AutoCodes – GMC – P0420 2018 GMC Sierra

P0420 2018 GMC Sierra Code - Catalyst System Low Efficiency Bank 1

Quick Answer

A 3-way catalyst controls emissions of hydrocarbons, Carbon Monoxide (CO), and Nitrogen Oxide (NOx). Common causes include Faulty Three-way Catalyst Converter Bank 1, Exhaust Tube, Intake Air Leaks. Severity is Moderate - service should be scheduled soon. The vehicle may still be drivable for short trips if it runs normally, but avoid hard driving and repair it soon.

What it means: P0420 2018 GMC Sierra indicates Catalyst System Low Efficiency Bank 1.
Common causes: Common causes include Faulty Three-way Catalyst Converter Bank 1, Exhaust Tube, Intake Air Leaks.
Severity: Moderate - service should be scheduled soon.
Can I drive?: The vehicle may still be drivable for short trips if it runs normally, but avoid hard driving and repair it soon.

Save to My Repairs Step-by-Step Diagnostic Guide My Repairs

AI answers can be wrong. Verify repairs with testing. Open full chat

Code Popularity 427

How popular is this code, among other codes? Low

Repair Importance Level

Moderate (Service soon) (2 / 3)

Repair Difficulty Level

Intermediate (2 / 3)

What is this?

Table of contents Created and updated by William Orellana on Jun 20, 2022

Description
Step-by-Step Diagnostic Guide
Possible Causes
How To Fix
Tech Notes
Cost to Diagnose
FAQs
Comments
Add Comment

💬 Need more help? Ask Coddy for instant answers — it's free!

Code P0420 2018 GMC Sierra Description

A 3-way catalyst controls emissions of hydrocarbons, Carbon Monoxide (CO), and Nitrogen Oxide (NOx). The catalyst within the converter promotes a chemical reaction, which oxidizes the hydrocarbons and the CO that are present in the exhaust gas. This process converts the hydrocarbons and the CO into water vapor and carbon dioxide (CO2), and reduces the NOx, converting the NOx into nitrogen. The catalyst also stores oxygen. The Engine Control Module (ECM) monitors this process by using Heated Oxygen Sensors (HO2S) that are in the exhaust stream before and after the 3- way catalyst. The HO2S produces an output signal that the ECM uses to calculate the oxygen storage capacity of the catalyst. This indicates the ability of the catalyst to convert the exhaust emissions efficiently. The ECM monitors the efficiency of the catalyst by monitoring the HO2S during an off-idle, Decel Fuel Cut-Off (DFCO) event. When the catalyst is functioning properly, the post-catalyst HO2S response to the fuel conditions during the DFCO event is slow compared to the response of the pre-catalyst HO2S. When the post-HO2S response is near that of the pre-HO2S, the oxygen storage capability and efficiency of the catalyst may be degraded below an acceptable threshold. The Diagnostic Trouble Code (DTC) will be set when the ECM determines that the efficiency of the catalyst has degraded below a calibrated threshold.

OBDII Code P0420 2018 GMC Sierra - Catalyst System Low Efficiency Bank 1 - AutoCodes.com — P0420 2018 GMC Sierra Code - Catalyst System Low Efficiency Bank 1

What does this mean?

What are the Possible Causes of the DTC P0420 2018 GMC Sierra?

Faulty Three-way Catalyst Converter Bank 1
Exhaust Tube
Intake Air Leaks
Faulty Oxygen (O2) Sensor
Faulty Fuel injector(s)
Leaking Fuel Injector (s)
Faulty Spark plugs
Improper Ignition Timing
Faulty Engine Control Module (ECM)
Dirty Air Filter

How to Fix the DTC P0420 2018 GMC Sierra?

Review the 'Possible Causes' above and visually examine the corresponding wiring harness and connectors. Check for damaged components and inspect connector pins for signs of being broken, bent, pushed out, or corroded.

What does this mean?

Code Tech Notes

As the code description implies the P0420 code means that the vehicle's control module has detected that the three-way catalytic converter is not working properly (it is not as efficient as the factory is expecting). Replacing the oxygen (O2) sensors may sometimes fix the code, but in most cases, the catalytic convert needs to be replaced to fix the problem. If the P0420 code is combined with other codes, try fixing the other codes first.

What is the Cost to Diagnose the Code?

Labor: 1.0

To diagnose the P0420 2018 GMC Sierra code, it typically requires 1.0 hour of labor. Rates vary by location, vehicle, and shop. Many shops charge between $80–$150 per hour; dealerships and metro areas may be higher, independents may be lower.

Need more information about the P0420 2018 GMC Sierra code?

Need more information on how to fix the P0420 2018 GMC Sierra code? Get Access to Factory Service Manuals

Need Help with the Code?

Get the P0420 2018 GMC Sierra code diagnosed by a professional: Find a repair shop in your area

About the author: William Orellana is a Master Mechanic and Mechanical Engineer with over 30 years of experience diagnosing and repairing vehicles. Learn more about his background.

Frequently Asked Questions about P0420 2018 GMC Sierra Code

1. What are the common symptoms of an OBDII code P0420 on a 2018 GMC Sierra?

The common symptoms include the check engine light turning on, decreased fuel efficiency, and potential engine performance issues.

2. What does the OBDII code P0420 "Catalyst System Low Efficiency Bank 1" mean for a 2018 GMC Sierra?

It indicates that the catalytic converter in Bank 1 (usually the side with cylinder 1) is not performing efficiently in reducing emissions.

3. What are the possible causes of an OBDII code P0420 on a 2018 GMC Sierra?

Potential causes include a faulty catalytic converter, oxygen sensor issues, exhaust leaks, engine misfires, or problems with the fuel system.

4. How can I diagnose the cause of an OBDII code P0420 on my 2018 GMC Sierra?

Diagnosis typically involves using a scan tool to monitor the oxygen sensor data, checking for exhaust leaks, inspecting the catalytic converter, and verifying the functionality of related components.

5. Can driving with an OBDII code P0420 on a 2018 GMC Sierra cause further damage?

Continued driving with this code may not cause immediate harm to the vehicle, but it can lead to increased emissions and potential long-term damage to the engine or catalytic converter.

6. How can I repair an OBDII code P0420 on my 2018 GMC Sierra?

Repair options may include replacing the catalytic converter, fixing any underlying issues with oxygen sensors or engine performance, and clearing the code with a scan tool.

7. Are there any common DIY tips for fixing an OBDII code P0420 on a 2018 GMC Sierra?

DIY tips may include checking for loose connections, ensuring proper fuel quality, changing spark plugs, and performing regular maintenance to prevent future issues.

8. Is it safe to continue driving my 2018 GMC Sierra with an OBDII code P0420?

While it may be safe to drive in the short term, it is recommended to address the underlying issue promptly to avoid potential damage to the vehicle and ensure compliance with emission regulations.

9. Can a malfunctioning catalytic converter cause an OBDII code P0420 on a 2018 GMC Sierra?

Yes, a failing or inefficient catalytic converter is a common cause of the P0420 code, as it indicates that the converter is not effectively reducing emissions as required.

10. How much does it typically cost to repair an OBDII code P0420 on a 2018 GMC Sierra?

The cost of repair can vary depending on the specific issue causing the code, but it may range from a few hundred to over a thousand dollars, with catalytic converter replacement being one of the more expensive options.