Error E03

Safety precautions:

1) If the boiler is showing E03, switch the boiler off at the programmer/thermostat and isolate electrical power at the fused spur or consumer unit if you can do so safely. Allow the boiler and system to cool for at least 30 minutes before touching pipework or components. Turn off the gas service cock to the appliance if you suspect internal faults and you are leaving it unattended. Do not attempt internal gas or electrical repairs yourself; these must be done by a qualified engineer.

Initial homeowner checks you can do (no tools or basic tools):

1) Look at the boiler display and the pressure gauge. Normal cold pressure is typically about 1.0–1.5 bar for domestic systems. If pressure is very low (<0.5 bar) or very high (>2.5 bar) note that and do not run the boiler until corrected.

2) Check that radiator TRVs and any zone or isolation valves are open. Closed valves can stop circulation and cause overheating.

3) Try to feel the flow and return pipes at the boiler (careful — they may be hot). If the flow pipe is very hot but the return is much cooler or barely warm, circulation may be poor.

4) Listen for the pump running or any unusual noises (grinding, loud vibration). A seized or failing pump can stop circulation.

5) Bleed the radiators (start with the highest radiators) to remove trapped air which can block circulation. After bleeding you may need to repressurise the system.

Specific diagnostic and basic fix steps you can attempt:

1) Allow the boiler to cool. Once cooled, perform a controlled reset using the boiler reset procedure in your manual. Observe if the fault returns immediately or after a period. If it returns immediately, do not keep resetting.

2) Check and correct system pressure: If pressure is low, use the filling loop as per the boiler manual to top up the system to around 1.0–1.5 bar. Open the valves on the filling loop slowly while watching the pressure gauge, then close them and securely stow the filling loop. If you are unsure where the filling loop is or how to use it, stop and call an engineer.

3) Bleed radiators again and then re-check pressure and circulation. Top up pressure after bleeding if necessary.

4) Ensure all heating circuit valves and motorised zone valves are in the correct (open) position. If your system has thermostatic or motorised valves, operate the thermostat call for heat and check they move/open.

5) If you suspect a blocked heat exchanger or scale build-up (you may notice noisy operation, vibration, or loss of heat flow), do not attempt to dismantle the heat exchanger. A chemical descaler or powerflush can clear scale but should be done by a qualified heating engineer. The user-reported remedy of adding a descaling agent or system cleaner (eg sentinel products) is something an engineer usually applies as part of a service or powerflush; follow the boiler manufacturer and product instructions.

6) If the pump appears not to run but you hear a faint hum, the pump may be seized or the impeller blocked. You can isolate the pump and attempt to free a stuck impeller only if you are competent and the boiler is isolated; otherwise call an engineer.

7) If the boiler shows other specific sensor or fault codes after reset, note them. E03 accompanied by sensor faults (flow/return thermistor errors) points to a faulty sensor or wiring that needs a professional.

When to call a professional and what to tell them:

1) Call a Gas Safe registered engineer if the fault persists after you have checked pressure, bled radiators and ensured valves are open. Also call a pro if you detect leaks, repeated pressure loss, pump failure, sensor errors, persistent noise/vibration, or if you are not confident performing the simple checks above.

2) Before the engineer arrives, note the exact fault code(s), what you tried (bled radiators, topped up pressure, reset), any noises or leaks, and when the fault started. This will speed diagnosis.

3) Do not attempt to open or repair gas valves, burners, control boards, heat exchanger internals or flue assemblies yourself — these are specialist, regulated tasks.

Final notes:

- Treat E03 as a sign of poor circulation, blocked flow or sensing failure until proven otherwise. Simple homeowner fixes: open valves, bleed air, top up pressure and reset. If the problem returns or any suspect components (pump, heat exchanger, sensors) are involved, get a qualified engineer. Repeated overheat events can damage the boiler, so do not ignore recurring lockouts.