American and United are competing with low fares from Southwest and Frontier Airlines with fares down to $43.10 each way between San Francisco and Phoenix.
United has fares at $43.10 and $48.10 from September 1 through December 9, mostly Tuesday, Wednesday. and Saturday flights.
American Airlines has fewer dates with the last $43.10 fare on October 14 for PHX-SFO and last date for SFO-PHX on November 3.
These are nonstop flights.
American Airlines San Francisco SFO – Phoenix PHX
$43.10 one-way September 15
United Airlines Phoenix PHX – San Francisco SFO
$86.20 round trip Saturday September 1 – Wednesday September 16