2020 Altima 2.5L - P0420 code and rotten egg smell

Hi Jessica,

The P0420 code on your 2020 Nissan Altima, combined with the rotten egg smell and reduced fuel economy, strongly points towards an issue with your catalytic converter's efficiency. P0420 specifically means 'Catalyst System Efficiency Below Threshold (Bank 1)'.

Here's a breakdown of what's likely happening:

1. Catalytic Converter Failure:

   • Cause: The rotten egg smell (sulfur) is a classic symptom of a catalytic converter that is failing or has failed. It occurs when the converter can no longer efficiently process sulfur compounds in the exhaust.
   • Efficiency: The P0420 code indicates that the catalytic converter isn't cleaning the exhaust gases as effectively as it should. This is usually determined by comparing the readings from the oxygen sensors before and after the converter.

2. Other Potential Causes (Less Common with Rotten Egg Smell):

   • Oxygen Sensors: A faulty upstream or downstream oxygen sensor can sometimes trigger a P0420 code by providing inaccurate readings, but typically wouldn't cause the sulfur smell.
   • Exhaust Leaks: Leaks before the catalytic converter can affect O2 sensor readings, but again, usually no smell.
   • Engine Misfires/Rich Fuel Mixture: While not the direct cause of P0420, persistent misfires or an overly rich fuel mixture can overload and damage the catalytic converter over time, leading to its failure.

When to See a Shop:

• Soon: While not immediately critical, a failing catalytic converter can eventually restrict exhaust flow, leading to further performance issues and potentially damaging other engine components. It also means your vehicle is emitting more pollutants.
• What they'll do: A technician will first confirm the P0420 code and check the live data from your oxygen sensors to verify the catalytic converter's efficiency. They will also inspect for exhaust leaks and ensure there are no underlying engine issues (like misfires) that could have caused the converter to fail. In most cases with a P0420 and sulfur smell, the catalytic converter will need replacement. This is generally not a DIY repair due to the cost and complexity.