Error D.78

Safety first

- Do not work on gas or sealed electrical parts. If you need to open the boiler casing, isolate mains electricity at the consumer unit first and only do so if you are competent. For almost all sensor/wiring/PCB work call a Gas Safe engineer.

Initial checks a homeowner can do

1. Note the displayed values: check D.20 (max DHW target), D.78 (DHW max flow temperature) and D.71 (max heating flow temp) as you have access to them. If D.78 equals your DHW setpoint (e.g. 55°C) the boiler is simply using that limit.

2. Check actual hot water at a tap: run hot water and measure or estimate temperature. If water reaches the expected temperature and nothing else faults, the code may just be informational.

3. Check for other fault codes on the display (for example F-codes or F.91) — these often indicate sensor or wiring faults that accompany D-codes.

4. Check the cylinder thermostat (if you have a separate cylinder stat or programmer) to ensure it is not set lower than the boiler limit.

5. Try a boiler reset using the normal reset procedure on the boiler front panel or app, then observe whether the D.78 message returns.

Basic visual checks (no internal work)

1. Look at accessible wiring and connectors to the hot water cylinder thermostat/sensor (at the cylinder head or immersion stat) for obvious corrosion, wetness, or loose connectors. Do not disconnect gas or live electrical connections.

2. Check the hot water cylinder top for obvious signs of overheating, leaks or discolouration.

If the problem persists or you suspect a sensor fault (when hot water is incorrect or other fault codes appear)

1. Note symptoms: are you getting no hot water, low hot water, or water that suddenly becomes too hot? Is D.78 constantly shown or only when trying to heat the cylinder?

2. If you are competent and have the boiler powered down at the consumer unit, you can visually inspect the sensor connector at the boiler PCB for a loose or obviously damaged plug. If unsure, do not probe further and call an engineer.

3. A trained engineer will measure the NTC resistance at ambient (NTC typically around 10–12 kΩ at ~20°C, though exact values can vary by sensor type) and check for short circuits or open circuits in the cable to the cylinder sensor. They will also check for related fault codes (for example F.91) and test the controller/PCB behavior.

4. If the NTC is out of tolerance or wiring is damaged the engineer will replace the sensor or repair the harness. If wiring and sensor measure OK the engineer may test the PCB/logic and settings and adjust parameters if necessary.

When to call a professional

- Call a Gas Safe registered engineer if you see other fault codes, if you have little or no hot water, if the cylinder is actually overheating, or if you suspect a faulty sensor, wiring or PCB. Do not attempt to replace sensors or work inside the boiler unless you are qualified.

Notes and practical guidance

- D.78 can be simply a setting (a limit) and not a fault; changing service menu limits should be done by an engineer because incorrect settings can create safety or performance issues.

- If you suspect the code is caused by a faulty NTC, a professional will measure resistances, inspect connectors and may replace the sensor or wiring; they will also clear and re-test fault history.

- Keep a record of displayed parameters and any accompanying F-codes and provide these to the engineer — it speeds diagnosis.

Remember: any work involving gas, internal electrical parts, PCB, or replacing sensors inside the boiler should be carried out by a Gas Safe engineer. If in doubt, call a qualified engineer rather than attempting internal repairs yourself.