How do i sync outside temp after replacing the battery?

Make sure that the channel (1,2,3) agrees between the sender and base. You might try a different channel due to interference. double check batteries. Then -- remove batteries from remote and base. Then put batteries in remote first (should be next to base) then insert batteries in the base. Be sure channel is in agreement between the units. Sync can take a minute or longer.

Bring outside temp sensor indoors. Remove batteries from both remote and base. replace outside first with fresh batteries (Alkaline recommended). Then the base. Make sure they are on the same channel. When display restored, you can take the remote back outside.

remove all batteries, bring the two units together and put the batteries in the base unit first, then the outdoor unit; they should sync within 2-3 minutes. Once they have exchanged temp and humidity once, you can separate them. Good luck.

What is an outside temp? More information please.

Hit reset next to batteries to have it sync with outside temp sensor. Push mode button to scroll thru date, time. Use up and down button on back to adjust numbers.

You have to push some buttons to tell the MAIN unit to scan, then put battery into the REMOTE, set channel, and hit reset through a little hole. And the MAIN should see it.

I had the same problem, Since the outside temp wouldn't read, I figured it was the outside unit batteries. Went through half a dozen AAA batteries and wouldn't read. It was the indoor unit. Had the same bats I installed 3 years ago. DUH! Replaced the AAA, and all was well. When the inside batteries die out, it has just enough juice to read indoor temp, but not enought to sense exterior signal.

Look in the manual, There should be a section on remote sensors, they usualy have a "sync" button on them. and hit main unit first then remote.