how to fix P0421 code on Volkswagen Passat B8 (2015-2022)

• •Check engine light illuminated
• •Poor fuel economy
• •Decreased engine performance
• •Increased emissions
• •Unusual exhaust smells (e.g., rotten eggs)
• •Engine stalling or rough idling

1. 1Scan for error codes using an OBD-II scanner to confirm P0421 and check for additional codes.
2. 2Inspect the exhaust system for leaks or damage, particularly around the catalytic converter and oxygen sensors.
3. 3Check the operation of the upstream and downstream oxygen sensors with a multimeter or scan tool.
4. 4Measure the temperature difference between the inlet and outlet of the catalytic converter using an infrared thermometer.
5. 5Inspect the catalytic converter for physical damage or signs of clogging.
6. 6Review engine parameters (fuel trims, air-fuel ratio) for any signs of improper combustion.

After completing the repair, verify that the issue has been resolved:

• ✓Test the affected component to ensure it functions correctly
• ✓Check for any error codes or warning lights
• ✓Monitor the vehicle for a few days to ensure the issue does not recur

To prevent this issue from occurring again:

• •Follow the manufacturer's recommended maintenance schedule
• •Address warning signs early before they develop into major issues
• •Use quality parts and fluids recommended for your vehicle
• •Keep detailed records of all repairs and maintenance

• •OBD-II scanner
• •Socket set
• •Oxygen sensor socket
• •Infrared thermometer
• •Multimeter

• •Upstream oxygen sensor
• •Downstream oxygen sensor
• •Catalytic converter (if faulty)