Error E.04 – .03

Safety first

1) If you smell gas or suspect a gas leak: do not stay inside. Open windows, turn off gas at the meter if safe to do so, leave the property, and call the national gas emergency number immediately. Do not try to light appliances or operate the boiler.

2) If the boiler is displaying the fault and is locked out, do not repeatedly press reset. Repeated resets can cause further damage. Turn the boiler off and follow checks below.

3) If you are not confident working around a boiler, call a Gas Safe engineer now.

Initial homeowner checks (safe, non-intrusive)

1) Check system pressure on the boiler gauge. Aim for around 1.0–1.5 bar (check your boiler manual for exact target). If pressure is low (under ~0.8–1.0 bar) top up the system using the filling loop per your boiler instructions. Re-check the fault after restoring pressure. If pressure is repeatedly low, call an engineer.

2) Look at radiators: are they completely cold or only warm at the top? Cold radiators or radiators hot only at the top indicate poor circulation or trapped air.

3) Bleed radiators to remove trapped air, starting with the highest radiators. After bleeding, re-check system pressure and repressurise if needed.

4) Ensure all radiator thermostatic valves and any external isolation valves (on the pump or flow/return pipes) are fully open.

5) If you can safely access the boiler casing and are comfortable doing so, listen to the boiler when a heating demand is made: can you hear the pump running (a low hum) or any unusual loud grinding/noise? Do not touch any wiring or internal parts while the unit is powered.

Specific diagnostic steps (what these checks tell you)

1) If topping up pressure, bleeding rads and opening valves clears the fault and the boiler runs normally, the cause was likely low pressure, an airlock or closed valve. Monitor for recurrence and call an engineer if it repeats.

2) If radiators remain cold and the pump makes no noise or sounds seized/grinding when the boiler calls for heat, the pump may be seized or electrically faulty. This usually requires professional replacement.

3) If radiators heat at the top but remain cold at the bottom, or one circuit is hot and another cold, suspect a stuck or faulty diverter/zone valve or a blockage/magnetic filter. These items often need an engineer to diagnose and repair/replace.

4) If the boiler shows signs of overheating (very hot to the touch around the case) or you see steam/leaks, isolate electrical supply and gas, turn off the boiler and call an engineer immediately.

5) If you have competent electrical skills and the boiler is isolated, you may visually check that the pump isolation valves are open and that pipework to/from the pump is not collapsed; do not remove covers, change wiring, or touch gas components unless you are Gas Safe registered.

Attempt a controlled reset

1) After doing the safe checks above (pressure, bleed, valves, listen to pump), try a single reset following the manufacturer: press and hold the reset button for the required time (usually 5–10 seconds) and allow the boiler to attempt restart.

2) If the fault clears and does not return, monitor the system closely. If the fault returns, stop resetting and arrange a professional visit.

When to call a Gas Safe engineer (and what to tell them)

1) Call a Gas Safe registered engineer if: the E.04 – .03 fault persists after basic checks; you discover a seized pump, noisy pump, blocked magnetic filter, stuck zone/diverter valve, repeated low pressure, or visible leaks; you are uncomfortable performing any checks; or you smell gas. This is a permanent fault code tied to overheating and circulation failure and should be treated as a high-priority safety issue.

2) When you call, report the exact error code (E.04 – .03), the steps you have taken (topped up, bled rads, reset attempts), whether radiators are heating, and any noises observed. This helps the engineer diagnose faster.

3) Do not attempt internal repairs on the pump, gas/air unit, PCB, sensors, or flue components yourself. These must only be worked on by a qualified Gas Safe engineer.

Final notes

E.04 – .03 indicates an overheating/circulation safety lockout. Some simple homeowner steps can rule out pressure or airlock issues, but if the problem is a faulty pump, blocked heat exchanger, sensor failure, stuck valve or wiring fault, a professional repair is required. Prioritise safety: gas smells, persistent overheating, or electrical faults mean isolate supplies and call a Gas Safe engineer immediately.