Purpose

This article outlines the procedures to diagnose and confirm Diagnostic Trouble Code (DTC) P0171 on a 2019 Hyundai Tucson and to replace the Heated Oxygen Sensor (HO2S) to resolve the issue.

Introduction

In cases where the Malfunction Indicator Lamp (MIL) on your vehicle’s dashboard is illuminated and the Engine Control Module (ECM) has stored DTC P0171 (System Too Lean (Bank 1)), it indicates the ECM has detected that the fuel trim average value exceeds the limit over certain period. A common cause of this DTC is the Heated Oxygen Sensor (HO2S).

The HO2S is a vital component in modern engines. Its primary function is to measure the oxygen levels in the exhaust gases of a vehicle. By monitoring the oxygen content, the sensor provides essential feedback to the ECM, enabling it to adjust the air/fuel mixture for optimum combustion efficiency. This helps improve fuel economy, reduce emissions, and ensure the engine operates within the desired parameters.

To correct DTC P0171, inspect for air leakage or restriction in the Intake or Exhaust System, check Fuel System operation, and replace the HO2S as needed to correct this issue.

Symptoms of DTC P0171 include:

• Malfunction Indicator Lamp (MIL) ON
• Reduced engine power and performance
• Rough idle
• Engine misfire

Possible causes of this fault are:

• Poor connection
• Intake/Exhaust System air leakage or restriction
• Faulty Heated Oxygen Sensor (HO2S)
• Faulty Manifold Absolute Pressure (MAP)
• Faulty fuel system
• Faulty Evaporative Emission System (EVAP)
• Faulty sensor signal
• Dirty oil or oil level too high

When DTC P0171 is present on a 2019 Hyundai Tucson 2.4L, replacing the HO2S may resolve the issue.

	Step Time required (hour) Sections Cautions	2 0.6 3 0

(This image is for illustrative purposes only and may not match the actual part number)

Diagnostic Procedures:

Initial Inspection

During your initial inspection, check for obvious mechanical signs such as torn inlet boots, missing or broken vacuum tees, and/or cracked, broken, or disconnected wire harnesses. Make sure all harness connectors are properly placed in the connectors and holders. This will prevent potential open or short-circuit issues. Also, check connectors for corrosion, frayed wiring, and damaged terminals. In addition, inspect the battery terminals and check the state of charge before attempting to diagnose possible electrical faults.

Ensure the vacuum hoses and hose fit tightly to a manifold vacuum source on the engine. Inspect all the hoses that direct and send air, especially between the Mass Air Flow (MAF) sensor and the throttle body. This fault may be caused by a vacuum leak, or a dirty or disconnected MAF sensor. Refer to the Vehicle Emissions Warranty Manual for time and mileage coverage for the emissions-related fault(s). For additional information, refer to the Service Manual or applicable Technical Service Bulletin (TSB).

Diagnostic Procedures

Inspect the rear Heated Oxygen Sensor (HO2S)

Use a Scan Tool in Live Data graphing mode and allow the engine to reach 158° F or greater. Hold the engine at 2500 RPM and monitor the HO2S voltage. The voltage should slowly fluctuate between 0.1–0.8V. Release the throttle, the voltage should rapidly decrease to approximately 0.1V then return slowly to 0.8V.

- If the voltage fluctuates rapidly between 0.2–0.8V, suspect a defective catalytic converter, then perform Repair Validation. If the same DTC sets, check for other components: Intake/Exhaust System for Restriction or Leakage, MAP Sensor, EVAP System and Fuel System.

- If the voltage did not follow this pattern, suspect a faulty HO2S. Replace/repair as necessary. Perform Repair Validation. If the same DTC sets, check for other components.

- If no fault is found, check for other components.

Replace the Heated Oxygen Sensor (HO2S)

Removal

Step 1 – Turn OFF the ignition switch and disconnect the negative (-) battery terminal.

Step 2 – Disconnect the HO2S sensor connector.

Step 3 – Unbolt and remove the HO2S sensor.

Installation

Installation is accomplished in the reverse order of removal.

Repair Validation:

1. Clear DTC(s) and Freeze Frame (FF) data.

2. Perform the Key ON Engine OFF (KOEO) and Key ON Engine RUNNING (KOER) test or perform the drive cycle requirements for the fault area.

3. Check for pending or stored DTCs. If none are found, the repair is complete.