My car has been giving me a P0131 code for a lean mixture on the drivers side, but the code is only triggered at high rpm, est 4000rpm or higher. Most of the time it also reads P0151, lean on the passenger side. I can drive all day under 3000rpm and not see the code come up in over 300 miles. I replaced the drivers side front oxygen sensor with a new denso piece and the code still came back.

Would it be safe to say that the fuel pump is not putting out enough pressure under load? I went to check the pressure at the rail yesterday but the gauge was broken. I should have 58-60psi at all times correct? The fuel filter was replaced less than two weeks ago but otherwise the fuel system is original 98' equipment.

 

Did you ever figure this out? I am having the same two codes after intake, headers, ory, lid, ls6 intake, and catback + tune. I have bosch 13111 O2s. Anybody else out there have these codes? I've been searching but amstill looking.

 

Try swapping your mass air flow sensor with a known good piece off another car. I never had a trouble code for the MAF but it was the source of all of my problems. Cleaning it made no difference.