Worcester Bosch Boiler Fault Codes
Gas Safe registered fault diagnosis for Worcester Bosch Greenstar, CDi, and i-series boilers. E9, A1, E1, F1, E5 — all explained with causes, repair costs, and safe homeowner checks.
Understanding Worcester Bosch boiler fault codes
Worcester Bosch boilers display fault codes on the digital panel when the system detects a problem and locks out. Unlike Vaillant's F-number system, Worcester Bosch uses a mix of letter-number codes (E9, A1, E1) and status codes (F1, H07). Each code maps to a specific fault detected by the PCB.
This guide covers the five most commonly searched Worcester Bosch fault codes, with technical explanations, causes, and what a Gas Safe engineer will do to fix each one. Reion Miller (Gas Safe 919881) carries Worcester Bosch parts and handles repairs across London.
Worcester Bosch fault codes explained
Overheat lockout
The safety thermostat has tripped because the boiler reached a dangerously high temperature. Often caused by limescale, a blocked heat exchanger, or a faulty pump.
Fan / flue fault
The PCB cannot confirm the fan is running at the correct speed, or the flue pressure switch has not triggered. Can indicate a blocked flue or fan failure.
Ignition failure
The boiler attempted to ignite but could not establish a flame within the required window. Gas supply, ignition electrode, and gas valve are all possible causes.
Low water pressure
System pressure has dropped below the minimum operating threshold. May resolve with a repressurise via the filling loop — but check for leaks first.
Flow thermistor fault
The flow temperature sensor is returning an out-of-range reading. Could be a failed sensor, a wiring fault, or limescale affecting the temperature reading.
Worcester Bosch fault code table
Extended fault code reference for common Worcester Bosch Greenstar models.
| Code | Meaning | Common cause | Action |
|---|---|---|---|
| E9 | Overheat lockout | Blocked HX, limescale, pump failure | Gas Safe engineer — do not reset repeatedly |
| A1 | Fan / flue fault | Fan failure, blocked flue, pressure switch | Gas Safe engineer required |
| E1 | Ignition failure | No gas, electrode, gas valve | Gas Safe engineer required |
| E2 | DHW thermistor fault | Hot water sensor failed or wiring | Gas Safe engineer |
| E5 | Flow thermistor fault | Flow sensor or wiring fault | Gas Safe engineer |
| E9 158 | Max temp exceeded (heat) | As E9 — overheating on heating circuit | Gas Safe engineer — check pump and HX |
| F1 | Low water pressure | Pressure below 0.5 bar | Repressurise to 1.2–1.5 bar (homeowner safe if no leak) |
| H07 | Water pressure low (new models) | Filling loop needed | Repressurise to 1.2–1.5 bar |
| CE | Temperature sensor fault | Thermistor short or open circuit | Gas Safe engineer |
| No display | No power to boiler | Fuse, wiring, or control board | Check fuse first; else Gas Safe engineer |
Why Worcester Bosch boilers overheat more in London
The E9 overheat fault is disproportionately common in London, and the reason is the water supply. Thames Water — which covers Wandsworth, Merton, Lewisham, Lambeth, Southwark, Greenwich, and Bromley — draws from chalk aquifers that carry high calcium concentrations.
Inside a Worcester Bosch boiler, calcium deposits as limescale on the plate heat exchanger. As the scale thickens, the heat exchanger cannot transfer heat efficiently to the water — so the metal itself gets hotter. When it exceeds the safety threshold, the overheat thermostat trips and the boiler locks out with E9.
Annual servicing, a Fernox or Adey magnetic filter, and a system inhibitor keep limescale from accumulating. Reion fits these on every new Worcester Bosch installation in South London as standard.
Worcester Bosch fault code questions
How do I reset a Worcester Bosch boiler after a fault code?
Why do Worcester Bosch boilers fault more often in London?
Are Worcester Bosch boilers reliable?
How much does a Worcester Bosch repair cost in London?
Which Worcester Bosch models are most common in South London?
Worcester Bosch fault showing on your boiler?
Reion is Gas Safe registered (919881), carries Worcester Bosch parts, and serves all of South London. Fixed price after the £80 diagnostic — no obligation to proceed.
