Well, the first question would be if this is a Cobalt 1 or 2. The Cobalt 2 has a faster processor and a much superior compass to the original Cobalt. It is a much improved design overall. Also, as the product matured, some early issues with cases leaks, high pressure seals, and rusting buttons were resolved or components were redesigned, so in general the newer it is, the better. The Cobalt 2 is now something I would consider (based on very few problems or returns) to be very reliable. That said, I talk to plenty of users who have many hundreds- in a few cases thousands- of dives on the original Cobalt.
If you can look at the Cobalt, navigate to the system info page (Main Menu- Settings- System Info) and check the serial # and the atmospheric pressure. That should ideally be within 10-20 mbar of actual pressure. You may need to use an online calculator like this:
Above Sea Level Barometer Reading Correction Calculator if you are above sea level to determine your actual local pressure. If it's way off, there may be a problem with the pressure sensor. That was an issue with some of the first Cobalt 2's produced, as Atomic received some unreliable sensors from the sensor manufacturer.
Would this still be within the original warranty period? I can't speak to Atomic's policies on used equipment or transferring warranty, but I can say in the years we have worked together on the Cobalt project (we are the original developers of the electronics and firmware) that they have been unfailingly generous and helpful to users of their gear. They have gone way over the top on many occasions to help users that had a problem, I doubt you will find better customer service anywhere.
-Ron
