Error F.85

Safety precautions:

- Do not remove the boiler casing or touch internal wiring unless you are a qualified engineer. There are live electrical parts and gas components inside. Opening the boiler yourself will often void warranties and can be dangerous.

- If you smell gas, evacuate the property, ventilate windows, do not operate electrical switches or phones near the leak, and call the gas emergency services immediately.

- For any checks that require isolation of electrical supply or gas, call a Gas Safe engineer.

Initial homeowner checks (safe, non-invasive):

1. Record the fault exactly as displayed and when it started (e.g., after a service, power cut, or DIY work). This helps the engineer.

2. Try a boiler reset using the boiler controls (consult your user manual for the reset procedure). Wait 5–10 minutes after reset to see if the fault returns. Do not repeatedly reset the boiler more than once or twice.

3. Check the system pressure on the boiler gauge. Low system pressure can affect circulation and produce implausible temperature readings. If pressure is low and you know how to use the filling loop safely, you can top up to the correct range (consult the handbook). If you are unsure, wait for a professional.

4. Bleed a radiator or two to remove trapped air in the system; airlocks can cause large temperature differences between flow and return.

5. Without opening the boiler, feel the external flow and return pipes where they enter the boiler (careful—they can be hot). You should normally feel the flow noticeably hotter than the return when the boiler is calling for heat. If both pipes are the same temperature or one is cold when the system is running, that indicates a circulation or sensor issue.

6. If any sensor or wiring plugs are visible and accessible outside the boiler casing (rare), you may check that they are seated firmly. Do not force or disconnect anything behind covers.

Diagnostic and fix steps for a professional (what an engineer will do):

1. Connect diagnostic equipment and read full fault memory and live sensor values from the boiler electronics to identify whether the fault is an open circuit, short circuit, implausible differential, or swapped sensors.

2. Visually inspect wiring harnesses, plugs and PCB connectors for corrosion, water damage, loose contacts or chafing. Repair or replace damaged connectors or loom as needed.

3. Measure each NTC thermistor’s resistance at ambient and with temperature change, then compare the readings to the manufacturer specification to confirm whether a sensor is defective. Check continuity of the wiring back to the PCB.

4. Confirm sensors are fitted to the correct pipes (flow sensor on the flow pipe, return sensor on the return pipe) and are correctly positioned and clipped to the pipework. Correct any swapped or incorrectly fitted sensors.

5. Check for system issues that can cause implausible readings: pump failure, blocked or stuck valves, closed zone valves, or airlocks. Restore circulation and re-bleed as required.

6. If wiring and sensors are good but readings are still implausible, check PCB inputs and the eBUS/communications for faults. Replace PCB only if proven faulty.

7. After repairs or component replacements, the engineer will clear the fault, run the boiler through several operating cycles, and confirm correct temperatures and stable sensor readings. They will also check for any reoccurrence under normal demand.

When to call a professional:

- Call a Gas Safe registered heating engineer if the error persists after simple non-invasive checks, if you suspect wiring or sensor failure, if you need the boiler case opened, or for any gas or electrical work. F.85 is a safety-related sensor fault and must be diagnosed and repaired by a qualified engineer to ensure the boiler operates safely.