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AutoCodes – GMC – P0420 GMC

P0420 GMC 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 GMC 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.
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Code Popularity 107,987
How popular is this code, among other codes? Medium
What is this?
Table of contents Created and updated by William Orellana on Jun 20, 2022
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Code P0420 GMC 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 GMC - Catalyst System Low Efficiency Bank 1 - AutoCodes.com
P0420 GMC Code - Catalyst System Low Efficiency Bank 1
What does this mean?

What are the Possible Causes of the DTC P0420 GMC?

  • 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 GMC?

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 GMC 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 GMC code?

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

Need Help with the Code?

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

Information for Specific GMC Models

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 GMC Code

1. What are the common symptoms of OBDII code P0420 GMC?

The common symptoms of code P0420 GMC include decreased fuel efficiency, a noticeable drop in engine performance, and the illumination of the check engine light on the dashboard.

2. What are the possible causes of OBDII code P0420 GMC?

Possible causes of code P0420 GMC include a faulty catalytic converter, damaged oxygen sensors, exhaust leaks, or issues with the engine's fuel system.

3. How serious is OBDII code P0420 GMC?

While code P0420 GMC does not pose an immediate threat to the vehicle's drivability, it can lead to increased emissions and potential long-term damage if left unresolved. It is recommended to address this issue promptly.

4. Can a faulty oxygen sensor trigger OBDII code P0420 GMC?

Yes, a faulty oxygen sensor can contribute to triggering code P0420 GMC as it may provide inaccurate data to the engine control module, leading to an inefficient catalytic converter operation.

5. Can I drive my vehicle with OBDII code P0420 GMC?

It is generally safe to drive your vehicle with code P0420 GMC in the short term. However, prolonged driving with this issue can result in increased emissions and potential damage to the catalytic converter.

6. How can I diagnose the root cause of OBDII code P0420 GMC?

Diagnosing code P0420 GMC typically involves inspecting the catalytic converter, oxygen sensors, and exhaust system for any visible damage or issues. Additionally, performing a comprehensive scan with a diagnostic tool can help pinpoint the exact cause.

7. What are some repair tips for resolving OBDII code P0420 GMC?

Repair tips for code P0420 GMC may include replacing the catalytic converter, repairing any exhaust leaks, replacing faulty oxygen sensors, and ensuring proper functioning of the fuel system components.

8. Can a clogged catalytic converter trigger OBDII code P0420 GMC?

Yes, a clogged or failing catalytic converter can trigger code P0420 GMC due to reduced efficiency in converting harmful emissions. It is essential to address any catalytic converter issues promptly.

9. How much does it cost to repair OBDII code P0420 GMC?

The cost of repairing code P0420 GMC can vary depending on the root cause of the issue, the extent of the damage, and the labor rates in your area. Repairs for this code may range from a few hundred to over a thousand dollars.

10. Will resetting the check engine light resolve OBDII code P0420 GMC?

Resetting the check engine light may temporarily turn off the warning indicator, but it will not resolve the underlying issue causing code P0420 GMC. It is crucial to diagnose and address the root cause of the problem to prevent further complications.

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