2000 outback losing power going uphill also gets louder , starts fine runs and revs smooth in park. Check engine light is on code 420 (cat) last time uphill check engine light was flashing
Anytime u have a lack of power uphill and a flashing check engine light its fuel related.. 99% of the time its lack of fuel(failing pump)
Lack of power up a hill could be an ignition miss or a plugged up catalytic converter.
Clogged converters, dirty mass airflow sensor, and severe ignition coil problems are causes but its more likely to be fuel pressure problems when dealing with uphill problems
There is a code indicating the cat.
P0420 Cat inefficiency.. computer doesnt like the oxygen storage of the cat... has nothing to do with it being blocked
Get a chrap scantool that has live data graphing and look at the oxygen sensor voltage, go for a test drive. During a heavy acceleration or uphill drive monitor the voltage and see what it is.. if it drops lean near 0 volts its a fuel issue