I have a nixon ultratide, screen is dead. tried replacing the battery, but i'm still getting nothing on the screen. What is the AC reset referred to on the sticker when you open the back of the watch?

All the watches and all the batteries. They have the Dork, but not Dork Too. They do have a battery replacement service though.

https://www.nixon.com/pages/watch-battery-replacement

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Pull the battery out of the watch, the number is stamped on the battery negative side, go on eBay and find the battery.

That is the most likely cause. The battery may have sat in the watch for a long time before it was sold and it may have been on the shelf for a long time before it was put in the watch. Replace it first and if it doesnt not last long, the watch may have a problem with its internal parts.

It may just be a coincidence that the battery is dead at the same time you changed the hours

Hi You don't fix it. Hopefully you mean it's only 3 days old from being new and you can take back to the shop for refund/replacement.
ROM - stands for Read Only Memory and is basically the chip that all the software and information is stored on. This runs the watch and it's this that has failed. You could try reseting the watch by removing the battery and then replacing it.

I have found the instructions on their web site but this alarm is not mentioned.

I suspect that it is a battery low indication; this would be consistent with a battery that is over a year old .

Try having the battery replaced.

Sounds like the battery is most likely dead. The plastic spacer is there specifically there to keep that from happening prior to sale.

I am guessing the battery went dead. Put the watch under a lamp about 10 inches from the bulb and leave it for 12 to 24 hours. This will recharge the cell. Then you will have to go through the setting procedure from the start. Either refer to the CD, your owners manuel or the Citizen website. You will need to do the "reset" procedure most likely. This ordinarily is not needed but in your case most likely will be needed as a result of your battery going completely dead.

Here some tips to deal with a Nixon Lodown:
1) Sleeping mood (display is off, all buttons lead to the same reaction: display shows time for some secs and then turns off again. This mode is for battery energy saving, switch it on and off by holding (c.a. 2 secs) the light button.
2) ROM FAIL: it happens that the lodown shows ROM FAIL and constantly restarts. It happened on my watch while I tried to change time zones. Here is what to do about it: Bring it to a professional watch repair store and get the battery replaced. The watch restarts automatically and works perfectly fine again. DON'T send it to Nixon, they will charge you for shipping, battery replacement, the new battery etc. Although it should be covered by the guarantee. Just get a new battery, it cost me 16 USD and the watch is good again, waterproof and works perfectly. Nixon authorized repair centers will charge you extra for pressure testing. I had this on my second Lodown, after I had it two weeks only...
3) Watch freezes: All digits are on at the same time, none of the buttons brings a reaction. I had this problem on my first lodown. Just get a new battery, that solves the problem. But again DON'T use authorized Nixon repair centers, it costed me almost 45 USD, just to get a new battery. Although the watch was only 1 year old and guarantee should have covered this case.
In general the Nixon Lodown is a good watch, looks good, has all features you need, as long as it works. The problem with the watch is, the above problems occur pretty often. I have my second Lodown now and I am very disappointed with the Nixon customer service. Although clearly guarantee cases, they will charge you for every single problem you have with the watch. My advice, bring the watch to a regular watchmaker.
After all, I don't understand why the Nixon Lodown manual does not mention all the above issues. The manual is completely useless, very poor from Nixon. You have to dig online to solve any issues on the watch. In summary: The Nixon Lodown watch is good, Nixon customer service and guarantee is a joke. ROM FAIL and freeze is solved with replacing the battery.

On most casio watch you can see AC (all clear) point , youy need to resset clock connecting this point with + on battery about 5 sec.

I have same problem but on my watch I do not see AC point. :)

regards
ststefan