| Test | Procedure | Pass Condition | Failure Implication | |-------|-----------|----------------|----------------------| | | Measure at fan connector (pin 1-2): 230V AC (mains models) or 24V DC (low-voltage models) | Within ±10% | Replace PCB or wiring | | PWM control signal | Measure between PWM pin and GND. Boiler calling for heat → 2–10V DC | Smooth variation | Faulty PCB | | Tacho feedback | Set multimeter to frequency (Hz). Fan running → 50–300 Hz | Stable frequency | Replace fan if absent, else PCB | | Fan winding resistance | Disconnect fan. Measure between winding phases (ohm) | ~10–50 ohms, balanced | Open or short → replace fan | | Hall sensor test | Rotate fan manually by hand → tacho voltage should pulse 0–5V | Pulsing DC | No pulse → replace fan |
| Cause Category | Specific Issue | Probability | |----------------|----------------|--------------| | | Water in fan housing or connector due to blocked condensate drain or leaky flue | High (>40%) | | Fan motor failure | Worn bearings, seized rotor, or shorted winding | Medium (25%) | | PCB / wiring fault | Loose connector, corroded pins, failed fan driver MOSFET on PCB | Medium (20%) | | Debris / obstruction | Foreign object in fan impeller (insulation, leaves, spider webs) | Low (10%) | | Flue gas recirculation | Incorrect flue length or terminal position causing pressure oscillation | Low (5%) | vaillant f64 fault code
