how to fix the P0421 code on my Volkswagen Golf MK7 (2015-2019)?

Solution

1. Preparation

• Tools Required: OBD-II scanner, multimeter, socket set, jack and jack stands, safety goggles, gloves
• Safety Warnings: Allow the engine to cool before working on the exhaust system. Disconnect the battery before performing electrical work.

2. Inspect and Replace Oxygen Sensors

• Sub-steps:
  1. Locate the upstream (pre-catalytic) and downstream (post-catalytic) oxygen sensors in the exhaust system.
  2. Disconnect the electrical connectors from both sensors.
  3. Use a socket wrench to remove the sensors from the exhaust pipe.
  4. Install new oxygen sensors and torque them to the manufacturer's specification (typically around 30 ft-lbs).
  5. Reconnect the electrical connectors to the oxygen sensors.

3. Inspect and Replace Catalytic Converter

• Sub-steps:
  1. Raise the vehicle securely using jack stands.
  2. Inspect the catalytic converter for any physical damage or excessive backpressure.
  3. If damaged, remove the bolts securing it to the exhaust system using a socket set.
  4. Install the new catalytic converter, ensuring proper alignment, and torque the bolts to the manufacturer's specification.
  5. Lower the vehicle.

4. Clear Codes and Test

• Sub-steps:
  1. Reconnect the battery.
  2. Use an OBD-II scanner to clear any stored trouble codes.
  3. Start the engine and allow it to reach operating temperature.
  4. Take the vehicle for a test drive to ensure no codes reappear.