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

Solution

1. Preparation

• Ensure the vehicle is parked on a level surface and the engine is cool.
• Gather necessary tools and parts.
• Disconnect the negative battery terminal to prevent electrical shorts.

2. Inspect and Replace Oxygen Sensors

• Tools Required: O2 sensor socket, ratchet, torque wrench.
• Sub-steps:
  1. Locate the upstream and downstream oxygen sensors (usually near the exhaust manifold and after the catalytic converter).
  2. Use the O2 sensor socket to remove the upstream sensor first; ensure to note its position.
  3. Install the new upstream oxygen sensor, torquing it to the manufacturer's specifications (typically around 30 Nm).
  4. Repeat the process for the downstream oxygen sensor.

3. Inspect and Replace Catalytic Converter (if necessary)

• Tools Required: Socket set, jack stands, exhaust pipe cutter.
• Sub-steps:
  1. Raise the vehicle using a jack and secure it with jack stands.
  2. Remove any heat shields or brackets securing the catalytic converter.
  3. Cut the exhaust pipes as required, ensuring to leave enough length for the new catalytic converter.
  4. Install the new catalytic converter, ensuring proper alignment with exhaust flanges.
  5. Reattach any heat shields and secure all components.

4. Clear Codes and Test Drive

• Tools Required: OBD-II scanner.
• Sub-steps:
  1. Reconnect the negative battery terminal.
  2. Use the OBD-II scanner to clear any stored codes from the ECM.
  3. Start the engine and let it idle for a few minutes.
  4. Test drive the vehicle to ensure that the check engine light does not reappear.