AutoCodes – Honda – P0420 1999 Honda Civic

P0420 1999 Honda Civic Code - Catalyst System Efficiency Below Threshold Bank 1

Quick Answer

The Engine Control Module (ECM) monitors the switching frequency ratio of heated oxygen sensors 1 (front O2 sensor) and heated oxygen sensors 2 (rear O2 sensor). 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 1999 Honda Civic indicates Catalyst System Efficiency Below Threshold 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 30,620

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 Aug 20, 2020

Description
Step-by-Step Diagnostic Guide
Possible Causes
How To Fix
Tech Notes
Cost to Diagnose
FAQs
Comments
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Code P0420 1999 Honda Civic Description

The Engine Control Module (ECM) monitors the switching frequency ratio of heated oxygen sensors 1 (front O2 sensor) and heated oxygen sensors 2 (rear O2 sensor).
A three way catalyst converter (Manifold) with high oxygen storage capacity will indicate a low switching frequency of heated oxygen sensor 2. As oxygen storage capacity decreases, the heated oxygen sensor 2 switching frequency will increase.
When the frequency ratio of heated oxygen sensors 1 and 2 approaches a specified limit value, the three way catalyst malfunction is diagnosed.

OBDII Code P0420 1999 Honda Civic - Catalyst System Efficiency Below Threshold Bank 1 - AutoCodes.com — P0420 1999 Honda Civic Code - Catalyst System Efficiency Below Threshold Bank 1

What does this mean?

What are the Possible Causes of the DTC P0420 1999 Honda Civic?

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 1999 Honda Civic?

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 (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 1999 Honda Civic 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.

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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 1999 Honda Civic Code

1. What does the OBDII code P0420 on my 1999 Honda Civic mean?

The code P0420 indicates that the Catalyst System Efficiency is Below Threshold in Bank 1 of your vehicle.

2. What are the common symptoms of a P0420 code in a 1999 Honda Civic?

Common symptoms of a P0420 code may include a check engine light illuminating on the dashboard, reduced fuel efficiency, and possible lack of engine power.

3. What are the possible causes of a P0420 code in a 1999 Honda Civic?

Potential causes of a P0420 code in a 1999 Honda Civic include a faulty catalytic converter, oxygen sensor issues, exhaust leaks, or engine misfires.

4. How can I diagnose a P0420 code in my 1999 Honda Civic?

To diagnose a P0420 code, you may need to use an OBDII scanner to retrieve the code and check the catalytic converter, oxygen sensors, and exhaust system for any issues.

5. Can I drive my 1999 Honda Civic with a P0420 code?

It is generally safe to drive your vehicle with a P0420 code, but it is recommended to address the issue promptly to prevent further damage to the catalytic converter and ensure optimal engine performance.

6. How can I repair a P0420 code in my 1999 Honda Civic?

Repairing a P0420 code may involve replacing the catalytic converter, oxygen sensors, or repairing any exhaust leaks. It is advisable to consult a professional mechanic for proper diagnosis and repair.

7. Will resetting the OBDII code P0420 on my 1999 Honda Civic fix the issue?

Simply resetting the code may temporarily turn off the check engine light, but it will not fix the underlying problem causing the P0420 code. Proper diagnosis and repair are necessary to address the issue.

8. Are there any maintenance tips to prevent a P0420 code in a 1999 Honda Civic?

Regular maintenance, such as timely replacement of oxygen sensors, monitoring engine performance, and ensuring a clean air filter, can help prevent issues that may lead to a P0420 code.

9. How much does it typically cost to repair a P0420 code in a 1999 Honda Civic?

The cost of repairing a P0420 code in a 1999 Honda Civic can vary depending on the specific issue causing the code and the labor rates in your area. Replacing a catalytic converter, for example, can be a significant expense.

10. Can a P0420 code in a 1999 Honda Civic cause emissions test failure?

Yes, a P0420 code can cause your 1999 Honda Civic to fail emissions testing, as it indicates that the catalyst system is not functioning efficiently, leading to higher emissions than allowed.