F8 error, the outdoor fan does not run, or the unit overheats and shuts down.

Safety: Turn off the outdoor isolator and verify that no power is present before working on the outdoor unit.

• Check fan rotation: Remove the top grille if necessary and try to rotate the fan blade by hand. It should spin freely. If stiff or seized, the motor bearings are likely faulty.
• Inspect capacitor (if fitted): Many Vaillant outdoor units use a run capacitor for the fan motor. Locate the capacitor in the control box, check for bulging or leakage, and replace it if suspected with an identical µF rating.
• Check wiring: Inspect the fan motor leads and connector for burns or loose connections. Reseat the connectors firmly.
• Test run: Restore power briefly and command cooling. If the compressor runs but the fan does not, and the F8 error appears, the motor or its drive circuit is faulty.
• Replace fan motor: Isolate power again. Remove the fan guard, the fan blade (note orientation), and the mounting bolts. Install a new outdoor fan motor, reconnect its wiring, and refit the blade and guard.
• Verify operation: Power up and run the unit. Confirm the fan starts reliably and the F8 error does not return.

If a new motor and capacitor do not resolve the F8 error, the outdoor PCB fan output may be defective and should be tested by a qualified professional.