Hi: Like many people with these modern cars, I have fought a long and debilitating battle over permanent eradication of "restricted performance" fault arising from lean mixture on one bank or the other. A local shop pretty much had it licked when their smoke test revealed some bad vacuum hoses on both sides of the 4.2 V8. I replaced them and drove around happily for months with no faults until I went to California last week. In so doing, I dropped from my local 2500' altitude to within a couple hundred feet of sea level. And BAM my lean code comes back. It came and went with no rhyme or reason during my weeklong trip but upon returning home to Tucson, it has gone away. Has anyone experienced this and is there a technical scientific explanation for what I have observed? Does the engine management system know what the barometric pressure is? Is it supposed to adjust for it? Or are different regional fuel formulations causing this? Thank you.

 

It knows the different pressure (BARO) and compensates so I'm afraid your car has a fault.

Whether it's worth hunting down is up to you...