I recently managed to get a 4iiii Precision left-only crank power meter. I fitted it and did a 40 minute indoor session. I have a dumb trainer, but I've found the power curve online. It was showing that I could expect ~200W for 30km/h on a certain setting. Instead, I was seeing 180W at best.
I checked the battery level, and the pressure of the turbo trainer against the back wheel. All seemed normal. I tried to zero the power meter a few times but nothing changed.
I was convinced the power was too low for what I expected, so I took to Google. Eventually I found this post from SRC1 on the BikeRadar forum:
I was getting low readings from my generation 1 4iiii precision, around 40w below what it should be. 4iiiis asked me to refit the battery with the polarity reversed, leave it in for 10 seconds, then refit properly. Fixed it for me.
Interesting... So I gave it a go and jumped back on my bike. 200W!! Amazing.