Do I need to change both batteries when one test's bad.

 

IMO I would change both. The other one is not far behind.

 

The problem with only replacing one is that like he said the other may not be far behind, and the driver's side is temp monitored for charging so if the state of health of the batteries isn't close, you'll either over or under charge the passenger side battery.

 

When you add a new one to an old one. It will want to pull power to the older weaker battery to keep the system balanced. This will in turn kill the newer battery quicker. I replaced both on mine.

 

Thanks for the info. I changed both.