Error E78

Safety precautions:

1. If you smell gas or suspect a gas leak, evacuate the property immediately and call the emergency gas number. Do not touch electrical switches or the boiler. 2. Isolate electrical power to the boiler before opening covers for inspection. For simple resets you can follow the manufacturer’s reset instructions without opening the casing. 3. Never attempt gas or PCB/electrical repairs unless you are a Gas Safe registered engineer.

Initial checks a homeowner can do:

1. Note the error code and any other display messages or LEDs. Write down the model and serial if visible. 2. Check the boiler pressure gauge (usually on the front panel). Normal cold pressure is about 1.0–1.5 bar. High pressure trips can be triggered if pressure is above the boiler’s high limit (often around 2.9–3.0 bar). 3. Look for visible leaks or water from the PRV discharge pipe (a plastic pipe that runs from the boiler to a safe drain point). If water is present the PRV may be operating and you have a real pressure problem. 4. Check whether the filling loop is connected and the isolation valves are closed (if the loop is open it can over-pressurise the system).

Basic reset and quick actions:

1. Perform a reboot: switch off the boiler using the isolator or mains switch, wait 30 seconds, then switch back on. Some faults clear after a power cycle. 2. Use the boiler’s reset button (hold for 3–5 seconds as per manual). If the code clears but returns, the fault is persistent. 3. If the pressure gauge reads high (above ~2.9 bar) and you are comfortable doing so, reduce pressure by bleeding a radiator (open a bleed valve using a radiator key until pressure falls to about 1.2–1.5 bar). Check the boiler pressure after bleeding. If you are not comfortable bleeding radiators, call an engineer.

If pressure is actually high (and/or PRV is discharging):

1. Turn the boiler off and isolate electrical power. 2. Do not continue to run the system. A continually discharging PRV or sustained high pressure requires an engineer. Likely causes include failure of the expansion vessel (lost charge), PRV fault, or system overfilling. These components are service items and require a qualified engineer to repair or replace.

If the pressure reading is normal but E78 persists:

1. Try a power reset and boiler reset as above. 2. Visually inspect the boiler’s accessible wiring and connectors for obvious loose connectors or corrosion (do not remove panels or probe inside unless you are qualified). 3. If connectors appear secure and the error remains, the pressure sensor (transducer) or PCB may be faulty — both require diagnostic tools and replacement parts fitted by an engineer.

Information to collect for the engineer:

1. Exact error code (E78) and any secondary codes or LEDs. 2. Pressure reading on the gauge when the code appeared. 3. Whether there was PRV discharge or visible leaks. 4. When the fault first appeared and whether it followed any other events (power cut, filling loop used, recent servicing).

When to call a professional:

1. Call a Gas Safe registered heating engineer if the fault persists after basic resets and pressure checks, if the PRV is discharging, if you find leaks, or if you are uncomfortable performing the simple actions above. 2. Do not attempt to replace the pressure sensor, expansion vessel, PRV or PCB yourself — these are gas appliance or electrical components requiring qualified repair. 3. If the boiler locks out repeatedly or the problem recurs soon after an engineer visit, insist on a full diagnostic and replacement of faulty components.

Final note: A temporary reset may clear a false sensor fault, but recurring E78 should be treated seriously because it can indicate failing components that affect safe boiler operation. Always use a Gas Safe registered engineer for any internal repairs or gas-related work.