Sennheiser Momentum 4 Wireless Won’t Power On

When trying to turn on the headphones, the power button has issues turning on the headphones, while it just flashes or does nothing at all.

The powered connection may need to be serviced and tweaked by taking the components apart.

The button may be stuck or inoperable and needs to be cleared of debris in order to function.

Try using a toothpick or an electronic device cleaning tool to run around/under the edge of the power button to clear debris.
Retry using the power button, holding it down until the button flashes white.

The Momentum 4 Wireless have multiple ways to turn on. This error might be the cause of the power button not being pressed correctly, issues with automatic switch-on, or gesture control via touchpad.

Automatic switch on: By lifting and putting on (LED display should flash white 3x)
Manual switch on: Pressing the multi-function button for 3 seconds (LED display should flash white 3x)
Gesture control via touchpad: Tap side of ear cover, horizontal swipe, vertical swipe, two-finger pinch. You can deactivate the touchpad if this power on function is not preferred.

The headphones not charging or needing to be charged constantly can be due to a faulty battery.

Take apart the earmuffs, separating it from the headphones.
Use a Philips screwdriver to unscrew the 8mm screws, take off cover.
Take the white box and pull to detach the wires connected to the battery and replace.