Error D.80

Safety precautions:

1) If the boiler is showing D.80, do not attempt internal repairs unless you are a Gas Safe registered engineer. Working on gas appliances, live electronics, or sealed piping can be dangerous and illegal for unqualified persons.

2) Before any external checks, turn off the boiler using the controls and isolate electrical supply at the fused spur if you will be removing any covers (only if you are competent and it is safe to do so). If in doubt, leave the boiler powered and only perform non-invasive checks.

3) Keep gas and water supply shut-off valves accessible and know where to close them in an emergency.

Initial checks a homeowner can do (no internal access required):

1) Record the exact error code and any other codes or messages shown, and note when it occurs (heating on, DHW on, start-up, after running for a while).

2) Try a simple reset: follow the boiler’s reset procedure (press and hold the reset button as specified in the user manual) and see if the code clears. If it clears but returns, the underlying fault remains.

3) Check boiler display for basic parameters: heating water pressure (should usually be around 1–1.5 bar when cold). Low pressure can cause faults; if pressure is low, follow the manual to top up the system unless you are unsure.

4) Ensure heating and return service valves (if visible) are open and that any external room thermostat or programmer is demanding heat when you expect it to.

5) Bleed any cold radiators to remove trapped air and re-check system pressure; airlocks or a low flow condition can affect sensor readings and pump performance.

6) Look for obvious signs of damage or disconnection on any external wiring to the boiler, and check for recent plumbing or electrical work that may have disturbed wiring.

Specific diagnostic and fix steps (to be completed by a Gas Safe engineer):

1) Verify symptom reproduction: engineer will run the boiler in heating mode while monitoring the flow temperature reading and the error behaviour.

2) Check connectors and harness: inspect and reseat the NTC thermistor plug at the sensor and at the PCB. Look for loose, corroded or disconnected plugs and for any damaged or chafed wiring in the harness.

3) Measure resistance and continuity: use a multimeter to measure the resistance of the flow NTC at ambient and check continuity through the wiring harness to the PCB. Compare readings to manufacturer specifications to confirm sensor health.

4) Inspect PCB connections: check that the board connectors are fully seated and that there is no moisture or damage on the PCB that could cause intermittent readings.

5) Check pump operation and system flow: confirm the pump is functioning and there are no blockages, airlocks or closed valves causing insufficient flow which can look like a sensor or temperature spread fault.

6) Replace suspect parts as required: if the NTC thermistor is out of spec, replace the sensor. If wiring or the connector is damaged, replace the harness or plug. If the PCB input is faulty and other components test good, the PCB may need replacement—this is a job for an engineer.

7) After repairs, the engineer should run a full system check: verify flow and return temperatures change correctly, check for leaks, confirm pressure and pump operation, and validate that the boiler operates normally under heating demand without the D.80 code returning.

When to call a professional:

1) If the error persists after basic resets and pressure/valve checks, call a Gas Safe registered engineer. Any internal checks, electrical testing, or replacements must be done by a qualified engineer.

2) If you smell gas, see signs of water leaks, or the boiler behaves dangerously (loud noises, visible damage, continuous lockouts), turn off the boiler, isolate supplies if safe and call an engineer immediately.

Note: Do not attempt to replace sensors, wiring or the PCB yourself unless you are a Gas Safe registered engineer. Always provide the engineer with the boiler model, serial number and a clear description of when the fault occurs and any steps you have already taken (resets, pressure checks, radiator bleeding).