What Does Code P0173 Mean?

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What Does Code P0173 Mean? – Understanding Code P0173: Causes, Symptoms, and Solutions GuideMechanic.Com Modern vehicles are equipped with sophisticated onboard diagnostic systems that monitor various components and systems to ensure optimal performance and emission control.

When a fault is detected, the system generates trouble codes to assist technicians in diagnosing the issue. One such code is P0173, which indicates a problem with the fuel trim system in Bank 2 of the engine.

In this article, we’ll delve into what code P0173 means, explore its possible causes, symptoms, and discuss potential solutions to address the issue effectively.

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What Does Code P0173 Mean?

What Does Code P0173 Mean

Code P0173 is a diagnostic trouble code (DTC) that indicates a “Fuel Trim Malfunction (Bank 2)” in the engine’s fuel delivery system.

This code specifically relates to the engine’s air-fuel mixture being too rich (too much fuel) on Bank 2, which typically refers to the side of the engine that does not contain the first cylinder in the firing order. Bank 2 is often found in V6 or V8 engines, where there are two separate exhaust manifolds or sets of cylinders.

Possible Causes of Code P0173:

Several factors can contribute to the triggering of code P0173. Some common causes include:

Faulty Oxygen Sensor:

A malfunctioning oxygen sensor, particularly the upstream oxygen sensor (sensor 1) on Bank 2, can inaccurately detect the air-fuel mixture’s composition, leading to improper fuel trim adjustments.

Vacuum Leak:

A vacuum leak in the intake system, such as a cracked or disconnected vacuum hose, can introduce unmetered air into the engine, disrupting the air-fuel ratio and triggering code P0173.

Faulty Mass Airflow Sensor (MAF):

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What Does Code P0173 Mean

A malfunctioning MAF sensor can inaccurately measure the amount of air entering the engine, leading to incorrect fuel trim adjustments and a rich air-fuel mixture.

Fuel System Issues:

Problems with the fuel system, such as a clogged fuel injector, fuel pressure regulator malfunction, or fuel pump failure, can disrupt the proper fuel delivery, resulting in a rich air-fuel mixture.

Engine Misfire:

An engine misfire on Bank 2 can cause incomplete combustion of the air-fuel mixture, resulting in unburned fuel entering the exhaust system and triggering code P0173.

Exhaust System Leaks:

Leaks in the exhaust system, such as a cracked exhaust manifold or damaged catalytic converter, can affect oxygen sensor readings and lead to improper fuel trim adjustments.

What Does Code P0173 Mean?

Symptoms of Code P0173:

What Does Code P0173 Mean

When code P0173 is triggered, drivers may experience the following symptoms:

Check Engine Light (CEL) Illumination:

The most common symptom of code P0173 is the illumination of the check engine light on the vehicle’s instrument panel.

Decreased Fuel Efficiency:

See Also: Engine Emission Warning

A rich air-fuel mixture can lead to decreased fuel efficiency, as excess fuel is not efficiently burned during combustion.

Rough Idle:

An overly rich air-fuel mixture may cause the engine to idle roughly or unevenly, leading to vibrations and instability at idle.

Engine Performance Issues:

The vehicle may exhibit sluggish acceleration, hesitation, or lack of power due to the incorrect air-fuel mixture.

Increased Emissions:

A rich air-fuel mixture can result in higher emissions of pollutants such as carbon monoxide (CO) and hydrocarbons (HC), which may fail emissions tests.

What Does Code P0173 Mean?

Diagnosis and Solutions:

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What Does Code P0173 Mean

Diagnosing and resolving code P0173 typically requires the following steps:

Retrieve and Record Freeze Frame Data:

Use an OBD-II scanner to retrieve freeze frame data associated with code P0173, including engine load, coolant temperature, fuel trim values, and oxygen sensor readings. This information can help pinpoint the underlying cause of the issue.

Inspect Oxygen Sensors:

Check the condition and operation of the upstream oxygen sensors (sensor 1) on Bank 2. Replace any faulty oxygen sensors as needed.

Check for Vacuum Leaks:

Inspect the intake system and vacuum hoses for signs of leaks, cracks, or disconnected hoses. Use a smoke machine or propane torch to detect vacuum leaks if necessary.

Test Mass Airflow Sensor (MAF):

Test the MAF sensor using a multimeter or MAF sensor testing tool to ensure accurate airflow measurement. Clean or replace the MAF sensor if it is faulty or contaminated with debris.

Inspect Fuel System Components:

Check the fuel injectors, fuel pressure regulator, and fuel pump for proper operation and fuel delivery. Clean or replace any faulty components as needed.

Check for Engine Misfires:

Use a scan tool to monitor engine misfire data and identify any misfiring cylinders on Bank 2. Address any misfire issues by inspecting ignition components such as spark plugs, ignition coils, and ignition wires.

Inspect Exhaust System:

Check the exhaust system for leaks, damage, or restrictions that may affect oxygen sensor readings and fuel trim adjustments. Repair or replace damaged components as necessary.

Clear Codes and Test Drive:

See Also: Check Engine Light Scanner

After addressing the underlying cause of code P0173, clear the diagnostic trouble codes using the OBD-II scanner. Take the vehicle for a test drive to ensure that the symptoms have been resolved and that the check engine light remains off.


Code P0173 indicates a fuel trim malfunction on Bank 2 of the engine, resulting in a rich air-fuel mixture. Understanding the possible causes and symptoms of this code is essential for diagnosing and resolving the underlying issue effectively.

By following the diagnostic steps outlined in this guide and addressing any faulty components, vehicle owners can restore proper engine performance, fuel efficiency, and emission control. If unsure about diagnosing or repairing the issue, consider seeking assistance from a qualified automotive technician.


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