Some OBD II readers show the exact measurements of each of the sensors. This should tell you which sensor is out of limits.
my $50 dollar one from Harbor Freight does this. You would need to get the sensor parameters for your exact year.
OBD II testerIt could be the idle control valve. Not sure if 8.1 has one. It acts like a choke for Fuel injection.
Bud
USAF Retired
Pace Arrow
2003 Chev Ice Road Tracker