P0420 and O2 sensor readings

[volvostud]

2010 XC70 T6 102k. I've been getting intermitant P0420 codes for years, going back to probably 70k miles. Always just cleared them and then a few months later they come back. Now they come back more often. Read my O2 sensor readings and get a consistent 1.0 on the upstream sensor and get a 0.14-.65 on the downstream (often holding in the .6-.63 range), and since clearing my code 40 miles ago, no code/CEL.

So how much of a difference should their be on the downstream? I've seen some reports for other cars but not Volvo.

Repair wise, I never thought that a Cat was low mileage replacement part but the local Volvo shop just said they go out at 100-120k and would be $2500 to replace. Nope.

[RickHaleParker]

2010 XC70 T6 102k. I've been getting intermitant P0420 codes for years, going back to probably 70k miles.

Have you checked for Air leak in the exhaust system? Perhaps something that opens and closes making the leak intermittent.
Intermittent electrical on one of the O2 sensors?
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ECM-P042000 Catalyst System Efficiency Below Threshold (Bank 1). General Failure Information. No sub type information

Diagnostic trouble code (DTC) information ECM-P042000

Condition
Engine Control Module (ECM) checks the efficiency of catalytic converter (bank 1) by switching between rich and lean fuel/air mixture. When the change-over is made, the signal from the rear heated oxygen sensor (HO2S) is checked and the time taken for the probe to switch.

The diagnostic trouble code (DTC) is stored if the engine control module (ECM) registers that the rear heated oxygen sensor (HO2S) switches too quickly and is interpreted as poor catalytic converter efficiency.

The diagnostic trouble code (DTC) can be diagnosed with the engine at operating temperature, approximately 15 minutes after the engine has first been started if the vehicle is driven (normal road load) at 70-90 km/h for 30-40 seconds.

Substitute value
Nothing.

Possible source
Air leak in the exhaust system.
Damaged front heated oxygen sensor (bank 1).
Damaged front catalytic converter (bank 1).

Fault symptoms
Malfunction indicator lamp (MIL) lit.

[volvostud]

I need to get it up on stands and roll under it to see what I can see/hear. The car otherwise drives flawlessly. I did just fill it up with 93 and I know there is some confirmation bias, but it feels like it idles smoother.

[RickHaleParker]

One can smoke test the exhaust system by injecting the smoke up the tail pipe.