98 Camry V6 P0171 Code Keeps Popping Up

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This is the story. About a month ago, the valve cover gaskets went bad and were leaking oil all over the place. So I had it fixed at the honda dealership I bought the car at because it was under an extended warranty there, and I figured, hey, how bad can someone mess up valve cover gaskets even if they don't know the engine well. Anyways, ever since then, the check engine keeps going off. The first time they said it was the bank 1 sensor 2 o2 sensor behind the cat, and I let them replace it, even though it wasn't under warranty and I had to pay $350. It ran well for a week without the light, but sure enough, the light came back on again. So I took it back in, and they said another o2 sensor was bad. At this point, I didn't want to spend another $350 to replace it, and I pulled the code, and it was p0171.

Without looking into what the code meant, I bought a couple o2 sensors and replaced them myself. The code still showed up. So I looked around olnline, and found that maybe cleaning the electrodes on the MAF might clear up the problem, and why I was there, I also sprayed some wd40 around to check for vacuum leaks. I didn't find any vacuum leaks, and the check engine light came back on again. I haven't had a chance yet to check the code again, but I'm sure it's p0171 like it was all the other times.

Anyone have any ideas? Is it possible they messed something up when they replaced the valve cover gaskets?

Thanks in advance,

Matt