Safety: Turn off the outdoor isolator and verify 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 failed.
Inspect capacitor (if fitted): Many Vaillant outdoor units use a run capacitor for the fan. Locate the capacitor in the control box, check for bulging or leakage, and replace if suspect with an identical µF rating.
Check wiring: Inspect the fan motor leads and connector for burns or loose connections. Reseat connectors firmly.
Test run: Restore power briefly and command cooling. If the compressor runs but the fan does not, and F8 appears, the motor or drive is faulty.
Replace fan motor: Isolate power again. Remove the fan guard, fan blade (note orientation), and mounting bolts. Install a new outdoor fan motor, reconnect wiring, and refit the blade and guard.
Verify operation: Power up and run the unit. Confirm the fan starts reliably and F8 does not return.

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

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

Possible Causes