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P0421 Mitsubishi Code - Warm Up Catalyst Efficiency Below Threshold- Cylinders 1-4

- Code Popularity: 167,750 Views
- Repair Importance Level: 2/3
- Repair Difficulty Level: 2/3
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What are the Possible Causes of the DTC P0421 Mitsubishi?

NOTE: The causes shown may not be a complete list of all potential problems, and it is possible that there may be other causes.
  • Warm Up Three Way Catalytic converter deterioration or malfunction
  • Exhaust system leaks
  • Loose front or rear HO2Ss
  • Faulty front heater O2 sensor
  • O2 sensors harness is open or shorted
  • O2 sensors poor electrical connection
⚠ Does your Mitsubishi have Safety Recalls? → Check here

How to Fix the DTC P0421 Mitsubishi?

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

Code Tech Notes

As the code description implies the P0421 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 P0421 code is combined with other codes, try fixing the other codes first.

There is a factory service recall for the following Mitsubishi Models:
2002 Mitsubishi Lancer
2003 Mitsubishi Lancer
2004 Mitsubishi Lancer
2005 Mitsubishi Lancer
2002−2005 Mitsubishi Lancer Factory Service Recall OBDII Code P0421

Search for P0421 Mitsubishi Questions and Answers: Search here →

What is the Cost to Diagnose the Code?

Labor: 1.0

To diagnose the P0421 Mitsubishi code, it typically requires 1.0 hour of labor. The specific diagnosis time and labor rates at auto repair shops can differ based on factors such as the location, make and model of the vehicle, and even the engine type. It is common for most auto repair shops to charge between $75 and $150 per hour.

When is the Code Detected?

The P0421 code is set when PCM has determined that the warm up catalyst efficiency for bank 1 is below threshold for the current engine operating condition. (Bank 1 identifies the location of cylinder #1, while bank 2 identifies the cylinders on the opposite bank)

What are Code Possible Symptoms?

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

Code P0421 Mitsubishi 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 P0421 Mitsubishi - Warm Up Catalyst Efficiency Below Threshold- Cylinders 1-4 - AutoCodes.com
P0421 Mitsubishi Code - Warm Up Catalyst Efficiency Below Threshold- Cylinders 1-4

Need More Information on the Code?

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