Worcester Bosch Boiler Faults & Error Codes Guide

The most common Worcester Bosch faults

Worcester Bosch is one of the UK's biggest boiler brands, and most homes run a Greenstar combi, system or heat-only model. The faults we see most often are rarely catastrophic — they tend to be the same handful of issues across the range. Knowing which is which helps you decide whether it's a five-minute fix or an engineer's job.

• Low water pressure — the gauge drops below about 1 bar and the boiler locks out. Usually a slow top-up cures it, though a persistent drop points to a leak.
• No hot water (but heating still works) — often a stuck diverter valve, a flow sensor, or the comfort/preheat setting being changed.
• An ignition or lockout fault — the boiler tries to fire, fails, and shows a code such as EA, then shuts down as a safety measure.
• A frozen condensate pipe — in cold snaps the external white plastic waste pipe ices up and the boiler stops, often with a gurgling sound.
• Fan, pump or overheat faults — internal mechanical faults that show codes like C6 (fan) or E9 (overheat) and need parts replacing.
• No power or no response — a tripped fuse, a flat thermostat battery, or the programmer not calling for heat.

Worcester Bosch error codes at a glance

Modern Greenstar boilers show a fault as a short code on the front display — a letter group followed by a number, such as EA, E9 or C6. The table below is a quick reference. The exact suffix digits and meanings vary between the Greenstar i, CDi and older Si ranges, so always cross-check against your manual or the label inside the boiler's casing flap.

For the full list with plain-English explanations of each code, see our dedicated guide to Worcester Bosch error codes.

What you can safely check yourself

Only a short list of Worcester faults is genuinely a homeowner job. None of these involve gas, the flue, the sealed combustion circuit or removing the casing — and they never should. Anything beyond this list is Gas Safe engineer work only.

1. Check the power and controls

Make sure the boiler has power, the fused spur is switched on, and any wireless thermostat or programmer isn't flat or dropped out of its settings. A tripped fuse or a flat thermostat battery can stop the boiler responding entirely — both are safe to check and put right.

2. Check (and top up) the pressure

Find the pressure gauge or digital reading on the front. Cold, it should sit at roughly 1 to 1.5 bar, rising towards about 2 bar when hot. Below 1 bar is low. You can top up via the filling loop — the braided silver hose (or internal keyed valve) underneath the boiler — opening the valves slowly until the gauge reaches about 1.5 bar, then closing them. If the pressure keeps dropping, you have a leak that needs investigating rather than endless topping up. Our guide to low boiler pressure walks through it.

3. Try one front-panel reset

If a lockout code such as EA is showing, press and hold the reset button on the front panel for a few seconds — once. If the boiler fires and stays running, keep an eye on it. If the same code returns, stop resetting and book an engineer. Our step-by-step guide on how to reset a Worcester Bosch boiler covers the exact button sequence.

4. Thaw a frozen condensate pipe

In freezing weather, the external white plastic condensate pipe can ice up and stop the boiler. You can safely pour warm — not boiling — water over the frozen section to thaw it, then reset once. See our guide to a frozen condensate pipe for the full method.

When you need a Gas Safe registered engineer

If the safe checks above don't fix it, the cause is almost certainly internal — and these are not DIY jobs. Anything involving the gas valve, the burner, the flame-sensing electrode, the flue, the sealed combustion circuit, the pressure relief valve, or removing the casing must be done by a Gas Safe registered engineer. It is illegal as well as dangerous for an unregistered person to attempt gas work. Book an engineer for:

• An error code that returns after a single reset.
• A stuck diverter valve, DHW flow sensor or thermistor causing no hot water — see our Worcester boiler no hot water guide.
• Fan (C6), pump or overheat (E9) faults that need parts replaced.
• A PCB or internal sensor fault (F0-type codes).
• Pressure that keeps dropping, suggesting a leak or expansion-vessel fault.

You can confirm any engineer is genuinely qualified at the Gas Safe Register. (Gas Safe replaced the old CORGI scheme in 2009, so always look for the Gas Safe ID card, not "CORGI registered".) For what these jobs typically cost, see our Worcester boiler repair guide.

How boiler cover helps with Worcester faults

The pricier Worcester repairs — a fan, a pump, a gas valve or a PCB — can each run to several hundred pounds in parts and labour, often more than a year of cover. A boiler cover policy turns that into a fixed monthly amount and gives you a number to call when the heating or hot water goes. If you're weighing it up, our guides to what boiler cover is, the best boiler cover and whether boiler cover is worth it set out what's actually included. Always check the boiler-age limit, the excess and any limescale or pre-existing fault exclusions before you buy.

Frequently asked questions

This article is general information, not advice, and reflects a selected panel of providers rather than the whole market. Prices and ranges are indicative for 2026. Always use a Gas Safe registered engineer for gas work.