"G" Sensor

The powertrain control module (PCM)/engine control module (ECM) receives rough road information from "G" sensor. The PCM/ECM uses rough road information to enable or disable misfire diagnostic. The misfire diagnostic can be greatly affected by crankshaft speed variations caused by driving on rough road surfaces. The "G" sensor generates rough road information by producing a signal which is proportional to movement of a small metal bar inside sensor. If a fault occurs which causes PCM/ECM to not receive rough road information between 30 and 80 mph (50 and 132 km/h), Diagnostic Trouble Code (DTC) P1391 will set.