This is long but this a complicated problem. My pos won't go into closed loop. I noticed it when my gta started guzzling gas real bad. Well i hooked the autoxray up and it confirmed that the car was in staying in open loop (WELL after it was warmed up). It said the o2 sensor was not ready and that the bolck learn was disabled. The thing is a)the sensor is NEW and b)i hooked the autoxray up earlier and the same sensor was functioning fine. The o2 voltage was right about 400 mv according to the autoxray when it was stuck in open loop. So i pulled the sensor and tested the ground at the manifold and the o2 sensor contacts with a multimeter and both were fine. The sensor still looked new. Just to see if the ecm and/or wiring was okay i disconnected the sensor from the harness(with the ignition ON) while monitoring the 02 voltage with the autoxray and it still said 400 mv, when presumably it should have been 0 since i opened the circuit. Someone told me that the ecm sends a reference voltage through the wire to the o2 sensor so i tested it with the multimeter (i grounded it the wire) and it didn't read anything. So whats going on? The CTS is functioning properly. I'm really getting sick and tired of this ********, and getting 10-13 mpg isn't going to cut it(okay so i can't expect it to get honda-like economy, but it was 18-19 mpg in combined city/highway driving before)........

 

Not to Crack, but is the car in open loop mode you didnt mention that, check that little harness above the heater box

 

is it a delco o2 sensor? when at operating temp, hold rpms in park about 1500 to 2000 and try inducing a vacuum leak(p.brake vac.line) and watch the pulse width on the injectors-are they increasing? then check the other direction use carb spray or propane (if you cant be sure that you wont set your car or self on fire, skip the carb spray test)and enrich the intake air thru a vac.line and ck. p.width are they decreasing? if the ecm adjusts p.w. you have closed loop fuel control happening no matter what the scanner says.do you know the wet finger and battery trick for the o2 signal wire-try that too, if you dont try this first then ill tell that test to you. typing sux