Error F.6

Safety precautions:

1. If you are not confident working around boilers, stop and call a qualified engineer. Do not attempt to repair gas or sealed electrical parts unless you are qualified.

2. Before any inspection that requires opening the casing, switch the boiler off at the mains and isolate the electric supply. Do not turn off or tamper with gas supply unless instructed by an engineer.

3. Only perform non-intrusive visual checks unless you are competent and qualified to work on gas appliances. Keep children and pets away while inspecting the boiler.

Initial homeowner checks (safe, do these first):

1. Note when the fault appears and whether any recent work, power cuts or freezing occurred. A persistent, repeatable F.6 is more likely a component fault.

2. Try a boiler reset following the user manual’s procedure. If the fault clears but returns soon after, further diagnosis is required.

3. Check for obvious visible issues: corrosion, water leaks, or rattling pump noises. Look for disconnected or visibly damaged cables at accessible plug-in points (do not remove covers that expose gas/electrics if you are unsure).

4. For this EcoMax Pro (a regular/open-vent boiler) you will not use a pressurisation filling loop; ignore combi filling-loop advice. Confirm central heating parts such as radiators and the feed/expansion tank are in their normal state.

Diagnostic and fix steps (for competent DIYers or for the engineer to follow):

1. If you are competent and have isolated mains power, remove the front casing to access the PCB and sensor connectors. Keep the gas isolated if you will work near gas components and only if you are qualified to do so.

2. Locate the flow NTC (typically on the flow side coming out of the heat exchanger; on many Vaillant units it is the front/red sensor). Identify the connector on the wiring harness and the socket on the PCB.

3. Visually inspect connectors and wiring for signs of corrosion, burning, pulled pins or loose connections. Reseat any accessible plug connections firmly.

4. With the mains isolated, use a multimeter to check continuity of the sensor cable from the sensor connector to the PCB socket and for unexpected shorts to earth/ground. An open or shorted cable indicates harness replacement or repair.

5. Measure the NTC resistance with the sensor disconnected (at room temperature). Compare the reading to the values in the service manual if available; an NTC should show a finite resistance that changes with temperature (resistance falls as temperature rises). If the sensor reads open circuit or a hard short, the sensor is faulty and should be replaced.

6. If you have a matching return sensor and are experienced, you can swap the two sensor plugs temporarily to see if the fault follows the sensor. If the fault code moves to the other channel, the sensor is faulty. Only perform this if you understand the wiring and have isolated power before unplugging connectors.

7. If sensor and wiring check out but the fault persists, inspect the PCB socket for damage. A damaged PCB connector or PCB fault can report an NTC error even with good sensor/wiring. At this stage a professional engineer should carry out PCB tests or replacements.

8. If sludge, scale or poor flow is suspected (radiators cold, pump noise, low flow), arrange system flushing or a engineer-led pump purge. Blocked heat exchangers or filters can affect temperature readings and should be addressed by a heating engineer.

When to call a professional:

1. If the fault persists after a reset and the basic visual checks, contact a Gas Safe / qualified heating engineer. Replacing sensors, repairing wiring harnesses, testing for PCB faults, and working on flues/gas components require professional qualification.

2. If you discover water ingress, corrosion, or damage to the PCB or gas components, stop and call an engineer immediately.

Note: Do not attempt to replace or adjust gas valves, burners or PCB-mounted components unless you are a qualified engineer. A competent engineer will test the sensor, harness and PCB, replace the faulty NTC or associated wiring, and re-check the boiler operation and safety interlocks before returning the appliance to service.