I was flying fine when it started acting squirrely as I was bringing it into land.  Ended up grabbing it because it was holding steady.  Got a vision sensor error.  DJI GO 4 app is telling me it is a "Downward Infrared Sensor Error (2)".  
Saw on Youtube where a guy was using DJI Assistant 2 to recalibrate sensors on a Mavic Air.  When I go to those menus for the Spark there is no "Calibration" selection for the Spark.

Can you calibrate them?

Currently trying to reload firmware to see what happens.

Our apologies for the inconvenience. Please make sure that the sensors are clean. Kindly restore the settings to factory defaults and update the firmware using DJI Assistant 2. After that, kindly calibrate the IMU. Please let me know if the issue persists. You can download the software here: https://www.dji.com/spark/info#downloads and follow the video tutorial below.

DJI Paladin Posted at 2018-9-18 02:38
Our apologies for the inconvenience. Please make sure that the sensors are clean. Kindly restore the settings to factory defaults and update the firmware using DJI Assistant 2. After that, kindly calibrate the IMU. Please let me know if the issue persists. You can download the software here: https://www.dji.com/spark/info#downloads and follow the video tutorial below.

https://www.youtube.com/watch?v=1Psa4hAImKw

same thing.  took it down to factory and then updated to newest version.  then calibrated the IMU.

Still "Vision System Error"

RepK Posted at 2018-9-18 17:21
same thing.  took it down to factory and then updated to newest version.  then calibrated the IMU.

Still "Vision System Error"

proceed to nearest DJI service center

Sounds similar to the problem that my Spark is experiencing, I get a downward sensor red error signal when at an inch or so above the ground during landings and it bounces onto the ground, not hard but it’s noticeable and can be corrected with a bit of power applied just on touchdown to arrest the rate of descent.

Been through all the suggested software resets and calibrations with no success.

Sending it back to DJI isn’t really a suitable option for me due to my remote location.

As mentioned above, it would be great to be able to somehow calibrate the downward sensors.

RepK Posted at 2018-9-18 17:21
same thing.  took it down to factory and then updated to newest version.  then calibrated the IMU.

Still "Vision System Error"

Sorry to hear that. I would recommend you to contact our support team to start up a case: (https://www.dji.com/support?site=brandsite&from=nav). We have the professional data analysis team who would do their best to find out the reason of this issue and then the corresponding resolution would be provided. You can provide your case number here in case you need further assistance.

DJI Paladin Posted at 2018-9-19 06:24
Sorry to hear that. I would recommend you to contact our support team to start up a case: (https://www.dji.com/support?site=brandsite&from=nav). We have the professional data analysis team who would do their best to find out the reason of this issue and then the corresponding resolution would be provided. You can provide your case number here in case you need further assistance.

If you ever did follow through with this action with DJI- it would be great to hear a summary of what if any problem was identified so that others like myself in this group might benefit.  I get the same error and have no solution as of yet.   It doesn't seem to effect too much though, so I guess that's the upside (so far).  It may effect features I don't often use.