You can know the automobile meaning of the engine P0391 Code and it is not helpful code if right meaning of the code is not found easily. The automobile meaning of the engine code is P for OBD-II Diagnostic Powertrain (P) Trouble Code For Engine. 0 for Intake Valve Control Solenoid Circuit Low. 3 for Engine Oil Temperature Sensor Malfunction. 9 for Cylinder 10 Contribution/balance Fault and 1 for Exhaust Gas Recirculation Sensor B Circuit High. However, read the car engine manual to know meaning of the code and according to the right meaning of the code solve the car engine.