Just bought a new bike. I transferred speed/cadence sensor to the new bike, but the Garmin can't fnd it. I have tried pairing, but that gets me nowhere. I have replaced the battery. Nothing works. Any ideas?

Is a combination speed/cadence or a speed sensor and a cadence sensor? Those are two different options in the Edge 800 sensor setup part of a bike profile and it won't find the sensors if you choose the wrong one. Assuming you created a new profile is your Garmin currently on that bike profile, there have been times where I couldn't figure out why my sensors weren't working when it turn out I had the Garmin set to the wrong bike.

It's a combo sensor. And yes, I have created a new profile. Even tried the old bike, but nothing works.

Wheel / crank arm magnets properly aligned? Could also try a fresh battery.

I had a similar problem once and Garmin tech support told me to take the battery out for a while. I did and then it worked.

This.

When you take the battery out of the sensor, hold the sensor upside down and shake out all the stray electrons.