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Like the title says ,

Have a 1999 Blazer , 4.3 , Black box PCM (16263494) , fighting an issue where none of the three O2 sensors shows a return ground , Pin 25, 26 and 7 are supposed to be the O2 sensors return ground , when I check the connectors on the sensors I have Power , ground for the heater and a signal back to the PCM , but no O2 ground return.

Hope that makes sense .

I checked the wiring to the sensors and the signal wires all make it to the PCM , (Pins 19,22 and 21) , and all of the return grounds make it back to the PCM (Pins 25,26 and 7) , so that ruled out a wiring problem to the sensors.

My question is , where does the PCM get the ground to supply to the three pins in question ? (25,26 and7) I can only find two PCM grounds at pin 17&18 , they are both good .

Anyone mess with the black box anymore that could provide some advice ?

If I jump grounds to pins 25,26 and 7 the O2 sensors will function normally . Swapped PCM's and same result .



Thanks

TOM