P0173 - OBD-II Trouble Code

Fuel Trim Malfunction (Bank 2)

Our emissions expert has put together the following information about the P0173 fault code. We have also included diagnostic procedures you can take to your repair shop if the mechanic is having difficulty analyzing the code.

OBD II Fault Code

  • OBD II P0173

Fault Code Definition
Long Term and Short Term Fuel Trim are the methods used by the Powertrain Control Module to continually adjust the air to fuel ratio in order to maintain the 14.7 to 1 (Stoichiometric) mixture that is ideal for combustion. Code P0173 is set when the Powertrain Control Module has reached its operational limit with these adjustments.


  • Check Engine Light will illuminate
  • In many cases, no abnormal symptoms may be noticed
  • In some cases, the vehicle may get poor fuel mileage
  • In some cases, the engine may hesitate during acceleration

Common Problems That Trigger the P0173 Code

  • Intake Manifold vacuum leaks
  • Defective Mass Air Flow Sensor
  • Defective Mass Air Flow Sensor wiring or connections

Common Misdiagnoses

  • Oxygen Sensor is replaced when the real problem is a vacuum leak
  • Oxygen Sensor is replaced when the real problem is a defective Mass Air Flow Sensor