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P0421 – Meaning, Causes, Symptoms, & Fixes


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What Does Code P0421 Mean?

  • P0421 definition: Bank 1 Catalytic Converter Efficiency is Below Threshold on Cold Start
  • Issue Severity: MODERATE – Extended driving with this code may cause internal engine damage.
  • Repair Urgency: Get this code fixed as soon as possible.
  • Diagnosis: 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.

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.

P0421 Causes

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

P0421 Symptoms

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

*In many cases there may be no noticeable issues with your vehicle

How Do I Fix Code P0421?

With a P0421 code, the first step is to get it properly diagnosed to figure out what is causing the malfunction related to the catalytic converter. Using the live data function of the FIXD sensor and app allows you to read and analyze engine data to properly diagnose a P0421 code.

If the sensors are all reading correctly and you’re not comfortable further diagnosing this issue at home, we recommend finding a RepairPal-certified shop to pinpoint the problem and give an accurate estimate for repairs.

These shops can not only help you figure out what’s going wrong before you waste time and money on the wrong parts, but they also offer a minimum 12-month, 12,000-mile warranty and stand behind all their estimates with guaranteed fair pricing.

> Find a RepairPal Certified Shop Near You

How Much Does It Cost to Fix Code P0421?

If you take your car to a shop for diagnosis, most shops will start with an hour of “diag time” (the time spent in labor diagnosing your specific issue). Depending on the shop’s labor rate, this typically costs somewhere between $75-$150. Many, if not most, shops will apply this diagnosis fee to any required repairs if you have them perform the repairs for you. From there, a shop will be able to give you an accurate estimate for repairs to fix your P0421 code.

Possible Repair Costs for P0421

When it comes to making repairs associated with the P0421 code, 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)

DIY Steps to Diagnose Code P0421

If you’d like to try to fix code P0421 at home without throwing money at parts, you’ll want to follow the steps below for proper diagnosis. Complete diagnosis may require some specialized equipment beyond what the FIXD Sensor can provide, but for the most part, this diagnosis and repair can be attempted by beginner DIYers.

DIY difficulty level: Intermediate

This repair requires mechanical knowledge and is not recommended for beginners.

Tools/parts needed (our top picks from Amazon):


Scan your vehicle to verify P0421 is the only code present. 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. If other codes are present, they must be addressed first.


Conduct a visual inspection of wiring and connector at the O2 and/or A/F sensors for fraying or disconnection.


Inspect the exhaust system for any evidence of a leak, which is usually an accumulation of soot on the exhaust pipe.


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.


If at this point the vehicle is still setting the same code, you may have a more serious problem with your catalytic converter, and you should bring the vehicle to a certified shop to have further diagnostic work performed.

Common P0421 Diagnosis Mistakes

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

Still Need Help Fixing Code P0421?

If you’ve followed the steps above and are still experiencing issues with the catalytic converter system and code P0421, please contact the FIXD Mechanic Hotline if you’re a FIXD Premium subscriber or find a RepairPal certified shop near you to get the right repairs at a fair price.

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Fixed It But The Check Engine Light Is Still On?

Check engine light sometimes need to be reset manually, check out our article:

How to Reset Your Check Engine Light | 4 Ways To Clear It (With or Without a Scanner)

Jeffrey N. Ross

Lifelong automotive enthusiast with a soft spot for offroading. Wrencher turned writer, but I still love to tinker on just about anything with an engine. Dream car: tie between a ‘71 Hemi ‘Cuda and a ’91 GMC Syclone. #GirlDad #SaveTheManuals

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We’re here to help you simplify car care and save, so this post may contain affiliate links to help you do just that. If you click on a link and take action, we may earn a commission. However, the analysis and opinions expressed are our own.


About the Author

Jeffrey N. Ross

Jeffrey N. Ross

Lifelong automotive enthusiast with a soft spot for offroading. Wrencher turned writer, but I still love to tinker on just about anything with an engine. Dream car: tie between a ‘71 Hemi ‘Cuda and a '91 GMC Syclone. #GirlDad #SaveTheManuals

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