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P0171 means that the pre-cat A/F sensor is sensing a lean condition...so lean that even with the ECU trimming to the maximum rich limit, it's still too lean.
The 5S-FE doesn't have a MAF, it's got a MAP (i.e., it uses manifold pressure to determine engine load, instead of measuring the airflow directly). The MAP is a pretty foolproof sensor, so it's not likely to be the problem.
Since you're in California, I'll assume you've got a California-spec emissions setup. Let me know if you think it's a Federal setup.
Any other codes other than the P0171? Usually best to troubleshoot other, more specific, codes before a general code like P0171.
Did you run out of gas right before the CEL came on? That can trigger a P0171.
Any exhaust mods from the exhaust manifold down to the pre-cat A/F sensor? Air leaks in that area can trigger a P0171.
Could be some clogging or restriction in the fuel system. If it's marginal, running some fuel system cleaner through might be enough to clear up the problem. Otherwise, you're looking at possibly troubleshooting the fuel filter or fuel pressure regulator.
Do you happen to know anyone with a good OBD scanner (not just a code reader). Getting the freeze frame data from when the P0171 occurred would help narrow things down.
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