DJI Mavic 3 Classic vision sensor error: calibration and fixes

A vision sensor error on the DJI Mavic 3 Classic means one or more of its obstacle-sensing cameras or infrared modules isn't giving usable data. Most of the time the cause is a dirty lens, poor light, a reflective or plain surface, or the drone having been stored a long time. Clean every sensor, restart outdoors in daylight and update the firmware. If DJI Fly asks to calibrate, it does so automatically.

The Classic's sensing system

The Mavic 3 Classic has a horizontal omnidirectional vision system, upward and downward vision systems, and two 3D infrared modules underneath, plus an auxiliary bottom light for low light. These hold position when GNSS is weak and brake for obstacles in Normal and Cine mode.

Fixes to try

Clean the sensors. Wipe every vision lens and the infrared windows with a soft, dry cloth. DJI says to keep the sensors clean and not tamper with them.
Check what's covering them. The storage cover must be off. Remove stickers, tape or anything near a sensor.
Restart in good conditions. Power on outdoors, in daylight, on textured ground. DJI lists dark surfaces under 15 lux, glare over 40,000 lux, water, glass, snow, plain colors and repeating patterns as conditions where the vision systems can't work properly. Starting up on a white table or a dark garage floor can trigger warnings.
Check the settings. Vision Positioning and Obstacle Sensing can be turned off in DJI Fly under Safety > Advanced Safety Settings. They turn back on by default after a restart. Obstacle Avoidance must be set to Bypass or Brake for the horizontal sensors to brake.
Update firmware in DJI Fly, or refresh it with DJI Assistant 2 (Consumer Drones Series) on a computer.
Let calibration run. DJI's manual says the vision cameras may need calibration after long storage, and that DJI Fly shows a prompt and calibrates them automatically. Follow it and keep the drone still.
Check the IMU and compass in DJI Fly's safety settings. A bad reading there can show alongside vision warnings.

Normal limits, not errors

Some "sensor not working" moments are expected:

Obstacle sensing is unavailable during RTH, auto landing and Intelligent Flight modes, per DJI
It's off in Sport mode
It can't reliably see thin branches, power lines, cranes or bridge cables
Fog and cloud can temporarily disable it

When to contact DJI

Start a repair if the error returns after cleaning, a daylight restart and a firmware update, if a sensor lens is cracked or scratched, or if the warning started after a crash. Vision modules are a DJI service part. Crash damage is covered by DJI Care Refresh if you have it.

Until it's fixed, fly as if obstacle avoidance is off. The general guide to DJI vision sensor errors covers other models, and the Mavic 3 Classic guide covers the drone.