P0171 Code - Toyota

P0171 TOYOTA - Air/Fuel Mixture System Lean Bank 1

P0171 Code - Toyota
P0171 Code - Toyota

Troubleshooting P0171 Code


Oftentimes, a P0171 means that your Toyota is having trouble with a dirty or faulty mass air flow (MAF) sensor, or a vacuum leak downstream of the MAF sensor (perhaps in the PCV line or in the valve itself). Of course, this code can also signify a bad O2 sensor in bank 1.

Additionally, it's rare (but still possible) that a P0171 code can be tripped by a plugged fuel injector, by a dirty fuel filter causing low fuel pressure, or even an exhaust leak before the first O2 sensor€¦isn't engine trouble code diagnosis fun?

Most of the time, the best way to respond to a P017 is to try cleaning the MAF sensor and the lines running to and from it. This will often resolve this issue.
When you clean the MAF sensor, be sure to inspect all of the lines including the vacuum and PCV hoses. Look for kinks, cracks, etc. and replace the lines as needed. Additionally, be sure to check all of the connections and hoses on the air intake and check for intake manifold leaks.
Finally, it's never a bad idea to change your air and fuel filter, especially if they're just about due for replacement anyways.
If cleaning the MAF sensor and inspecting the lines doesn't solve the problem, it may be time to replace the O2 sensor on bank 1. Consult your Toyota's manual for its exact location, which is specific to model. If that doesn't do it, it's probably time to enlist the help of a professional.

P0171 Toyota Possible causes

  1. Intake air leaks
  2. Faulty front heated oxygen sensor
  3. Ignition misfiring
  4. Faulty fuel injectors
  5. Exhaust gas leaks
  6. Incorrect fuel pressure
  7. Lack of fuel
  8. Faulty Mass Air Flow (MAF) sensor
  9. Incorrect Positive Crankcase Ventilation (PCV) hose connection

Possible symptoms 

 
  • Engine Light ON (or Service Engine Soon Warning Light) 
  • Excessive Fuel Consumption 

P0171 Toyota Description 


With the Air/Fuel Mixture Ratio Self-Learning Control, the actual mixture ratio can be brought closely to the theoretical mixture ratio based on the mixture ratio feedback signal from the heated oxygen sensors 1. The Engine Control Module (ECM) calculates the necessary compensation to correct the offset between the actual and the theoretical ratios. In case the amount of the compensation value is extremely large (The actual mixture ratio is too lean.), the ECM judges the condition as the fuel injection system malfunction and light up the MIL (2 trip detection logic).