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A P0420 indicates the catalytic converter isn't working properly. The power-train control module (PCM) determines that the oxygen storage capacity of the three-way catalytic converter (TWC) is degraded past a calibrated value.
Anytime I have had this code, the converter needed replaced. Here is a link that shows in general how one is replaced:
Here are the directions specific to your vehicle. The attached pictures correlate with the directions.
2002 Chevy Truck TrailBlazer 4WD L6-4.2L VIN S
Vehicle Powertrain Management Emission Control Systems Catalytic Converter Service and Repair Procedures
Catalytic Converter Replacement
1. Raise the vehicle. Refer to Vehicle Lifting.
2. Remove the nuts that secure the converter pipe to the exhaust manifold.
3. Remove the nuts that secure the converter pipe to the muffler.
4. Slide the catalytic converter out of the exhaust insulators and out of the vehicle.
1. Install the converter hangars into the exhaust insulators in order to secure the pipe to the vehicle.
Notice: Refer to Fastener Notice in Service Precautions.
2. Install the converter pipe to the muffler and secure the pipe with the nuts.
Tighten the converter pipe to muffler nuts to 45 Nm (33 ft. lbs.).
3. Install the converter pipe to the exhaust manifold with the exhaust seal, and secure the pipe with the nuts.
Tighten the converter pipe to the exhaust manifold to 50 Nm (37 ft. lbs.).
4. Lower the vehicle.
Let me know if this helps.
Images (Click to enlarge)
Friday, October 4th, 2019 AT 8:46 PM