my dyson v8 turns on for a second then immediately shuts off. it’s fully charged.

Diagnosis

The Dyson V8 is programmed with a "pulsing" or "rapid shut-off" safety mechanism. When the motor triggers a sudden spike in amperage—usually caused by a physical obstruction or a lack of airflow—the onboard logic board cuts power instantly to prevent the brushless motor from burning out. If the machine runs for exactly one second and stops, it is almost never a battery failure, but rather a critical airflow restriction. This happens because the vacuum operates on a pressure-differential system. If air cannot move freely from the cleaner head through the bin and out the exhaust, the motor creates a vacuum lock. This increases the load on the motor exponentially, triggering the safety cutoff. You can confirm this by removing the wand and the cleaner head; if the motor runs continuously without the attachments, the problem is a blockage in the pipes. If it still shuts off with nothing attached, the issue is likely a saturated filter or a faulty trigger switch.

How to Fix It

To resolve this, you must perform a comprehensive "clear path" inspection. Follow these steps in exact order:

1. Isolate the Motor: Remove the long wand and the cleaner head. Press the power trigger. If the vacuum now runs without stopping, the blockage is in the attachments. If it still shuts off, proceed to step 2.
2. Deep Filter Clean: Remove the pre-filter (the fabric wrap around the top) and the post-filter (the pleated filter at the back). Even if they look clean, microscopic dust can clog the pores. Wash them under lukewarm water until the runoff is clear. Let them air dry for a full 24 hours. Using a damp filter will trigger an immediate shut-off.
3. Clear the Inlet Valve: Look at the opening where the wand connects to the main body. Check the small rubber flap (the inlet valve). Use a flashlight to see if a coin, pebble, or clump of hair is lodged right at the entry point. Use a pair of needle-nose pliers to pull out any debris.
4. Wand Inspection: Drop a coin or a small ball through the wand to see if it passes through freely. If it gets stuck, use a broom handle or a flexible plumbing snake to push the clog out.
5. Brush Bar Maintenance: Detach the cleaner head. Flip it over and remove the side end-cap using a coin to unlock it. Pull out the brush bar and use scissors to cut away the hair and string wrapped around the axle. A seized brush bar puts too much torque on the motor, causing a shutdown.

If That Didn't Work

• Battery Voltage Drop: While you mentioned it is fully charged, the V8 batteries can suffer from "cell collapse." This means the battery shows a full charge (voltage), but cannot sustain the current (amperage) required to spin the motor. If you have a multimeter, check if the voltage drops precipitously the moment you pull the trigger. If it does, replace the battery pack (Part No: 968997-01).
• Trigger Switch Failure: The V8 uses a plastic trigger lever that can crack or wear down, causing an intermittent connection. If the vacuum shuts off when you apply slight pressure to the trigger, the internal plastic switch may be snapped. This requires a trigger replacement kit.
• Internal Cyclone Clog: Occasionally, debris bypasses the bin and gets stuck inside the cyclone assembly. This is a "hidden" clog. Gently tap the cyclone assembly against a hard surface to dislodge any trapped debris, or use compressed air to blow out the internal channels.
• Motor Controller Fault: If the machine has been exposed to moisture or "wet vacuuming," the circuit board may have a short. Look for any signs of corrosion near the battery terminals or the trigger assembly.

Heads Up

• Never use a wet filter: Using a filter that is even slightly damp will cause the vacuum to shut off and can permanently damage the motor by pulling moisture into the electronics.
• Avoid "Max" mode for long periods: Constant use of Max mode on the V8 generates significant heat; if the motor hits its thermal limit, it will shut down for 15-30 minutes to cool.
• Check the Flap: Always ensure the rubber inlet flap is sealing correctly; if it stays open or is torn, it can affect the suction pressure and trigger the sensor.
• Regular Maintenance: Wash your filters once every month to prevent the "pulsing" shut-off behavior from returning.