Pulled code p0420 on 99 chevy astro, 2wd, v6

From what i am finding the code means o2 sensors. they recommend replacing them and if this does not work than replacing the cat converter

The catalytic converter has an oxygen sensor in front and behind it. When the vehicle is warm and running in closed loop mode, the upstream oxygen sensor waveform reading should fluctuate. The downstream O2 sensor reading should be fairly steady. Typically the P0420 code triggers the Check Engine Light if the readings of the two sensors are similar. This is indicative of (among other things) a converter that is not working as efficiently as it should be (according to specs). It is part of the vehicle emissions system.

causes

A code P0420 may mean that one or more of the following has happened:

• Leaded fuel was used where unleaded was called for
• An oxygen sensor is not reading (functioning) properly
• The engine coolant temperature sensor is not working properly
• Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe
• Retarded spark timing
• The oxygen sensors in front and behind the converter are reporting too similar of readings

• Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.
• Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
• Inspect the downstream heated oxygen sensor (HO2), replace if necessary
• Replace the catalytic converter

While it is hook up to the scanner, scroll to the o2 output readings

compare bank one senor one to bank one sensor two reading to each other. If they are inversely porportional they are working If one stays pretty mush where it is at, it is bad. Same goes for bank two. One way for you to locate the bad sensor is to go to the one you think it is and unhook it and read the scanner. If you have o volts you unhooked the right one. This is how the armatures can know which of the four sensors is the right one.

There is usually a Cat inefficacy code if the cat is bad. but not always.

hope this helps.

Did they stick the probe up the tailpipe or just plug into the computer? With that many miles, it could be a catalytic converter, but if the o2 sensors have never been changed it could be them. If it is a sensor its probably the one behind the converter.