Purpose

This article outlines the procedures to diagnose and confirm Diagnostic Trouble Code (DTC) P0420 on a 2017 GMC Yukon, and to replace the Catalytic Converter(s) 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 P0420 (Catalyst System Low Efficiency), it indicates the ECM has detected that the catalytic converter on Bank 1 is not functioning properly.

The catalytic converter is a component of the exhaust system. Its purpose is to reduce harmful emissions by converting them into less harmful substances. Bank 1 refers to the side of the engine where cylinder 1 is located. Operating your vehicle with a malfunctioning catalytic converter will result in increased tailpipe emissions, and may damage your vehicle’s engine.

To correct this issue, you will inspect the vehicle’s exhaust system, test the oxygen sensors and catalytic converter(s), and replace the catalytic converter(s) as needed.

Symptoms of DTC P0420 include:

• Malfunction Indicator Lamp (MIL) ON or flashing
• Reduced engine power and degraded performance
• Poor fuel economy
• Exhaust emits rotten egg smell
• Failed emission test

Possible causes of this fault include:

• Damaged or defective catalytic converter(s)
• Damaged or defective oxygen sensor or wiring
• Exhaust leak or damage
• Incorrect air-fuel ratio or fuel pressure
• Oil or contamination in the catalytic converter(s)
• Incorrect fuel type

On a 2015 - 2018 GMC Yukon V8-5.3L, replacing catalytic converter may resolve DTC P0420.

	Step Time required (hour) Sections Cautions	5 1.6 3 0

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

Inspection and Diagnosis

Step 1: Initial Inspection

During initial inspection, check for obvious signs of mechanical damage, such as:

• Torn inlet boots
• Missing or broken vacuum tees
• Cracked, broken, or disconnected wire harnesses

Make sure all harness connectors are securely attached and are properly placed in holders. This will prevent potential open or short-circuit issues. Also, check connectors for corrosion, frayed wiring, and damaged terminals.

Inspect the exhaust system for obvious leaks, damage, and Heated Oxygen Sensor (HO2S) condition. Check for exhaust leaks at the exhaust manifold and exhaust pipes, specifically at or around the HO2S, or Air/Fuel Ratio (A/F) sensor. Inspect for visual or physical damage, severe discoloration, substrate rattling noise and/or potential partial restrictions. Refer to the Vehicle Emission Warranty Manual for time and mileage coverage for the emission-related fault(s). For additional information, refer to the Service Manual or applicable Technical Service Bulletin (TSB) #16-NA-111 prior to continuing diagnosis.

Step 2: Diagnostic Procedures

1. Inspect the ignition coil wire(s) or connector(s) to ensure they are securely attached to the spark plugs. Make sure the coil(s) and housing fit properly into the valve cover, cylinder head, and spark plug(s). Check the ignition coil harness connector(s) for proper fit into the connector(s) and holders. This will prevent short-to-voltage, short-to-ground, or interference with other wires. While the engine is running at idle, spray a fine water mist onto the wires and coil(s) and observe for evidence of arcing or changes in RPM. Occasionally, fuel contamination may occur, and when the fuel level is low, the dirt or contamination can enter the fuel injection system.

2. Start and run the engine at idle until the engine reaches normal operating temperature. Check Long Term Fuel Trim (LTFT) at idle, 1500, 2000 and 2500 RPM. Confirm the value is no more than ±10%. If the value is greater than specification, correct the condition before continuing Check for faults in the following areas and repair as necessary:

• Fuel Pressure
• Leaking injectors
• Vacuum leaks
• Manifold Absolure Pressure (MAP) sensor
• Mass Air Flow (MAF) sensor

3. Increase engine speed to 2,000 RPM for approximately 3 minutes. Confirm that Closed Loop operation has been achieved.

4. Return the engine to idle. Using a Scan Tools, check that Heated Oxygen Sensor (HO2S) B1S2/B2S2 voltage slowly switches between 0.600-0.900 volts.

5. If the voltage fluctuates rapidly between 0.200-0.800 volts, a defective catalytic converter(s) may be indicated.

Replace the Catalytic Converter

Removal

Step 1 – Use a lift to raise the vehicle.

Step 2 – If the vehicle is equipped with 4-wheel drive (4WD), detach the front U-joint yoke retainer bolts from the front axle propeller shaft.

Step 3 – Disconnect all heated oxygen sensors on bank 1 and bank 2.

Step 4 – Remove the exhaust front pipe from the exhaust muffler.

Step 5 – Remove the right catalytic converter from the right exhaust manifold.

Step 6 – Remove the left catalytic converter from the left exhaust manifold.

Installation

Installation is accomplished in the reverse order of removal.

Validate the Repair

1. Erase DTCs. Turn the ignition off for one minute, then restart the engine.

2. Perform the drive cycle requirements for catalytic converter validation. Optionally, operate the vehicle within the conditions recorded in the Freeze Frame data when DTC P0420 was initially set.

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

NOTE: Some new catalytic converters may require a break-in period of approximately 100 miles.