I'm having this annoying issue with my Ptap G3 where it reads non-zero (1-30 w) when coasting. I've tried all the obvious stuff (battery, pairing to different head units etc) to no avail. I've had this issue before actually (different hub) and it was resolved by sending it in for calibration service.
I sent this one in to Sram/Quarq (new owners) and when I received it back it still had the same problem.... This will be fun. We'll see what kind of customer service I get and how they propose to solve the problem. I've spent $200 on this calibration service already and don't really want to spend any more; the ball is in their court. I've got a month before I want to begin training in earnest so I'll be patient - for now.
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"Your beauty is an aeroplane;
so high, my heart cannot bear the strain." -A.C. Jobim, Triste