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Q:

po code 174, 304, and 304 on 2000 Volvo S70

Rookie cbe0621eac06868b3efe0d8d1d3611e23c60d3114864ea2ec19a68cfbd3eebab
ii thinking...maf sensor, or vacuum leak?
(1) Answer
If it is a fault code for a lean condition and misfire I would recommend:
Smoke test the intake system for leaks, repair any leaks.
If fault returns it could possibly have a faulty Mass Air Flow Meter
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