Quality used car & truck parts, engines, and transmissions from our vehicle salvage yards. Do it yourself for less with recycled auto parts--the B&R way!. The dreaded Check engine light comes on - now what? What is a trouble code anyway? Surely does not sound good. Here's some lists and unasked for advice.. The transmission adapts the output of the engine to the drive wheels. Transmissions usually have multiple gear ratios (“gears”) with the ability to switch between.