OBD2 Codes

P0421 – Meaning, Causes, Symptoms, & Fixes


Code P0421 Definition

Bank 1 Catalytic Converter Efficiency is Below Threshold on Cold Start

What Does P0421 Mean?

When this code is triggered the ECM (Engine Control Module) has deduced, through programmed rationality, that the Bank 1 Catalytic Converter isn’t warming up sufficiently; therefore, it’s not cleaning up the exhaust as well as it is supposed to. It does this using the values of the pre and post catalyst Oxygen or Air/Fuel sensors, and compares them to what they should be as the engine warms up using the engine coolant temperature. The most likely failure that causes this code is the Catalytic Converter, but oftentimes the Catalytic Converter can fail due to the engine running poorly. Always make sure the engine is running properly before replacing the Catalytic Converter.  

What Are The Symptoms Of Code P0421?*

  • Check Engine Light
  • Rotten egg smell from exhaust
  • Poor emissions
  • Lack of power

*No noticeable adverse conditions in some cases

What Is The Cause Of Code P0421?

  • O2 Sensor
  • A/F Sensor
  • Bank 1 Catalytic Converter
  • Spark Plugs
  • Ignition Coil(s)

How Serious Is Code P0421? – Moderate

This trouble code may not have any effect on the driveability of the vehicle, but if left unresolved for a long period of time can result in engine damage and driveability concerns. Also, you may notice a rotten egg smell from the exhaust. We recommend getting this fixed before it causes bigger concerns.

Code P0421 Common Diagnosis Mistakes

Replacing the catalytic converter without resolving an engine running concern which caused the catalytic converter to fail.

Tools Needed to Diagnose:

  • FIXD
  • Basic Hand Tools
  • Vehicle Specific Service Manual
  • Multimeter

How To Diagnose And Repair Code P0421?

Difficulty of Diagnosis and Repair (3 out of 5)

  1. Check to see if there are any other codes along with P0421 and clear your Check Engine Light with FIXD.
  2. Check the freeze frame data in order to pinpoint the issue.
  3. Conduct a visual inspection of wiring and connector at the O2 and/or A/F sensors for fraying or disconnection.
  4. Check for exhaust or intake leaks.
  5. Make sure the vehicle does not have a misfire. If it is misfiring or running poorly, resolve this concern first before a further diagnosis is conducted.
  6. If the vehicle is running normally, check the voltages of the Bank 1 O2 and/or A/F sensors and compare them to the vehicle specifications. If the Post-Catalyst O2 sensor has a rapidly changing voltage between .1V and 1V, then the Bank 1 Catalytic Converter has failed and is in need of replacement. 

Estimated Cost of Repair

For error code P0421, one or more of the below repairs may be needed to solve the underlying issue. For each possible repair, the estimated cost of repair includes the cost of the relevant parts and the cost of labor required to make the repair. 

  • Catalytic converter $400-$2400
  • Air fuel sensor or oxygen sensor $200-$300
  • Spark plugs $40-$640 (some cars require Intake manifold removal)
  • Ignition Coil(s) $230-$640 (some cars require Intake manifold removal)


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  1. There are some of the mechanics terms or language that I did not understand but in overall I did understand the issue and how to resolve it .
    Thank you .

  2. I appreciate your concern for my vehicle, so far you have treated me right, again I thank you

  3. I have already had the issues repaired. Cost was 530.00. to replace 2 sensor’s. Can definitely tell it’s fixed. Way more power, no hesitation any longer either. Ppl so funny when you tell them the issues that need repair. And ask how you are aware of it. Tell them I have Fixed. Oh well we’ll double check it to be sure that is the case. Mechanic do not like the general public having this kind of information. I love it. Thanks again for a great product

  4. How did you know my 06 jeep liberty was having this issue? Is it common at this point of life of the car?

  5. Yes I have all that thing your talking about, and l check manifold from the head where it’s attached to the head on 09 Chevy Malibu 2.4 and it has a crack where it leak bad l check 02 sensor that also had, and kind hard to it out, is there better away to it out.

  6. Helps grandson buy & sell used cars.He is a licensed dealer in FL.

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