"A downstream O2 sensor CANNOT cause a 0171 code."
Yes it can it did on mine.
He showed me the computer hooked up to it, the upstream was switching from "lean" to "rich" every second, the downstream was stuck on "lean". he said it should be switching because of the different fuel readings, he says no way should it be stuck on lean.
Who told you PO171 cant be the downstream O2?
