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RD, ASE Certified Technician
Category: Car
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Experience:  Ase Master 35 years
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ML320: MB..Fault codes P0173 Fuel Trim Malfunction (bank 2) codes

Customer Question

I have failed my smog check for my 1998 MB ML320 follow reasons:
Fault codes P0173 Fuel Trim Malfunction (bank 2)
codes P0170 Fuel Trim malfinctions (bank 1)
Please, let me know how I can fix it and how much it will be cost if I will change it.
Thank you.
I will wait for you answer.
Submitted: 6 years ago.
Category: Car
Expert:  RD replied 6 years ago.


Welcome to JA!

P0170 Fuel Trim Malfunction (Bank 1)


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 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).


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

When is the code detected? - Fuel injection system does not operate properly.
- The amount of mixture ratio compensation is too large. (The mixture ratio is too lean.)


Possible Causes - Intake air leaks
- Front Heated oxygen sensor may be faulty
- Injectors may be faulty
- Exhaust gas leaks
- Incorrect fuel pressure
- Lack of fuel
- Mass air flow sensor may be faulty
- Incorrect PCV hose connect


Possible Solution Dirty air filter of faulty air flow sensor are common causes of the problem.



P0173 Fuel Trim Malfunction (Bank 2)


Fault Code Definition
Long Term and Short Term Fuel Trim are the methods used by the Power Train 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 Power Train 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

I would look for a vaccum leak first

If your 02 sensers are fairly old i would replace them

Here is a link for oem 02 sensors.

There is some less price ones but i would replace them with good ones



Let me know if i can help more