I keep getting the ses light for low voltage on the upstream sensor. I replaced both sensors but the light came on during my recent trip to Las Vegas as soon as I started going up into the mountains.The book says its running too lean and goes into a default mode.

may be a bad fuel filter or bad fuel pressure?
Willwren should will chime in and get you better info.

what brand of o2 did ya use??????

Does the higher altitude have any affect? :?:

What is the exact code? What book are you referencing?

Is this book the Factory Service Manual? This is what Echo is asking I think.

The code I am getting is PO 131 O2 sensor circuit low voltage bank one sensor one. The new O2 sensors are AC Delco