The gas tank cover locks up and does not open even when the door locks are locked and unlocked several times. After many tries the tank capopens up. What is the reason?

I had TWO that had a problem that was repairable with little effort.

Take a good look at the lock pin the next time you have it open. There is a rubber grommet around the lock pin - in two of our 850s this grommet was not fully installed and pushing the pin to one side. Both bricks were '95s, one failed, the other worked OK, but I repaired it anyway with a dental pick.

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96 855R, 95 855,854, 90 744 Ti - 343,000 Volvo miles put on 7 bricks

My '94 850T Wagon has the same problem. I was told bt the dealer that there is a small motor inside the gas door and it needed to be replaced. Less than $200.00. The dealer then showed me how to manually pull open the door. I'm waiting to find one of those motors at the junk yard. To pull it manually, (on a wagon) pull the passenger side floor board at the rear of the cargo area, Then undo the side panel revealing the hook which will open the door. I suspect that there is a similar hook on the sedans. Good luck