About a year ago my 90K 01 Catera started turning on the check engine light, code po171 and 74. Lean condition left and right bank. I started by checking for vacuum leaks, replacing vacuum lines, new spark plugs, news coil packs, new throttle body, by-passed the coolant that flows thru it, all to no avail. All you guys thinking bad mass air flow would be correct, but stay tuned. Even bought a new aftermarket MAF, from Japan for $69, wouldn't keep the CEL off. Sent it back for a refund and put the "bad" one back in. Kept driving it because it ran so good, but the CEL annoyed me no end. A new OEM Bosch MAF on ebay was $166 plus ship and I just couldn't justify it only to turn off a light. Eventually I found a used sensor only, no housing on ebay for $32, bought it just to see, swapped it out on sunday. So far it has not turned the light on, the old one would have by now, but it has a quite a bit less bottom end grunt below 40 mph than it had. My conclusion is the new one works correctly, the old one had bad low speed computer input causing the computer to try and compensate for a lean condition by lengthening the injector pulse...causing more fuel to be injected at low speed. I may get a little better gas mileage but i miss the off the line scoot it caused, may change it back....haha good luck, just wanted to share that little hot rod tip. The bad MAF would make it rev up to 1600-1700 rpm for a few seconds at start-up, a good one does not.

Update: Still no CEL after several days and now that the computer has gotten "used" to this sensor the bottom end grunt has improved. I prob should have unhooked the battery and let it learn fresh but dont like unhooking it as it makes the trans learn all over too.