my dyson v8 keeps pulsing and then shutting off after a few seconds, what's wrong?

The pulsing is the automatic cut-out activating due to a blockage. The motor will pulse and not function until the blockage is cleared.

• Disconnect from the charger before starting.
• Check for blockages in this order:
  1. Empty the bin if dirt is above the top line.
  2. With the bin open, examine the cyclones and the tool/wand attachment point.
  3. Remove the wand and cleaner head. Check both ends of the wand for blockages.
  4. Turn the cleaner head over and check the brush bar and air inlet for obstructions.
• Remove any debris you find.

IF THAT DIDN'T WORK: Wash the filters. A clogged filter can also cause performance issues. The Pre-filter and Post-filter both need to be washed under running water until clear, then dried for at least 24 hours. They must be completely dry before reinsertion.