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If the cadence sensor has been installed according to instructions
And if the Edge 520 and the cadence sensor are both new
And if the cadence sensor has its battery/batteries properly installed, and they are new batteries and of the right type for the cadence sensor
... then if all that is true, but the cadence sensor is still not responding, I would say that would mean that you got a defective cadence sensor from the store, and would be justified in returning it promptly for an exchange or refund... the cadence sensor at least, but perhaps the whole Garmin 520 package and set, should be returned before the return/exchange period runs out.
(The return/exchange period usually does not last longer than 2 weeks after purchase, and sometimes less, e.g. a week or 10 days after the purchase. So doing this return-and-exchange ASAP is advisable.)
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