If your Worcester Bosch boiler has stopped working and is showing a code on the display, don’t panic. Worcester Bosch boilers are designed to shut down and flag a fault code when something goes wrong — it’s a built-in safety mechanism, not a sign that the boiler is beyond saving.
The code tells you, and us, exactly where the problem lies, which means faster diagnosis and a quicker fix.
This guide covers the most common Worcester Bosch error codes across the full range, what they mean, and what’s likely causing them. Some you can resolve yourself; others need a Gas Safe registered engineer. Never open your boiler casing or attempt internal repairs yourself — it’s dangerous, and it can invalidate your warranty.
If your boiler is regularly faulting, over 10–12 years old, or costing more in repairs than it’s worth, it may be time to consider a replacement boiler.
Full Worcester Bosch Greenstar Error Code List
These codes apply across the core Worcester Bosch Greenstar range — including the 4000, 8000, CDi, and i/Si series. Most models share the same alphanumeric fault codes.
| Fault Code | Meaning | Likely Cause |
|---|---|---|
| EA / EA 227 | No flame detected after ignition | Gas supply issue, faulty ignition electrode, failed gas valve |
| E9 | Safety temperature limiter tripped | Blocked pump, failing heat exchanger, limescale, faulty PCB |
| E2 / E2 222 | Flow temperature sensor fault / short circuit | Damaged or wet NTC sensor or connecting leads |
| E5 218 | Flow temperature too high | Pump or circulation issue |
| EF 349 | Central heating boil detected | >18°C difference between flow and return — circulation issue |
| A1 / A1 281 | Pump fault — stuck, dry running, or air in system | Low pressure, airlocked pump, or pump failure |
| A7 | Domestic hot water sensor defective | Faulty hot water NTC sensor or damaged leads |
| A8 | EMS / heat control module communication error | Check electrical connections and communication cable |
| F7 / F7 228 | Flame detected before burner starts | Wet or dirty electrode, PCB issue, flue problem |
| FA / FA 306 | Flame detected after gas shut-off | Gas valve fault — Gas Safe engineer required immediately |
| FA 364 | Gas valve EV2 leak test failed | Gas valve leak — do not repeatedly reset |
| FB 365 | Gas valve EV1 leak test failed | Gas valve fault — engineer required |
| FD / FD 231 | Power interruption / reset pressed in error | Press reset to clear; if recurring, check mains supply |
| F0 / F0 237 | Internal error / gas valve or control box fault | Control board or PCB issue |
| C6 / C6 215 | Fan running too fast or too slow | Fan lead/connector fault, fan may need replacing |
| C7 / C7 214 | Fan not running on start-up | Fan failure or wiring fault |
| C1 / C1 264 | Airflow stopped during operation | Flue blockage, fan fault, or condensate issue |
| C4 / C4 273 | Fan failed to stop / airflow in last 24 hours | Control board or fan relay issue |
| D4 / D4 271 | Flow and safety sensor temperature difference too high | Circulation problem or sensor fault |
| D1 / D1 240 | Return sensor error — short circuit or wet/damaged | Sensor needs replacing or drying out |
| B1 | Code plug not detected | Code plug loose, missing, or incompatible |
| B7 / B7 257 | Internal error / control board problem | PCB or burner control unit fault |
| 9U / 9U 233 | Heat Control Module (HCM) error | Code plug problem or HCM failure |
| CC | Outdoor temperature sensor defect | Sensor or connecting lead fault |
| 2951 V | Loss of flame too frequent. | Refer to the blocking error(s) that lead to this locking error. Check the fault history on appliance. |
| 1101 | Thermostat not connected | Boiler has lost communication. Usually down to batteries requiring replacing |
Worcester Bosch 8000 Series — Numeric Fault Codes
The Greenstar 8000 uses a numeric code system rather than alphanumeric. Here are the codes most commonly seen on 8000 series boilers in Edinburgh homes:
| Fault Code | Meaning | Notes |
|---|---|---|
| 1017 W | Water pressure too low | Top up via filling loop; check pressure sensor |
| 1065 B | Pressure sensor defective or disconnected | Check connection; may need replacing |
| 1073 W | Flow temperature sensor short circuit | Sensor likely needs replacing |
| 1074 W | No signal from flow temperature sensor | Check plug connection or replace sensor |
| 1021 B | Hot water temperature sensor defective | Check plug connection to hot water sensor |
| 2924 V | No feedback from modulating gas valve | Press reset; if recurring, gas valve needs replacing |
| 2927 B | Flame failed during burner operation | Check gas supply, ionisation electrode, flue integrity |
| 2948 B | No flame signal | Check CO2 valve settings; engineer likely required |
| 2965 B | Flow temperature too high | Pump or circulation issue |
| 2964 B | Flow rate in heat exchanger too low | Pump or blockage issue |
| 2970 B | Pressure drop in system too rapid | Check for leaks; vent the system |
| 2971 B | System pressure too low | Top up water; check pressure sensor |
| 2946 V | Incorrect code plug detected | Check and replace code plug |
| 360 V | Incompatible code plug | Check and replace code plug |
| 227 V | No flame signal after ignition | Burner control unit may need replacing |
| 224 V | Safety temperature limiter tripped | Top up water pressure to reach reset threshold |
How Worcester Bosch Fault Codes Work
Worcester Bosch uses a structured fault code system designed to make diagnosis straightforward for engineers. When the boiler detects a problem, it locks out or blocks operation and displays a code pointing to the affected component — whether that’s a flow temperature sensor, heat exchanger, gas valve, pressure sensor, or burner control unit.
There are two core error types you’ll encounter:
Locking errors — The boiler shuts down completely and won’t restart without a manual reset. A flashing fault code appears alongside a triangle symbol, usually with a static cause code. The reset should only be done once the underlying fault is fixed — repeatedly resetting a locking fault can cause further damage.
Blocking errors — The boiler stops temporarily but may restart automatically once the fault clears (a thawed condensate pipe, for example). No fault code shows on screen immediately; press the return button to access the cause code.
Beyond these two, Worcester Bosch also uses:
- Fault error codes — Alphanumeric codes covering a wide range of component issues
- Maintenance codes — Begin with H; the boiler keeps running but flags that attention is needed soon
- Cause codes — Three-digit numbers accessed via the return button that give more detail on blocking and locking faults
The Most Common Worcester Bosch Fault Types Explained
Before getting into the full code list, these are the five fault categories our Edinburgh engineers deal with most frequently. Understanding what each covers helps make sense of what your boiler is trying to tell you.
1. Flow Temperature Sensor Faults
Flow temperature sensor faults are among the most common call-outs we attend. The sensor monitors the primary flow temperature in your central heating system, and when it reads incorrectly — due to a short circuit, damaged connection, wet sensor, or incorrect installation position — the boiler will fault to prevent overheating. A Gas Safe engineer will check the sensor position, test the connections, and verify whether the sensor needs replacing or just reseating.
2. Safety Temperature and Overheating Faults
These occur when the safety temperature limiter or high-limit thermostat detects abnormal temperatures inside the heat exchanger — often caused by a pump failure, blocked heat exchanger, limescale build-up, or a differential sensor reading outside the normal range. The boiler cuts out to prevent damage. This one always needs an engineer; it’s not a fault to reset your way through repeatedly.
3. Gas Valve Faults
Gas valve faults are serious and should never be ignored or repeatedly reset. They typically relate to the modulating gas valve, gas valve relay, or burner control unit — and can indicate a gas valve leak, flame signal irregularity, or ignition lead issue. If you’re seeing FA, FB, or F0 codes, call a Gas Safe engineer without delay.
4. System Pressure Faults
Low system pressure is behind more fault codes than most homeowners realise. When the pressure sensor detects pressure outside the acceptable range — usually caused by a leak, recent bleeding of radiators, or a faulty pressure sensor connection — the boiler will stop operating. This is one of the few faults you can check yourself: look at the pressure gauge on the boiler front. It should read between 1 and 1.5 bar when the system is cold.
5. Fan and Flue Faults
Fan and flue faults cover issues with airflow, fan speed, flue gas thermostat operation, and vent system blockages. Edinburgh winters can contribute here — debris, ice, or bird nesting in external flue terminals are more common than you’d expect. An engineer will test fan speed sensors, inspect the flue terminal, and verify the vent system is clear and properly sealed.
What to Do When Your Worcester Bosch Shows a Fault Code
Before calling an engineer, a few quick checks are worth doing yourself:
- Check boiler pressure — The gauge should read 1–1.5 bar when cold. Below 1 bar, top up via the filling loop. Low pressure is behind a surprising number of fault codes
- Try a single reset — Press and hold the reset button for 3 seconds. If the fault clears and doesn’t return, it may have been a one-off. If it comes back within 24–48 hours, something needs attention
- Check your gas supply — Try another gas appliance in the house. If the hob won’t light either, the issue is with your gas supply, not the boiler
- Check for a frozen condensate pipe — In Edinburgh winters, external condensate pipes freeze regularly. Thaw it with warm (not boiling) water poured along the pipe — this resolves most winter lockouts without an engineer visit
Call a Gas Safe engineer without further resets if you’re seeing FA, FB, F7, EA codes, if the fault returns immediately after reset, if multiple codes are appearing together, or if you notice any smell of gas. These situations need professional diagnosis, not repeated resets.
Got a different make of boiler? Our guide to boiler error codes for every major brand lines up the same faults across Vaillant, Worcester Bosch, Ideal, Baxi and Glow-worm.
Need a Worcester Bosch Repair in Edinburgh?
Our Gas Safe registered engineers cover Edinburgh and the surrounding areas and work on the full Worcester Bosch range — from older CDi models to the latest Greenstar 8000. If you’ve spotted a fault code and aren’t sure what to do next, call us on 0131 510 8500 or book a boiler repair online.
If your boiler is repeatedly faulting or the repair cost is climbing, it’s worth getting a replacement quote at the same time. A new Worcester Greenstar 4000 installed by The Edinburgh Boiler Company starts from £2,550 — with a 10-year guarantee included.
Get a free quote today or contact us and we’ll get it sorted.
