Safety comes before troubleshooting
If you smell gas, suspect a gas leak or have a carbon monoxide concern, stop the checklist. Move to a safe place and call the National Gas Emergency Service on 0800 111 999. Do not operate electrical switches or use flames in the affected area. For suspected carbon monoxide, move everyone into fresh air.
Do not reset an appliance showing signs of unsafe operation. Water reaching the boiler’s electrical parts, scorching or a damaged flue needs professional attention. Do not restart an appliance an engineer has isolated as unsafe.
Checks you can make without opening the boiler
- Record the make, model and exact displayed code before changing anything. Photograph the display if you can safely do so.
- Check whether the display has power and whether the home has a power interruption. Do not remove electrical covers or keep resetting a breaker that trips.
- Check the thermostat setting, timer, heating mode and clock using the user manual. After an interruption, schedules may need correcting.
- Check whether a cold tap has water. Note any supplier interruption or meter warning already visible; do not test the gas supply by lighting another appliance.
- Read the pressure gauge and compare it with the model’s user instructions. Do not use a universal target or refill a system you cannot identify.
- Only perform a user reset if the manual permits it and no safety warning is present. If the fault returns, stop rather than repeatedly resetting.
Older boilers with a permanent pilot light
Some older gas boilers use a continuously burning pilot light; modern electronic-ignition models generally do not. If your appliance has a viewing window, you can note whether the pilot is visible without opening the casing. A pilot that is out, repeatedly goes out or appears abnormal is information to give a Gas Safe registered engineer.
Use only the specific user instructions for that model. Do not use a match, remove covers, adjust gas valves or repeatedly attempt relighting. If the manual is unavailable, the pilot will not stay lit, or there is any gas smell or carbon monoxide concern, stop and get appropriate help. The gas-emergency instructions above take priority.
No heating, no hot water, or neither?
Say whether every radiator is cold, just one room is affected, or hot water still works normally. Explain whether your system has a hot-water cylinder rather than assuming every home has a combi boiler. A fault code is useful information, not a complete diagnosis.
Make a short note of when the fault began, any recent servicing and any visible leaks. Keep the manual and previous service record available so the engineer does not have to reconstruct the history from memory.
Cold weather and condensate faults
A frozen external condensate drain can stop some boilers. Look only from a safe position at ground level, without climbing or dismantling anything. A code associated with ignition or lockout can have other causes, so do not assume ice is responsible.
Northern Gas Networks advises gentle warming of an accessible frozen condensate pipe with a warm-water bottle or warm, not boiling, water. Only follow the instructions appropriate to your installation and manufacturer. If access is unsafe, you cannot identify the pipe or the boiler remains stopped, call a suitably qualified engineer.
When you need an engineer
Gas boiler work requires a Gas Safe registered engineer qualified for that appliance. Verify the attending engineer’s details; ordinary plumbing attendance is not confirmation that gas work is included. Tell us about vulnerable occupants and lack of heating when discussing availability.
Our standard-hours boiler repair guide is £90–£350 for labour only. Callout, parts and materials are separate, with the work and price agreed before it starts. Routine servicing from £60 is not a breakdown repair or a separate landlord gas safety certificate. For emergencies or out-of-hours work, call rather than waiting for a provisional online booking.
Further reading and sources
General guidance for homeowners. Diagnosis and work requirements depend on your property and equipment.
