A catalyzer was a part of the compression coil that made the engine turn, generating power. The catalyzer and the coil rotated as part of the main engine. When Serenity's catalyzer blew, Kaylee said it was the port one. The crew of the SS Walden came aboard with a new catalyzer, and that vessel's captain shot Malcolm Reynolds, with the intention of repairing and stealing Serenity. Mal was barely able to chase them off the ship and install the new catalyzer, repairing the engine just before the clock ran out on the O2.