how to fix P0420 code on Ford Mustang S550 (2015-2023)

Solution

1. Preparation

• Gather tools and parts needed for repair.
• Disconnect the vehicle's battery to prevent electrical shorts.
• Allow the engine to cool down before starting work on the exhaust system.

2. Inspect and Replace Oxygen Sensors

• Tools Required: O2 sensor socket, ratchet, torque wrench
• Remove the upstream (pre-catalytic) and downstream (post-catalytic) oxygen sensors.
• Inspect the sensors for contamination or damage.
• If they are faulty, replace with new sensors, ensuring proper torque (typically around 30 lb-ft).

3. Check and Replace Catalytic Converter

• Tools Required: Socket set, jack stands, exhaust gasket, torque wrench
• Lift the vehicle using a jack and secure it with jack stands.
• Remove any heat shields and bolts securing the catalytic converter.
• Disconnect the exhaust pipes from the catalytic converter.
• Inspect the converter for blockages or internal damage; replace if necessary.
• Install the new catalytic converter with a new gasket, tightening bolts to the manufacturer’s specifications (typically around 40 lb-ft).

4. Clear the Codes and Test

• Reconnect the battery and use an OBD-II scanner to clear the codes.
• Start the engine and allow it to reach operating temperature.
• Take the vehicle for a test drive to ensure the check engine light does not reappear.