I have a higher mileage 250,000 km (155,000 mile) Crosstrek.  Owned for one year.  Intermittent codes come that suspends Cruise control.  At a mechanic shop, they found P0420 and said post converter 02 sensor is crossing over .45 volts steady.  ( they say should be at .45)  they said no external damage but temp equal across converter therefore internal converter damage.  They also suspect cat 02 sensor is seized.  
They recommend replacing convertor.  $2000 Can.   ($1500 US).  Anyone with insights if this is coherant.   
any other options?  I called local Subaru dealership. They said I could try Cat cleaner (Sea Foam). Independent  Mechanic said this will gum up Cat Converter and cause it to fail sooner.   
any advice?

Simply put: 
1. you can ignore it without issues if the engine is asymptomatic. 
2. If you absolutely need the cruise control or to pass inspection, I would do "tiny converter" O2 sensor insert (not the spacer kind but the "tiny converter" kind.  This is for example only I'm not recommending this one or this company but I would try it: 
https://www.racdyn-usa.com/PROD/130+10+10+012.html?gclid=EAIaIQobChMIgZiku6DQ6wIVRr7ACh251gpFEAQYBiABEgIrdvD_BwE

3. A shop around rust areas should have torches available for extracting the sensor.  They do get REALLY bad though with rust so it could be a lost cause.  Can't tell without seeing it.

4. Subaru converter and be done with it

5. Get another car....might be a reasonable optionif rust is starting to eat away the exhaust and rear quarters. 

6. Aftermarket exhaust - local shops can cut and weld in a conveter for $200 or less.  But it'll be low grade and might not work or could fail in a year. 

More details.  P0420 is mechanically *benign*.  You can drive 100,000 more miles all day long without issues.  This doesn't mean to ignore other issues. If you have any *other* existing issues like bad gas mileage, hesitation, cylinder misfires - those need fixed or addressed.  If the vehicle has no other issues it'll run fine with the code and causes no long term issues. 

This can often be fixed without replacing the converter but it takes someone who's really knowledgeable to track it down without egregious guesswork.   That person is not me nor does anyone who frequently posts on any subaru forum online typically have the time to walk someone through it.  If there was I'd love to learn more effectively how to diagnose it. 

1. Make sure you have all recalls taken care of...if any. 

2:  If you plan on keeping the car for awhile - consider doing routine maintenance on it now:  replace the spark plugs, change the air filter, make sure there are absolutely no: 
a. vacuum leaks
b. exhaust leaks
c. check engine codes (other than the P0420). 

 

Thanks.   I have driven it over this past year, with no other problems.  It is simply annoying.  Thanks for insights and explanations.