The Nissan Path

I recently purchased my 03 Pathfinder. It had 74k miles, 1 previous owner, and I thought I found a diamond in the ruff. However once I got it home, I was having problems starting the vehicle. It seemed as though the battery was dying. ( engine would just make a clicking sound when turning the ignition.) Being that the vehicle is 12 years old I figured it might be that the second battery is on the end of it's life cycle so I replaced it. The next morning it started fine but after work that day I just barely got it to start. I think something is sucking the juice from it but with little electrical experience I'm wondering where I start troubleshooting?

topic moved to the correct category.

You can measure the draw on the battery by using a multimeter in the fuse box. With the key off, just have the black on ground and the red touch to each fuse and anything over 0.1 amps or so is the culprit.

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Actually, anything over 50 milliamps (or 0.05 amps) is considered excessive draw. Start with your basics. Have the battery tested; just because it's new doesn't necessarily mean it's good. Check your battery cable connections; R51's are notorious for have issues due to battery cable terminals that can't tighten-up sufficiently. Check the charging system voltage. Then check for parasitic draw.