Here’s the deal, just installed the power option for my Polar S720, bought the power sensor used (great deal from Joe W. on the classified’s here). My first ride this morning, everything is working fine, everything reading out as it was supposed to. Get to the point where I am going to start going hard, hit the lap split button and all of the sudden no more wattage reading, no more cadence. Speed and heart rate are still reading out fine though. All the wires seem to be fine. The little indicator light on the power sensor that is supposed to be green is showing red when I pedal so obviously there is a problem somewhere. Could the battery in the mounting unit be bad? Would the speed still read out if the battery is dead? Or does the speed rely more on the watch battery to work? Any ideas on where to start?
Had similar a while back Mike and replaced the battery in the mount which did not solve the problem - sent it to Polar (out of Warantee) and they fixed it for the cost of shipping - when I asked what the problem was they pretty much blew me off - although I didn’t push it as my unit was repaired. FYI - the speed unit works on the watch only - do that on my race bike. The cadence and power work off the sensor mounted on the chain stay…
Check the cadence magnet.
If you still have speed but lost power and cadence readout then the cadence is the problem. Without the cadence you lose the power too.
It seems odd that you would have missed it since it is so prominently featured in the manual 
You have to mount the magnet with the Power Sensor where is says Cadence on the sensor. For a normal cadence sensor those area to mount are great. The pedal spindle works great, but not in his case
You can, if… everything measures correctly. If the Sensor is mounted exactly in the middle where it states middle, and if your spindle happens to line up directly where it states Cadence then you got lucky. I have to mount it on my crank so it lines up with the word Cadence.
problem solved. After all the talk about the cadence magnet I checked that. At first glance everything looked fine, but upon closer inspection, the magnet had slipped down about a half an inch and was no longer aligned properly. Since the crank arm tapers towards the middle from both ends, it just naturally gravitated toward the center. Just need to find a better way to anchor it.
Thanks for all of your collective wisdom!!
I don’t want to turn this into who knows more or a flame fest, but on page 4 of the Power Sensor manual under section 2, it states place it in the middle.
Now thats a productive exchange of information! Good Job!