Toyota Repair: poor mpg, failing emission tests, emissions test, failure notice

Question
Hello Ted, Heres the situation, 1991 Toyota, Pickup 4wd 3.0L V-6 Manual Transmission, recently developed a rough idle, poor MPG; about 17 mpg's, and tipping the emissions scale at 157.00 CO's; minimum 20.00 CO. When diagnosed by a shop they determined that there was two dead holes; #2 and #6, and the truck needed a valve job at around $2500.00. Having done this repair on other V-6's I replaced the heads with rebuilts, due the cost. After the repairs were done I retested emissions at 20.43 CO, failing again. A couple of shops said that I need to drive it in order to "break" in the valves. Retest # 2 came in at 67.00 CO's, every one suggested a plugged cat converter, after this repair I tipped the scale at a 20.06, failing again. This time I wondered how accurate the test station was and took the truck to a more reliable State run emissions test station. This time 97.00 CO's and a failure notice again. Their suggestion was to check the Oxygen Sensor because there was no fuel control under a load, such as climbing a hill. The self diagnosis has revealed no codes, the timing is perfect, and advances when the jumper is removed. I have run some pinpoint tests on the O2 sensor and when cold the sensor has a resistance of 4.7 ohms, spec says 5.1 to 6.3 ohms. Test # 2 Inspection of feedback voltage; according to this test an analog meter should fluctuate 8 times or more. In my case none, it will sits still at about 5 volts, no fluctuation. Thus failing the second pinpoint test. I bought a new, after-market, O2 sensor and the resistance on this one is also out of spec, 3.9 ohms.  The problem I have is that I don't want to diagnosis this problem with a check book and I don't want to replace something that isn't broken. Sorry for the book but I need some kind of direction to take, next to spending a large amount of money on a repair shop. Thank you for your time.

Answer
The 5 volts you are reading is the reference voltage supplied to the O2 sensor, you are correct the voltage should fluctuate above and below the reference voltage, it is not going into closed loop operation in which case the o2 sensor sends a signal to the computer related to oxygen content of the exhaust gas, the sensor has to reach a certain temperature in order for closed loop operation to start, the sensor has a heater circuit to heat it up quickly after cold start and then the exhaust temp will take over, I'm not sure what is going on, maybe you should try a sensor from the toyota dealer, also make sure the engine reaches normal operating temperature, if it doesn't the computer will not go into closed loop.