A powertrain control module (PCM) is a device that manages the engine and transmission control systems. It controls fuel delivery, ignition timing, and emissions control systems to ensure that the engine runs efficiently and safely. A PCM rarely needs replacement, and it may be necessary only if it fails. A faulty PCM could cause several symptoms such as engine stalling or misfiring, inconsistent acceleration, and increasing emissions. A professional mechanic can diagnose a faulty PCM and recommend replacement if necessary.