Error E78

Safety precautions:

- If you smell gas, evacuate the property immediately and call the gas emergency number. Do not attempt any checks or resets.

- Turn off electrical power to the boiler before accessing internal panels. If you are not comfortable isolating electrical supplies, stop and call a professional.

- Do not dismantle gas components, the burner, or safety devices unless you are a Gas Safe engineer. Working on pressurised heating systems carries risks.

Initial checks a homeowner can do:

1) Note exactly when the E78 appeared and any recent work or changes (topping up, radiator bleeding, frozen external pipes, power cuts).

2) Look at the boiler's pressure gauge. Normal operating pressure for most Baxi combi/system boilers is about 1.0–1.5 bar when cold. If it is below about 0.8–1.0 bar the boiler may lock out.

3) Press the reset button on the boiler (hold for 5–10 seconds as per manual). A temporary glitch can clear with a reset, but do not keep resetting repeatedly if the fault returns.

If pressure is low (DIY top-up):

1) Locate the filling loop (usually a braided hose and two isolation valves under the boiler or near the boiler). Check the boiler manual for exact location and instructions for your model.

2) With the boiler powered and following the manual, open the filling loop valves slowly and watch the pressure gauge. Close both valves as soon as pressure reaches 1.0–1.5 bar. Do this slowly to avoid overfilling.

3) Run heating and check radiators to see if the system heats normally and if the E78 clears after a reset.

4) If the pressure keeps dropping after topping up, inspect for visible leaks at radiators, pipework, valves and the boiler. Persistent pressure loss indicates a leak, failed filling loop, or expansion vessel problem and needs an engineer.

If pressure is correct but E78 remains or returns:

1) Power-cycle the boiler (switch off, wait 30 seconds, switch on) and try a reset once. Note if the fault is intermittent or permanent.

2) Bleed a radiator to remove airlocks which can cause poor circulation and false sensor readings. Re-check pressure after bleeding and top up if needed.

3) Check for circulation issues: feel if the pump is running (gentle vibration) and whether flow pipework gets warm when the boiler calls for heat. Poor circulation leading to high flow temperature or dry-fire conditions can trigger pressure-related faults.

4) Visually inspect accessible wiring and connectors to the pressure sensor and PCB for loose connections, corrosion, or damage. Do not disconnect or probe internal wiring unless you are competent and have isolated electrical power.

When to call a professional (required):

- The pressure will not stay up after topping up or you find a leak.

- The E78 persists after correct pressure has been restored and the boiler reset.

- You suspect the pressure sensor, expansion vessel, pressure relief valve, pump or PCB is faulty.

- You detect signs of overheating, repeated lockouts, or abnormal noises from the boiler.

- Any work involving gas, internal electrical repairs, component replacement, or sealed system faults.

What an engineer will do:

- Pressure-test the system, locate leaks, test and recharge the expansion vessel, and inspect the pressure relief valve.

- Test the pressure sensor and its wiring, and replace the sensor or repair wiring if required.

- Diagnose pump or circulation faults and check flow/return temperatures and safety thermostats.

- Replace PCBs or safety components if the fault is due to internal electronics.

Other important notes:

- Do not repeatedly reset the boiler; this can mask a developing fault and cause further damage.

- Keep a record of the fault code, what you tried, and any pressure readings to give to the engineer. This saves diagnostic time.

- For safety and warranty reasons, arrange a Gas Safe registered engineer for any repairs beyond basic topping up, bleeding radiators, and visual checks.