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The C5's use the same rear O2 sensors from '00 to at least '02 so the ones from your '02 Trans Am might work. I would give it a shot, nothing to lose. Check to see that your wiring is good as that could cause your problem.
If you need to get a new one then I would stick with the AC Delco ones as I have heard of others, notably Bosch, that have caused some people problems. The GM# is 25317697 and the AC Delco# is 213-810. Shop around the internet and you'll find a good price. As I recall, the price at gmpartsdirect was pretty decent
Check the grounds for the sensors. There is a ground stud on the drivers side frame rail below the brake vacuum booster. There is a braded ground wire attached to it. That wire attaches to the engine. The ground piont is below # 5 spark plug on the cylinder block! I would remove the wire and clean the connections. The connection on the block can only be accessed from below,