Gas Safe: Gas work, flue work, and PCB replacement on a gas boiler must be carried out by a Gas Safe registered engineer. Homeowners can check obvious items such as heating pressure, condensate overflow, and whether a control is unplugged — then call an engineer.
Always confirm the part number against the boiler GC number and the installation manual before ordering.
Overview
F.01 is the return-pipe version of F.00. The PCB has lost the signal from the heating return NTC (open circuit or disconnected). The boiler cannot prove return temperature, so it locks out.
Vaillant meaning: Return temperature sensor interruption
Possible causes
- Return NTC plug not fitted, or pulled off the sensor or PCB multiplug
- Break in the NTC loom (often at the X20 / NTC connector)
- Return NTC open circuit (typical 12 kΩ at 20 °C)
- Defective PCB input (less common — prove the sensor and loom first)
Possible solutions
- Identify the return NTC on the heating return pipe (clip-on or in-pocket, depending on range).
- Unplug and refit the sensor connector and the PCB multiplug. Check for bent pins and moisture.
- Measure resistance at 20 °C; open circuit or wildly out of range means replace the NTC.
- Continuity-test the two cores back to the PCB. Repair or replace the harness if open.
- If sensor and loom are good, the PCB is the remaining item.
Possible parts needed
- Flow/return NTC 193592 (common clip-on ecoTEC / ecoMAX type)
- NTC 0020068120 (later / alternative clip-on)
- Wiring harness / NTC lead
- PCB (only after sensor and loom are proven)
