Overview
On a Worcester Bosch CDI Highflow combi boiler, a steady "Light on" indication is generally not a serious fault code — it usually means the boiler has received a heating demand and is in an operational or standby state related to that demand. In other words, the control thinks the heating system should be calling for heat. Depending on model and exact lamp behaviour, a plain steady light can simply mean the boiler is "on" and waiting or running as expected. It is different from flashing sequences which indicate specific faults (ignition failure, sensor faults, overtemperature, etc.). Severity is low when the light is steady and the boiler is actually producing hot water or heating. However, if you have a heating demand (thermostat calls for heat) and the light is on but the radiators remain cold or the boiler will not fire, that points to a secondary problem — external controls, water pressure, a stuck pump, or an internal fault — and should be investigated. Basic checks and a single reset are reasonable for a homeowner, but any work that involves the gas supply, internal components, or repeated lockouts should be left to a Gas Safe registered engineer.
Possible Cause: Heating demand on boiler is OK.
Troubleshooting Steps
Safety precautions:
1. If you smell gas, evacuate the building immediately, call your gas emergency number and a Gas Safe engineer — do not attempt any checks.
2. Do not remove covers or attempt internal electrical or gas repairs. Only carry out external and visual checks and simple user actions such as reset, bleed radiators, or top up pressure if you are comfortable doing so.
3. Turn the boiler off at the mains before touching any exposed pipework or electric isolators.
Initial checks a homeowner can do:
1. Confirm heating controls: Ensure the programmer/clock is set to heating ON, the room thermostat is set above room temperature, and any smart controls or wireless devices are calling for heat.
2. Check boiler display and lights: Note whether the light is steady or flashing and, if flashing, record the exact flash sequence (number of short and long flashes and the gaps). That sequence is the real diagnostic code for engineers.
3. Check system pressure: Look at the boiler pressure gauge — typical operating pressure is around 1.0–1.5 bar when cold. If pressure is below ~0.7 bar the boiler may lock out or not circulate.
4. Look for obvious issues: Frozen/blocked condensate pipe in cold weather, visible leaks, tripped external isolation switches, or a blown fuse on the boiler spur.
Simple diagnostic and fix steps you can try:
1. Reset the boiler once: Follow the manual — either press the reset button for about 3 seconds or turn the control knob to the reset position and hold for 3 seconds, then return to the normal setting. Do not repeatedly reset; if the fault returns, stop and call an engineer.
2. Confirm heating call: Turn the room thermostat well above the room temperature and set the programmer to a continuous call for heat. Listen for the boiler and pump to run. If nothing happens but the light is on, continue with the checks below.
3. Top up system pressure if low: If the pressure is low (typically below 0.7 bar) and you know how to use the filling loop, you can carefully top up the system until the gauge reads about 1–1.5 bar. Follow the manufacturer instructions in the manual. After topping up, re-check for leaks and then try a reset.
4. Bleed radiators: Air in the system can prevent circulation. Bleed radiators starting from the lowest floor to remove trapped air, then check pressure again and re-pressurise if required.
5. Check motorised valves and pump: If you can hear the pump running and feel pipework warming after a call for heat then the boiler is firing but the system may not be circulating correctly. If the pump is silent but mains power is present, do not try to dismantle it — call an engineer.
6. Inspect condensate discharge (winter): If the condensate trap/pipe is frozen or blocked it can cause lockouts. A frozen condensate pipe can sometimes be thawed with warm (not boiling) water; follow guidance in the manual.
7. Look for additional indicators: If the boiler shows any other flashing code or displays an EA/ E9 / F0 type code, do not attempt internal repairs. Note the code and call a Gas Safe engineer.
When to call a professional / final notes:
1. Call a Gas Safe registered engineer if the boiler still will not heat after the above basic checks, if the boiler locks out repeatedly after reset, if you record a specific flash fault sequence, or if any internal component (igniter, gas valve, fan, sensors, control board) appears to be at fault.
2. Always call a professional if you smell gas, see a water leak, observe repeated pressure loss, or the boiler displays serious error codes (for example EA codes or overtemperature faults).
3. A Gas Safe engineer must carry out any work on the gas supply, ignition system, flame detection, internal PCBs, or component replacements. Provide the engineer with the exact light/flash pattern and any steps you have already taken — that information speeds diagnosis and repair.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Worcester Bosch CDI Highflow Combi Boiler.