Complex sentence: Poole takes one of the extra-vehicular pods and swaps the AE-35 unit, but when Bowman conducts tests on the removed AE-35 unit, he determines that there was never anything wrong with it.
Split into simpler sentences: Poole takes one of the extra-vehicular pods and swaps the AE-35 unit. But when Bowman conducts tests on the AE-35 unit that has been replaced, he determines that there was never anything wrong with it.