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They’re solid vehicles. Prices seem to be subsiding. If you can wait a few more months, don’t buy now tax returns still in play. Economy will continue to stall and people will be spending on summer travel houses pools land scaping and cars are second fiddle.

Yea. It’s 100% benign for pre 2005 models. The ECU doesn’t use that data for fuel trim. It’s not indicative of anything but emissions. the biggest issue is you can’t tell when the code trips again. It’s routinely ignored in states that don’t check the check engine light for emissions or inspections for 100s of thousands of miles. Install an extender $10 on eBay and the code will go away. It’s just a spacer - remove rear O2 sensor, install spacer and reinstall O2 sensors into the spacer. It just makes the rear O2 sensor sit a little off the exhaust stream which is enough to trick 90% of them to not trip the code. repair usually includes replacing the converter. Aftermarkets sick and can cause the same issue in 1 month or 2 years. Subaru converters are $500+. So it’s not economical to repair if it’s not needed. it can often be caused also (conflating causes with an tires converter) due to an exhaust leak, fuel trim, vacuum leak, etc. but it’s nearly impossible to track down unless you’re a signals wizard. Most of the times the repair will be installing a new converter and making sure the engines working properly.

I get the P0420 on my 2002 EJ251 every few months, and then I delete the code. My car normally just does local runs. But once in a while, I go on the freeway at high speed; if it's a hot day, the CEL comes on and it's a P0420 code. So I delete it. Been doing this for years. The engine runs beautifully!

IF fuel trims and knock sensor and vacuum, and performance, etc. are all OK, you could ignore the code. Problem is, many locales would never allow the car to pass emissions/safety test, AND, the cel would be on all the time - masking any new problem detection. just eliminate every other possible cause before jumping to a new after-cat sensor or new cat. converter.

Hopefully Dad has an OBDII reader for her and has taught her how to use it. Easy to clear.