I have a big-box-bike 18 speed that I ride around the neighborhood for exercise. Recently it stopped staying in the rear gear I put it in -- I roll the gear selector where I want it, but as soon as I let go the gear slides back to 6. If I hold the gear I want the bike stays in that gear, but as soon as I let go the spring in the derailleur drags it back to six. I assume there's some sort of a ratcheting mechanism or pawl somewhere that holds the derailleur in the gear I select, I just don't know where it is.
I'd rather just replace the broken part then get a new bike -- but before I order something I thought I'd ask the professionals -- what should I be ordering to fix this problem?