Error D.77

Safety precautions:

1) Do not attempt internal electrical or gas work unless you are a qualified Gas Safe engineer. Working inside the boiler can be dangerous. 2) Before any visual checks, turn the boiler off at the programmer and at the isolator/power switch. Do NOT disconnect gas fittings. 3) If you see signs of burning, water ingress or smell of gas, isolate the gas supply and call a professional immediately.

Initial homeowner checks you can safely do:

1) Note the exact code(s) shown (D.77 and any accompanying F codes like F.91). Take a photo for the engineer. 2) Try a single reset: press the boiler reset button and allow the boiler to restart. If the code does not return, monitor the boiler — if it reappears later call an engineer. 3) Check for obvious external issues: is the boiler area wet or damp, are sensor cables visible and obviously damaged, is there recent building work or disturbance around the boiler that might have pulled connectors? 4) Check boiler water pressure and basic operating conditions (pressure should typically be around 1–1.5 bar when cold). Low pressure can cause other faults to appear.

What the engineer will check and do (do not attempt unless trained):

1) Verify fault memory and any related codes on the PCB to determine whether the fault is an interruption (open circuit), short-circuit, or a coding/communication error. 2) Visually inspect and test sensor plug connections (heating flow, heating return, cylinder/actoSTOR) and the wiring harness for loose plugs, corroded pins, water ingress or chafing against the casing. 3) Measure NTC thermistor resistances (example reference: heating return NTC roughly 12 kΩ at ~20°C) and check for continuity/shorts in the cable harness. 4) Check the coding resistor and PCB connections if the diagnostic points to coding resistor output range or gas family mismatch. 5) Test for short circuits to chassis/earth in sensor plugs or cables. 6) If a defective sensor or cable is confirmed, replace the NTC and/or wiring harness. If PCB connector or PCB faults are found, the engineer will advise on repair or PCB replacement. 7) After repair/replacement the engineer will clear faults, run the boiler through cycles and confirm correct operation (including cylinder charging and flow temperatures).

Key notes and when to call a professional:

1) If the D.77 persists after a reset, or appears repeatedly, book a Gas Safe registered engineer — this fault requires measurements and component replacement that must be done safely and correctly. 2) Do not try to bypass or ignore sensor faults; the boiler uses these sensors for safe temperature control. 3) If you are unsure at any stage, or observe other symptoms (no hot water, heating not charging cylinder, repeated lockouts, water leaks, burning smell), call a Gas Safe engineer immediately.

Remember: do not attempt internal repairs or replacements on the gas, electrical or PCB components yourself. A qualified Gas Safe engineer is required to diagnose and fix D.77 faults safely.