How to fix a code P0420

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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:

https://www.2carpros.com/articles/catalytic-converter-replacement

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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
Procedures
Vehicle Powertrain Management Emission Control Systems Catalytic Converter Service and Repair Procedures
PROCEDURES
Catalytic Converter Replacement

Removal Procedure

pic 1

1. Raise the vehicle. Refer to Vehicle Lifting.
2. Remove the nuts that secure the converter pipe to the exhaust manifold.

pic 2

3. Remove the nuts that secure the converter pipe to the muffler.

pic 3

4. Slide the catalytic converter out of the exhaust insulators and out of the vehicle.

Installation Procedure

pic 4

1. Install the converter hangars into the exhaust insulators in order to secure the pipe to the vehicle.

pic 5

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.).

pic 6

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.

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Let me know if this helps.

Take care,
Joe

As Joe said a P0420 is 99.9% of the time a bad converter. If you have a scan tool that can read live data you can watch the voltages of the O2 sensors at both ends of the converter. The voltage in the front O2 should switch rapidly while the one behind it should stay at a very even voltage. If the rear O2 tracks with the front one the converter is bad. Another method is to use an infrared thermometer to measure the front and rear of the converter.
A "dead" converter will show the same or very close temperatures. A working one will show at least a 100 degree difference front to rear, most will show more.

That depends on what emissions system your truck has, Federal or California. You might check with RockAuto. Com to get an idea of prices, then see if any of your local parts stores have similar items. The replacements are all bolt in items and are not difficult to change, and the directions above apply.