In my experience, 8 times out of 10, I find it's a sending unit (I'm sure I've replaced way more left senders than right)... Once out of 10 it's an alternator, and once out of 10, it's a transfer pump... )When they're working right, I find the levels are very close to one another. I have seen people that fill only the left tank, and wonder why it doesn't transfer to the right tank... I once filled a right side tank full, while the left tank was nearly empty, and it did transfer.