Body Fat % on Index S2 Smart Scale Seems Inaccurate

It should be possible to measure if the scale is sending a small electrical current?

I just got on it with rubber shoes, it refused to give a fat value.

It is clearly that the scale uses the electric impedance in some way. I saw it giving slightly different variations of the BF% for the same weight. Also, sometimes BF% goes down with the weight, and sometimes BF% goes up when the weight goes down. But whatever it measures, it is clearly incorrect. It clearly uses some statistical values to estimate the bone mass (although it is funny that the bone mass still varies day to day) and lean muscles mass, and the impedance is used to calculate the balance between the fat and the body water in some fairly narrow range. But it clearly assumes that everyone has the same lean muscle mass for their sex and age, and doesn't seem to take fitness into the account at all.

The scale must use electrical impedance in some way. You can easily test this by stepping on the scale in shoes with a rubber sole. In this case, the scale only displays the weight and BMI.

Garmin-Kevin Can you please email me with additional information? I'm noticing some strange behavior with body fat now too.

It also refuses a fat value if you use the guest profile with no shoes.  It's still just an inaccurate algorithm on a very expensive weight scale.

Yes... the scale is *** compared to Xiaomi and others that cost only around $50 (and Garmin costs $160). I tried once weighing myself twice. First just me and second holding a 500ml bottle full of water (extra 0,5 kg)... Garmin S2 measured the same weight, so even sensors aren't accurate. Also the screen... its laggy and it doesn't scream "high quality". Garmin seriously needs to fix this, because they can not charge 160$ for something thats not even worth 30-$40. Whats even the point of collecting the data if its wrong? I was hoping the S2 will be a major improvement and at least accurate to some degree, but as it seems it's just a big disappointment.

Yep, same behavior over here. All was fine and working smoothly and for some reason on the 28th of Feb my body fat % measurement spiked from 22 to 31%. I didn't eat anything special that day and ever since it always reads something around 30%. My wife is super athletic and she's getting 40% body fat!! 
Honestly, this is unacceptable for a device of this cost. Garmin do something and get this fixed!!!  At the end of the day this is just a calculation which surely is far from accurate but at least is a reference on how we are doing and a good indicator of loosing/gaining fat over time. Even if it is a fake value, please make something up that at at least allows us to keep track of our training condition!

Why can't Garmin allow some entries of simple body measurements into the Garmin Connect app (Waist, neck, hips, wrist etc) alongside age, height, activity level.  This would then enable the algorithm it uses to have more parameters for greater accuracy because it could take into consideration more physical attributes.  I am sure it does use electrical impedence in some way but the upshot is for most who use this forum who are probably more aware of the facts than your average user there is a consistent inaccuracy in what the scales are saying with regard to body fat