lean mixture

In-laws 97 Gr. Marquis Check engine light came on and codes read Lean mixture in both intake manifolds. Car has 47.000, never had fuel filter replaced. Garage thought it might be fuel pump going bad. Wasn't sure if it was one of those of the wife running the tank down to 1 or 2 gallons. Codes were reset and stayed off when keeping the tank full but came on again 1 night when the tank got dwon to 10 gallons. Any ideas?

have auto zone scan codes and check them with a repair manual or call dealer service and ask them what codes mean.