Runs great except uphill!
P0420 (inefficient catalytic convertor) and later 0171 (too lean) and 0172 (too
rich)
Replaced downstream O2 Sensor - Solved P0420
Test drove for 100 miles and ran great sustained at 80MPH. Got to a
mountain pass and car lost power, would not go over 50 up the hill.
Returned to town and Check Engine Light turns on 1 mile from home. Only
code now is P0172 (too rich)
No Vacuum Leaks
Is upstream O2 next or is there other ideas first.