P0173 OBD-II Trouble Code: Fuel Trim Malfunction (Bank 2)

P0173 code definition?

P0173 Fuel Trim Malfunction (Bank 2)

What the P0173 code means

Code P0173 is triggered by the engine control module (ECM) when it detects that the air-fuel ratio is too lean or too rich. For most lean and rich conditions, the ECM can compensate for the lack or air or gas entering the engine. However, if the amount of compensation is too much, a P0173 code may set.

Fuel trim is what the ECM implements to adjust the air-fuel ratio of the engine according to changing driver conditions. The air-fuel ratio should stay at, or very near to 14.7:1. It must stay at this range during idling, going down hills, cruising, and at highway speeds.

There are two forms of air-fuel ratio adjustment: short-term and long-term. Short-term fuel trim happens almost instantly, while long-term fuel trim is a gradual adjustment that happens over longer durations in contrast to the instant adjustment provided by short-term fuel trim.

The ECM can only adjust the air-fuel ratio to a certain extent. If the ECM can not compensate for the required adjustment, a P0173 code may set.

What causes the P0173 code?

• A dirty or faulty mass air flow sensor
• Leaking intake manifold or torn intake boot
• Vacuum leak
• Faulty fuel injector
• Faulty fuel pressure regulator
• Pinched fuel line

What are the symptoms of the P0173 code?

• A Check Engine Light
• Increased smog or soot from the exhaust
• Loss in acceleration
• Hesitation or misfire during idle
• Increased fuel consumption

How does a mechanic diagnose the P0173 code?

• Visually inspects the mass air flow sensor
• Tests the resistance of the mass air flow sensor and circuitry with a digital multimeter
• Performs a fuel pressure test
• Inspects the wiring of the fuel injectors for fraying
• Inspects the intake manifold and vacuum lines for leaks
• Uses a scan tool to specify the fuel trim out of range (short-term or long-term)

Common mistakes when diagnosing the P0173 code

• Replacing the Mass Air Flow Sensor without cleaning or testing it first

• Deeming a component faulty without checking for more common causes, such as vacuum leaks

• Not using a scan tool to determine if there is an issue with either the short- or long-term fuel trim

• Failing to inspect the wiring of the component thought to be at fault

How serious is the P0173 code?

• The life of the catalytic convertor could become shorter because it will have to filter more pollutants in the exhaust gases

• Gas consumption will increase

What repairs can fix the P0173 code?

• Replacing the mass air flow sensor
• Replacing the fuel pressure regulator
• Replacing the intake manifold gaskets
• Repairing or replacing the engine air filter intake system
• Replacing a vacuum line connected to the throttle body
• Repairing a leaking brake booster
• Replacing a damaged fuel line
• Replacing a faulty fuel injector

Additional comments for consideration regarding the P0173 code

It is suggested to always check the wiring of a component and clean it before deeming it faulty. It’s also a good idea to check the intake system and vacuum lines for leaks.

Need help with a P0173 code?

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