenstar 4000 myself?
Yes, you can press the reset button for most faults, but avoid resetting repeatedly if fault code E5 (flue temperature too high) appears, as this suggests a fan fault requiring professional inspection. Continuous resetting without addressing the underlying issue wastes time and could mask a safety concern.
What does it mean if pressure keeps dropping on my Greenstar 4000?
If you've topped up the system to 1.2 bar but the pressure falls again within a few days, there's a leak somewhere in the heating circuit—possibly a faulty valve, pipework, or radiator. You'll need a Gas Safe engineer to locate and repair the leak, as leaving it unaddressed will continue to cause pressure faults.
Is the Greenstar 4000 still under warranty?
The boiler comes with a 5-year parts and labour warranty if registered within 30 days of installation through Worcester Bosch's website. This extends to 8 years if you arrange an annual service plan, so always keep your service records to protect your cover.