Safety first: Remove the battery and keep the unit disconnected from the charger while inspecting.

Step 1 – Hard reset: Remove the battery for at least 5 minutes. Press and hold the power button for 10 seconds (with battery removed) to discharge residual power. Reinstall the battery and test.
Step 2 – Inspect for liquid damage: Check around the power button, display, and seams for signs of liquid ingress, corrosion, or residue. If present, do not attempt to power the unit repeatedly.
Step 3 – Check connectors: If comfortable opening the housing, remove screws and carefully separate the halves to access the main PCB. Verify that all connectors are fully seated and that there are no burnt components.
Step 4 – Replace main board: If no obvious external cause is found and E9 persists, the main control board must be replaced with a genuine Dirt Devil PCB designed for your model.
When to stop: Board replacement and sealing against future moisture are best handled by a qualified technician.

All LEDs flash rapidly and the vacuum will not start, even with a known-good battery

Possible Causes