how to fix P0421 code in Volkswagen Golf MK7 (2015-2019) catalyst issue?

Solution

1. Preparation

• Gather tools and parts required for the repair.
• Disconnect the negative battery terminal to prevent electrical shorts.
• Allow the engine to cool before beginning any work.

2. Inspect and Replace Oxygen Sensors

• Tools Required: O2 sensor socket, torque wrench, penetrating oil
• Remove the upstream oxygen sensor from the exhaust manifold.
  • Sub-steps:
    • Apply penetrating oil to the sensor threads if needed.
    • Use the O2 sensor socket to unscrew and remove the sensor.
• Install the new upstream oxygen sensor.
  • Sub-steps:
    • Torque to manufacturer's specifications (typically around 30 Nm).
• Repeat the process for the downstream oxygen sensor.

3. Inspect and Replace the Catalytic Converter

• Tools Required: Socket set, torque wrench, exhaust gasket kit
• Remove the exhaust pipe or components connected to the catalytic converter.
  • Sub-steps:
    • Loosen and remove bolts securing the exhaust components.
    • Support the exhaust system to prevent damage.
• Remove the catalytic converter.
  • Sub-steps:
    • Disconnect any oxygen sensors connected to the converter.
    • Remove mounting bolts and carefully detach the converter from the exhaust system.
• Install the new catalytic converter.
  • Sub-steps:
    • Ensure a new gasket is used to prevent exhaust leaks.
    • Torque bolts to manufacturer's specifications.
• Reinstall any exhaust components removed during the process.

4. Final Checks

• Reconnect the negative battery terminal.
• Start the engine and check for any leaks in the exhaust system.
• Clear the trouble codes using the OBD-II scanner.