Ford Expedition

Tonight when driving home from work my 1997 Expy Eddie Bauer with 5.4L check engine light came on.

I noticed no change in performance - no shuddering, hesitation, etc.

I went to Autozone and borrowed the code reader and the code pulled was P1131.

Where would you start on the list below?

Or should I just replace the HO2S-11 Oxygen sensor?

We just got the vehicle on Dec. 1st - 73K miles on it now.

This link shows:
http://v8sho.com/SHO/TSB0197HO2SServiceTips.htm

"P1131 - Lack of HO2S-11 Switch, Indicates Lean

When an HO2S sensor indicates lean at the end of a test, the system is trying to correct for an over-lean condition. The code is set when the fuel control system no longer detects switching for a calibrated amount of time.

See Possible Causes for DTC P1130"

Possible Causes:

Electrical:
· Short to VPWR or VREF in harness or HO2S
· HO2S circuit shorted to Ground
· Water in harness connector
· Open circuit
· Corrosion or poor mating terminals and wiring
· Damaged HO2S
· Damaged PCM (other DTCs should be present)
Fuel System:
· Excessive fuel pressure (stuck fuel pressure regulator, restricted fuel return lines, etc.)
· Leaking/contaminated fuel injectors or fuel pressure regulator
· Low fuel pressure or running out of fuel (fuel pump concern, fuel supply line restrictions, low fuel level, etc.)
· Vapor recovery system (stuck VMV, etc.)
Induction System:
· MAF contamination
· Air leaks between MAF and throttle plate
· PCV system / Other vacuum leaks
· Improperly seated engine oil dipstick
EGR System:
· Leaking gasket
· Stuck EGR valve / Leaking diaphragm or EVR
Base Engine:
· Oil overfill
· Incorrect cylinder compression
· Exhaust leaks before or near the HO2S
· Secondary air stuck on