DJI Mini 4 Pro obstacle avoidance not working

DJI Mini 4 Pro obstacle avoidance is most often "not working" because it's switched off: it doesn't run in Sport mode, it needs Obstacle Avoidance set to Bypass or Brake in DJI Fly, and it stops working at night. Fly in Normal or Cine mode, check the setting under Settings > Safety, and keep the sensors clean. Even then, it can miss thin branches, wires, glass and water.

Will Mini 4 Pro avoid obstacles? — From DJI's Mini 4 Pro manual and specs. The vision system needs light, texture and modest speed. It's a backup, not a replacement for watching where you fly.

Check these settings first

Flight mode. DJI's manual says the vision systems are disabled in Sport mode, so the drone can't sense obstacles. Use Normal or Cine.
Obstacle Avoidance action. In DJI Fly, go to Settings > Safety and set Obstacle Avoidance to Bypass or Brake. DJI says the omnidirectional vision system only activates in Normal or Cine with one of these selected. Off disables avoidance.
Advanced Safety Settings. Vision Positioning and Obstacle Avoidance can be turned off under Safety > Advanced Safety Settings. Make sure they're on.

Know the limits

The Mini 4 Pro uses an omnidirectional binocular vision system plus a 3D infrared sensor underneath. DJI's specs say:

Speed: forward, backward and sideways sensing works at flight speeds up to 12 m/s. Up and down sensing only to 5 m/s.
Light: the side and upward cameras need surfaces with clear patterns and more than 15 lux. DJI's manual says the vision systems become unavailable at night.
Range: forward sensing measures from 0.5 to 18 m. Brake early, because the drone needs room to stop.

DJI lists situations where the vision systems can't work properly, including:

Plain single-color surfaces (pure black, white, red or green)
Water, glass and highly reflective surfaces
Repeating patterns such as identical tiles
Small obstacles like tree branches and power lines
Very dark (under 10 lux) or very bright (over 40,000 lux) scenes, and fast-changing light
Lattice structures and cables: towers, cranes, suspension bridges

Clean and check the sensors

Wipe every vision lens and the infrared window with a soft, dry cloth. DJI says not to use cleaners containing alcohol.
Remove stickers, film or anything blocking a sensor.
Don't fly in rain, smog or visibility under 100 m.
If DJI Fly shows a vision sensor warning, see Mini 4 Pro sensor error.

In Intelligent Flight modes and RTH

ActiveTrack, MasterShots and QuickShots rely on the same sensors and have the same blind spots. During Advanced RTH, DJI says the drone switches to Preset RTH and can't bypass obstacles if the light or environment isn't suitable. Set an RTH altitude higher than nearby obstacles. See Mini 4 Pro obstacle avoidance for how the system works.

When it's a fault

If avoidance does nothing in daylight, in Normal mode, with Bypass set and clean sensors, update firmware in DJI Fly. If it still fails, or a lens is cracked, contact DJI or start a repair. For more on the drone, read the Mini 4 Pro guide. Want better sensing in low light? The Mini 5 Pro adds forward LiDAR.