P0173 medium Severity

P0173: Fuel Trim Malfunction (Bank 2)

May cause rough idle, hesitation on acceleration, reduced fuel economy, and increased emissions; often triggers the check engine light.

Overview

P0173 indicates a fuel trim malfunction on Bank 2 where the PCM/ECM cannot maintain proper air-fuel ratio due to excessive corrections, often caused by vacuum leaks, sensor issues, or fuel delivery problems.

Common Symptoms

  • Illuminated check engine light (MIL)
  • Rough or uneven idle
  • Hesitation/stumble on acceleration
  • Reduced fuel economy
  • Strong fuel smell from exhaust

Most Common Causes

Vacuum leak on Bank 2 intake or unmetered air entering system high

Unmetered air causes lean conditions forcing extreme fuel trim adjustments.

Faulty or contaminated Mass Air Flow (MAF) sensor high

Incorrect airflow measurement leads to improper fuel trim correction.

Faulty Bank 2 upstream oxygen (O2) sensor high

Inaccurate O2 sensor readings can mislead PCM about air-fuel ratio.

Fuel delivery issues (pressure regulator, pump, injectors) medium

Clogged injectors or low/high fuel pressure disrupts proper mixture.

Exhaust leaks before Bank 2 O2 sensor medium

Leaks can cause false lean readings, prompting excessive corrections.

Diagnostic Steps

  1. Retrieve live fuel trim data with an OBD-II scanner
    Expected: Long-term and short-term fuel trim values significantly positive or negative on Bank 2
    Interpretation: Confirms abnormal fuel trim behavior indicating issue is present.
  2. Inspect intake system for vacuum leaks using a smoke test
    Expected: Smoke escaping from hoses/intake seals
    Interpretation: Presence of leaks explains unmetered air causing lean condition.
  3. Clean or test the MAF sensor
    Expected: Restored normal airflow readings if sensor is clean/functional
    Interpretation: Incorrect airflow readings suggest MAF sensor fault.
  4. Test Bank 2 upstream O2 sensor response
    Expected: Voltage should cycle between lean and rich rapidly under varying conditions
    Interpretation: Slow or incorrect response indicates O2 sensor fault.
  5. Measure fuel pressure at the rail
    Expected: Fuel pressure within OEM specifications
    Interpretation: Out-of-spec pressure points to fuel delivery component issues.

Repair Solutions

Seal or replace vacuum lines and intake gaskets on Bank 2 medium
Estimated Cost: $50 - $200

Fixing vacuum leaks often restores proper trim without expensive parts.

Clean or replace the MAF sensor medium
Estimated Cost: $80 - $220

Cleaning may suffice; replacement if contaminated or faulty.

Replace Bank 2 upstream O2 sensor medium
Estimated Cost: $90 - $250

Use OEM quality sensor for reliable readings.

Repair or replace fuel delivery components (pump/regulator/injectors) hard
Estimated Cost: $200 - $800

More complex and costly; requires precise diagnosis.

Common Misdiagnoses

Avoid these common diagnostic mistakes:

  • Assuming a bad O2 sensor without checking for intake/exhaust leaks first
  • Replacing the MAF sensor prematurely without confirming readings
  • Ignoring potential fuel pressure issues leading to incorrect root cause

Frequently Asked Questions

What is the definition of the P0173 engine code?

Fuel Trim Malfunction (Bank 2) is the standard definition for P0173. This diagnostic code is triggered when the ECU detects a problem in the Fuel System.

Can P0173 cause engine damage?

This is a medium priority issue. May cause rough idle, hesitation on acceleration, reduced fuel economy, and increased emissions; often triggers the check engine light.

How do I know if I have code P0173?

You may experience the following: Illuminated check engine light (MIL), Rough or uneven idle, Hesitation/stumble on acceleration.

Common triggers for the P0173 error

The most common causes are: Vacuum leak on Bank 2 intake or unmetered air entering system, Faulty or contaminated Mass Air Flow (MAF) sensor, Faulty Bank 2 upstream oxygen (O2) sensor.

Is P0173 expensive to fix?

Expect to pay around $200 if the issue is a Seal or replace vacuum lines and intake gaskets on Bank 2, but up to $800 if the Repair or replace fuel delivery components (pump/regulator/injectors) needs replacement.

Does P0173 affect Ford, Toyota, or Honda?

This is a universal code. You might see P0173 on Ford, Chevrolet, Toyota, Honda, Nissan, or any other OBD-II vehicle.

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