2002 Toyota Prius Diagnosis Code help !?

suggested of answers previous questions prius 2002 176,000+ mileage, brought prius toyota dealer's service , have them check out. here 2 diagnosis codes came up:

po420 , po171

first 1 kind of problem actuator (part number 17230-21010), , 2nd 1 mass air flow (22204-21010). both of them makes check engine light appear constantly, , reasons make gas mileage become low (33 mpg on average). suggested replace both of them $600+ (!). when brought car back, check engine light came off ! don't know what's happening :|

, have problem ac - has cooling effect, of times doesn't. fan on not ac. , mechanics there's no codes those, , found out problem ac compressor (part number 88310-47030-84) , expansion valve (88515-47020). suggested replace both of - shocking cost.

wonder there alternative ways problems? haven't replaced car tires yet. btw, tire of size 175/65r14 s+m now, not xl. haven't asked dealer's mechanics check throttle body, spark plugs, or o2 sensor. should have local mechanics check things out ?

as suggested of answers previous questions prius 2002 176,000+ mileage, brought prius toyota dealer's service , have them check out. here 2 diagnosis codes came up: po420 , po171 first 1 kind of problem actuator...

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