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AutoCodes - Generic - P0420

P0420 Code - Catalyst System Efficiency Below Threshold Bank 1

- Code Popularity: 661,669 Views
- Repair Importance Level: 2/3
- Repair Difficulty Level: 2/3
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By - Updated Aug 27, 2022
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|Code P0420 Possible Causes

  • Gas leak from exhaust system
  • Faulty Three-way Catalyst Converter Bank 1
  • Faulty Air Fuel Ratio (A/F) Sensor Bank 1 (Sensor 1 or Front Sensor or Upstream Sensor)
  • Faulty Heated Oxygen Sensor Bank 1 (Sensor 2 or Rear Sensor or Downstream Sensor)
  • Faulty Fuel Injector(s)

|How do I Fix the Code P0420?

Check the "Possible Causes" listed above. Visually inspect the related wiring harness and connectors. Check for damaged components and look for broken, bent, pushed out, or corroded connector's pins.

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|Code P0420 Tech Notes

As the code description implies the P0420 code means that the vehicle's Engine Control Module (ECM) has detected that the Three-way Catalyst 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 Three-way Catalyst Converter needs to be replaced to fix the problem. If the P0420 code is combined with other codes, try fixing the other codes first.

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|What is the Cost to Diagnose the Code P0420?

Labor: 1.0

The cost of diagnosing the P0420 code is 1.0 hour of labor. The auto repair labor rates vary by location, your vehicle's make and model, and even your engine type. Most auto repairs shops charge between $75 and $150 per hour.

|What are Code P0420 Possible Symptoms?

  • Engine Light ON (or Service Engine Soon Warning Light)

|Code P0420 Description

The Engine Control Module (ECM) uses sensors mounted in front of and behind the Three-Way Catalytic Converter (TWC) to monitor its efficiency.
The first sensor, the air-fuel ratio sensor, sends pre-catalyst information to the ECM. The second sensor, the heated oxygen sensor, sends post-catalyst information to the ECM.
In order to detect any deterioration in the three-way catalytic converter, the ECM calculates the oxygen storage capacity of the three-way catalytic converter. This calculation is based on the voltage output of the heated oxygen sensor while performing active air-fuel ratio control.
The oxygen storage capacity value is an indication of the oxygen storage capacity of the three-way catalytic converter. When the vehicle is being driven with a warm engine, active air-fuel ratio control is performed for approximately 15 to 20 seconds. When it is performed, the ECM deliberately sets the air-fuel ratio to lean or rich levels. If the cycle of the waveform for the heated oxygen sensor is long, the oxygen storage capacity is great. There is a direct correlation between the heated oxygen sensor and the oxygen storage capacity of the three-way catalytic converter.
The ECM uses the oxygen storage capacity value to determine the state of the three-way catalytic converter. If any deterioration has occurred, the ECM illuminates the MIL and stored the DTC.
This system determines the deterioration of the entire catalyst system (including the front and rear catalysts), by using the oxygen storage capacity value of the front catalyst, which is more sensitive than the rear catalyst, as the representative value. Therefore, be sure to replace the front and rear catalysts together when catalyst replacement is necessary.
OBDII Code P0420 - Catalyst System Efficiency Below Threshold Bank 1 -
P0420 Code - Catalyst System Efficiency Below Threshold Bank 1

|Need More Information on the P0420 Code?

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

|Information for Specific Makes

|Need Help with the P0420 Code?

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|Code P0420 comments

Help us improve Leave a comment below or tell us if the information above helps you fix the code. What are the symptoms on your vehicle for the P0420 code? Have you replaced any parts?
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