I have a 91 5.0 5spd mustang I got as a basket case, and I've just been putting it together as parts come my way. I've driven it a few weekends without the O2s hooked up, and it runs fine, kinda rich smelling, and the cel comes on after driving a bit, but no big deal.
So I'm digging through my Ford Hoard, and I find an O2 sensor harness. I cleaned it up, disconnected the battery and plug it in. The car starts right up, immediate cel, and wants to idle at 2500rpm. So I shut it down, loose the harness, now when I start it, it finds its idle but has the cel on.
I remember something from when I was making a 5.0 5spd Ranger that if you use an automatic PCM in a 5spd application, the 02s are wired different and it spells imminent doom for the PCM. I was checking diagrams last night, and no diagram makes any reference to a 5spd or automatic specific 02 wiring. Anyone else ever heard of this? I'm bringing my pre OBD connectors to check codes today