■ Spark control systems. Spark control systems havebeen in use since the earliest gasoline engines. Itwas discovered that the proper timing of the ignition spark helped to reduce exhaust emissionsand develop more power output. Incorrect timingaffects the combustion process. Incomplete combustion results in HC emissions. High CO emissions can result from incorrect ignition timing.Advanced timing can also increase the production of NOx. When timing is too far advanced,combustion temperatures rise. For every 1° ofoveradvance, the temperature increases by 125°F(51.57°C). Spark control on today’s engines ishandled by the PCM. Through input signals fromvarious sensors, the PCM adjusts ignition timingfor optimal performance with minimal emissionslevels.