99 taurus code p1131

Lack of Upstream Heated Oxygen Sensor Switch - Sensor Indicates Lean - Bank No. 1
Does this mean I have a bad O2 sensor?

Welcome to the forum, fellow Iowan!

Not necessarily. The code means the sensor is indicating a lean condition on bank 1. This could be due to a restricted or bad fuel injector or a vacuum leak. It could also point to a crack in the bank 1 exhaust manifold or a bad O2 sensor. Other than the diagnostic code does the engine idle smoothly and run well?

-Rod

Except for the check engine light having come on, the car is runnig fine.I reset the fault code and my daughter drove it to and from work yesterday (50) miles. The car have 156k on it and I haven't replaced any of the O2 sensors. It has received regular servicing since we bought it new in fall of 98

Per the factory service manual, P1131 indicates the fuel/air ratio is correcting rich for an overly lean condition on bank 1. It certainly sounds like you have a vacuum leak on bank 1 more than a faulty O2 sensor. The factory service manual suggests the code is determined by measurements on the B1S1 oxygen sensor, but lists the following possible causes (notice that the O2 sensor is not listed as a possible cause):

Fuel system
-- Low fuel pressure.
-- Contaminated fuel injector(s).

Induction system
-- Air leaks after the MAF sensor assembly.
-- Vacuum leaks.
-- PCV system.
-- Improperly seated engine oil dipstick.

Base engine
-- Oil overfill.
-- Cam timing.
-- Cylinder compression.
-- Exhaust leaks before or near the HO2S(s).

-Rod