Engine-Codes - Honda - P0420 Honda

P0420 Honda Code - Catalyst System Efficiency Below Threshold

- Code Popularity: 463,433 Views
- Repair Importance Level: 2/3
- Repair Difficulty Level: 2/3
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What are the Possible Causes of the DTC P0420 Honda?

NOTE: The causes shown may not be a complete list of all potential problems, and it is possible that there may be other causes.
  • Faulty Three-Way Catalytic Converter (TWC)
  • Faulty Secondary Heated Oxygen Sensor (Secondary HO2S (Sensor 2))
  • Secondary Heated Oxygen Sensor 2 harness is open or shorted
  • Secondary Heated Oxygen Sensor 2 circuit poor electrical connection

Code Tech Notes

As the code description implies, the P0420 code means that the vehicle's Powertrain Control Module (PCM) has detected that the Three-way Catalyst Converter is not working properly (it is not as efficient as the factory expects). Replacing the oxygen (O2) sensors may sometimes fix the code, but in most cases, the Three-way Catalyst Converter needs to be replaced to resolve the problem. If the P0420 code is combined with other codes, try addressing the other codes first

There is a Factory Service Bulletin for the following Honda models:
2003–2005 Honda Civic Hybrid ULEV (KA)
2002–2006 Honda Insight With M/T
Honda Civic, Insight Factory Service Bulletin OBDII Code P0420





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How to Fix the DTC P0420 Honda?

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 pins.

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What is the Cost to Diagnose the Code?

Labor: 1.0

The cost to diagnose the P0420 Honda code is 1.0 hour of labor. The diagnosis time and labor rates at auto repair shops vary depending on the location, make and model of the vehicle, and even the engine type. Most auto repair shops charge between $75 and $150 per hour.

What are Code Possible Symptoms?

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

What Does the P0420 Honda Code Mean?

The Three-Way Catalytic Converter (TWC) converts Hydrocarbons (HC), Carbon Monoxide (CO), and Oxides of Nitrogen (NOx) in the exhaust gas to water vapor, Carbon Dioxide (CO 2), and Dinitrogen (N 2). The Powertrain Control Module (PCM) fluctuates the air/fuel ratio temporarily to detect the performance of the TWC and measures the degree of fluctuation in the secondary Heated Oxygen Sensor (Secondary HO2S (Sensor 2)) output. If the accumulated value for a set time is too great, deterioration of the performance of the catalyst is detected, and the PCM detects a malfunction and stores a Diagnostic Trouble Code (DTC).

P0420 Honda Code - Catalyst System Efficiency Below Threshold

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