My engine light is on, on my 2000 Subaru Legacy l.code are po420 and po171, They said to add gas cleaner but light still on. Any suggestion, much appreciated. Thank you:

Code p 0420 refers to cat converter system bank 1 --efficiency below threshold ===causes ---cat converter faulty---wiring --- heated oxygen sensor (HO2S)
I would be inclined to check the cat converter as the report is generated by the oxygen sensor to the ECM in that the desired results from a correct mixture are not with parameters
code p 0171 refers to system too lean bank 1===causes--intake /exhaust manifold leak---air system---fuel pressure / pump---MAF/VAF sensors---injectors--HO2S
the connection here could be a cracked flex section of the exhaust pipe before the cat converter
it would give both codes
bothe codes refer to the same bank
forget the fuel cleaner idea as a fix as it is not relevant to the codes