The Engine Control Module (ECM) receives rough road information from the G sensor. The ECM uses the rough road information to enable or disable the 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 the movement of a small metal bar inside the sensor. If a fault occurs which causes the ECM to not receive rough road information between 50 and 132 km/h (30 and 80 mph), DTC P1391 will set.