fast efi problem

How many wires on your O2 sensor? If 5-wire, it is a wideband type. Sounds like that since your computer has a linear reading for it. If 4-wire, it is a normal on/off type (or narrow-band). Either above will have a dedicated ground thru the wiring. If a 1-wire or 3-wire type, it grounds thru the exhaust pipe.

For more detail, two of the wires in the 3-wire type are for an internal electrical heater. The 4-wire type adds a dedicated ground for the signal. I think the 1-wire type were only in 1980's cars. A Megasquirt page explains all.