96 mustang coupe with 3.8, Error code P1151

I have a 96 mustang with 3.8.. After topping off fuel tank, I got check engine light which had error code P1151 "manufacturer control fuel / air mixture..
I confirmed gas cap was tight, cleared error code with scanner. Same error code keeps coming back. I've checked for dipstick tight, oil cap tight, checked for vacuum leak using brake cleaner. EGR is operational when direct vacuum supplied to diaphragm. I've yet to get on a lift to check ohms of O2 wiring and sensor. When checking I/M sensors key on engine running at operating temps, everything shows "ready" except catalyst.... this is after a 15 minute interstate drive. Car runs great...(I've reset 3 times two errors each, in the past 60 miles always same code) Any educated ideas? The list I get from google is long from this code.. about all sensors, egr valve, egr valve control computer, vacuum leak downstream from mass air flow sensor etc.. I've also got to check my Hayns manual, it's all I have besides here and google..
Here's the list from internet.. http://www.obd-codes.com/trouble_codes/ford/p1151.php
All that follows is what's on that page..
P1151 OBD Trouble Code Technical DescriptionLack of HO2S-21 Switch, Sensor Indicates Lean
What does that mean?A Heated Exhaust Oxygen (HO2S) sensor indicating lean at the end of a test is trying to correct for an over-rich condition. The test fails when the fuel control system no longer detects switching for a calibrated amount of time.
SymptomsYou will likely not notice any drivability problems.
CausesA code P1151 may mean that one or more of the following has happened:
Electrical: Short to VPWR in the harness or HO2S Water in the harness connector Open/shorted HO2S circuit Corrosion or poor mating terminals and wiring Damaged HO2S Damaged PCM Fuel System: Excessive fuel pressure Leaking/contaminated fuel injectors Leaking fuel pressure regulator Low fuel pressure or running out of fuel Vapor recovery system Induction System: Air leaks after the MAF Vacuum Leaks PCV system: Improperly seated engine oil dipstick EGR System: Leaking gasket Stuck EGR valve Leaking diaphragm or EVR Base Engine: Oil overfill Cam Timing Cylinder compression Exhaust leaks before or near the HO2S(s) Possible SolutionsSee the above list of causes and go from there. Sources: MotorCraftService.com
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In checking the "I/M" status on various sensor components, I find that the Catalyst is "not ready" almost continuously since start of this problem. Sometimes it shows the 02 and HO2 "not ready". Is there a temperature sensor or some other sensor inside the catalytic converter? I've now used about 3/4 of "that" tank of gas, and plan on topping off at another station shortly and getting under car to check wiring to 02 sensor prior to going to dealer... I do notice that the converter isn't getting as hot as "normal". I normally feel floor getting warm on long trips. Part of the troubleshooting for code "P1151" makes reference to running rich, trying to make up for a perceived "lean" condition... Chuck
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