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It could also be a bad 02 sensor, a bad fuel injector, or a weak fuel pump. but like 90% of the time it's a vacuum leak. (literally just went through this on another vehicle of mine, replaced a vacuum line just this morning).
But being that it's on both banks? that leads me towards a vacuum leak.
I also did this on my truck when I bought it years ago, I bought silicone vacuum line in an off color (green) and went through and replaced all my vacuum system, so that way I could easily see what I had/had not replaced, my vacuum canister was cracked and leaking so I bypassed it with a check valve, but then my fuel pump was weak, and had to be replaced, and that's what cleared my codes. I had a P0171 and 174.
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1998 Mazda B4000
Mesa, AZ