VL53L4CD - Signal is below the defined threshold

[DrD00m]

Hey,

I have a custom made board/device which is using 4x VL53L4CD Tof Sensors to measure the distance. I have trouble to calibrate the sensors correctly or get correct values from them from time to time. Sometimes it works well - sometimes it does not.
What I can see is, that for sensor where the distance is not accurate I am receiving the result status " Signal is below the defined threshold", as described in the datasheet.
What exactly does that mean and how can I fix that? What is causing this issue?

The signal threashold is set to 5000 and the sigma is set to be 10.

I noticed when I am running the offset and xtalk calibration - this issue can disappear but it takes multiple calibration iterrations and sometimes it does not fix the problem.

 

And as well: what exactly is a 17% reflective material!? (as mentioned in the datasheet to calibrate offset and xtalk)

[DrD00m]

@John E KVAM I got the issue again where the sensor never clears its interrupt no matter what I do. 
This can only be solved by removing power entirly from the board (and therefor from the sensor) and restarting it. Somehow the Sensor can be stuck in this very strange state where it delivers bad values like below 200 even tho the next object (my ceiling) is 1800mm away combined with the interrupt which can not be cleared.
The interrupt is LOW immediately.

I don't know how the sensor can be brought into this state and I also don't know why this is always only on sensor 2 of my 4 sensors where all running the same code-path. 
Electrically the board looks fine. I think this is an I2C timing issue because adding 5ms after each i2c write, lowering the communication speed from 400kHz to 300kHz and also wait ~1s after each sensor initialisation seem to help somehow. 

Any idea what could cause this on the sensor?