2000 S80 2.9 Is the connection where the exhaust and catalytic converter meet threaded on both the exhaust and the catalytic converter, or does the bolt just go through both and have a nut on the end? I'm replacing the catalytic converter on mine and the bolts are so rusty I cannot get them off. I was wondering if I could just cut them off (the bolts holding them together) and pull the catalytic converter away from the exhaust.

Rear Oxygen Sensor
A rear or downstream oxygen sensor is installed in the exhaust after the catalytic converter. It measures the amount of oxygen in the exhaust gases coming out of the catalytic converter.

It is probably where the exhaust manifold meets the tail pipe but it will be in front of the catalytic converter.

This code sets because the onboard computer senses that the exhaust quality before vs. after the catalytic converter do not meet design specs. It determines this via the upstream and downstream O2 sensors in the exhaust.
Possible causes are:
Poor engine performance.
Damaged/defective converter
Damaged/defective O2 Sensor(s)
Exhaust leaks

One before the catalytic converter screws directly into the exhaust pipe, the other is located after the catalytic converter it also screws directly into the exhaust pipe. Sometimes the one before the cat. converter is a pain to get out depending on exactly where it's located on the exhaust pipe. You can't miss it, they each have a wire coming out of them connecting to a connector to the vehicle wiring system. Its usually the saying if One goes out so will the other. Might as well replace them both, and use some Anti Seeze on the new ones so they aren't so hard to get out if you have to do it again later in years. Just make sure you don't get any of the anti seeze on anything else but the threads you screw into the exhaust pipes. Good luck.

If you have the V6 engine, then bank 1 is on the passenger side. Sensor 1 is between the engine and the catalytic converter, sensor 2 is after the cat.

Catalyst System Efficiency Below Threshold (Bank 1)

A code PO420 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 reading

• 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

There are 3 different engines available in that vehicle, and the location of the O2 sensors vary depending on which engine you have. The sensors are before and after the catalytic converter in the exhaust system. So the best way to find them is to find the catalytic converter and find the little plugs, which have wires on them, screwed into the exhaust pipe in front of and behind the converter. If you have the 2.7L V6 you will find that you have two converters which are bolted directly to the exhaust manifold and hence there are 4 sensors.

Catalytic Converter Removal & Installation 2.2L Engine To Remove:
Catalytic converter 2.2L engine

1. Raise and safely support the vehicle.
2. Disconnect the heated oxygen sensor (HO2S) electrical connector.
3. Remove the exhaust manifold pipe flange mounting nuts from the catalytic converter.
4. Remove the catalytic converter flange mounting nuts from the muffler.
5. Remove the catalytic converter from the muffler.
6. Remove the catalytic converter from the rear hanger bracket insulator.
7. Remove the catalytic converter.
8. Remove the (HO2S) from the catalytic converter.

To Install:

1. Install the (HO2S) to the catalytic converter.
2. Install a new gasket and the catalytic converter to the muffler.
3. Install catalytic converter to the catalytic converter hanger.
4. Install the catalytic converter flange mounting nuts to the muffler flange.
   • Tighten to 26 ft lbs (35 Nm).
5. Install the catalytic converter flange mounting nuts to the exhaust manifold pipe flange.
   • Tighten to 26 ft lbs (35 Nm).
6. Connect the heated oxygen sensor (HO2S) electrical connector.
7. Lower the vehicle.

------ Catalytic Converter Removal & Installation 4.3L Engine To Remove:

1. Raise and safely support the vehicle.
2. Raise and support the transmission.
3. Remove the transmission support
4. Disconnect the left and right pre-converter heated oxygen sensor (HO2S) electrical connector.
5. Disconnect the post-converter (HO2S) connector. Muffler flange and gasket
   
   
6. Remove the catalytic converter flange mounting nuts from the muffler.
7. Remove the catalytic converter from the muffler.
8. Remove the exhaust manifold flange mounting nuts from the catalytic converter. Catalytic converter 4.3L engine
   
   
9. Remove the catalytic converter from the support bracket insulator.
10. Remove the catalytic converter and gaskets.
11. Remove the heated oxygen sensor.

To Install:

1. Install the heated oxygen sensors as needed.
2. Install a new gasket and the Install the catalytic converter to the support bracket insulator.
3. Install the catalytic converter to the exhaust manifold flange. Install the mounting nuts.
   • Tighten to 39 ft lbs (53 Nm).
4. Using a new flange gasket, install the catalytic converter to the muffler. Install the mounting nuts.
   • Tighten to 30 ft lbs (40 Nm).
5. Connect the left and right pre-converter heated oxygen sensor (HO2S) electrical connectors.
6. Connect the post-converter (HO2S) electrical connector.
7. Install the transmission support.
8. Lower the transmission.
9. Lower the vehicle.

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It looks like you can cut them and replace them with new bolts and nuts.

there should be more than one 02 sensor so make sure to replace the correct one, but anyways they screw into the exhaust pipe usually near where the pipe meets the exhaust manifold

The code PO420 you have is Catalyst System efficiency is below threshold bank one. Usually replacing the O2 sensor that's the front most in the exhaust system will cure your problem,

Your Check Engine light is on and you find a P0420 code for a catalytic converter fault. Does that really mean your converter has reached the end of the road and needs to be replaced? A new converter can cost $600 to $1000 or more if a new car dealer or repair shop does the job, or maybe half that amount if you do it yourself. It's an expensive fix that may or may not be necessary. The problem, in most cases, is an emissions issue, not a performance issue that affects the way the engine runs.

A P0420 diagnostic trouble code is a "generic" fault code that is set when the Onboard Diagnostic II (OBD II) system sees a drop in converter efficiency. The OBD II system monitors catalyst efficiency by comparing the switching activity of the upstream and downstream oxygen sensors in the exhaust. The upstream O2 sensor in theexhaust manifold reflects the condition of the exhaust gases as they exit the engine. The downstream O2 sensor in or behind the catalytic converter reflects the condition of the exhaust after it passes through he converter.

The catalytic converter is like an after-burner. It oxidizes (burns) any residual fuel vapors (unburned hydrocarbons or HC) in the exhaust. It also burns any carbon monoxide (CO) in the exhaust. The exhaust must meet federal emission standards, and if a problem exists that causes emissions to exceed the federal limits by 150%, the OBD II system is supposed to catch the fault, set a code and turn on the Check Engine light.

Thanks