how to fix P0420 code on GMC Sierra T1XX (2019-Present)

• •Check engine light illuminated
• •Decreased fuel efficiency
• •Engine stalling or rough idling
• •Increased emissions
• •Unusual exhaust odors (sulfur smell)

1. 1Scan the vehicle's OBD-II system for error codes to confirm P0420 and check for additional related codes.
2. 2Inspect the exhaust system for any visible leaks or damage, particularly before and after the catalytic converter.
3. 3Measure the voltage output of the upstream and downstream oxygen sensors using a multimeter while the engine is running.
4. 4Check the condition of the catalytic converter by inspecting for any signs of physical damage or overheating.
5. 5Verify fuel trims to ensure the engine is running within the proper air-fuel ratio.
6. 6Test the fuel pressure to confirm the fuel system is functioning correctly.

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
• •Multimeter
• •Socket set
• •Torque wrench
• •O2 sensor wrench

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