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My knowledge on OBD2 computers is not as good as OBD1, but I think I can still shed a little light.
Yes there are only 2 O2 sensors on Foci just like the mechanic said.
First off, the basic CPU is probably the same for all vehicles from X manufacturer. This means that on models with fewer devices monitoring the engine, there will be ports open. Yes it is possible to put a computer designed for more O2 sensors in a car with fewer sensors- however, I doubt it would even start since there are apt to be many other issues than just the different number of O2 sensors.
Voltage low, voltage gone then reappearing: sounds like a connection issue. I'd have him check the bulkhead connector inside the driver's fenderwell. Ford did not put any connection grease on the pins to protect the connection from corrosion, and the connector is not completely weather proof. As far as I know nobody has ever had a problem with the bulkhead connector, so it's not for sure, but it does fit part of your issues. I'm assuming, of course, that your mechanic physically checked the wiring from the O2 sensors to the bulkhead connector. An O2 sensor creates it's own voltage of less than or equal to 1v through a chemical reaction to oxygen in the exhaust, and it doesn't take much resistance, corrosion, or loose connection to stop less than 1v.
I hope that helps, and please keep us posted if he figures it out.
Yes there are only 2 O2 sensors on Foci just like the mechanic said.
First off, the basic CPU is probably the same for all vehicles from X manufacturer. This means that on models with fewer devices monitoring the engine, there will be ports open. Yes it is possible to put a computer designed for more O2 sensors in a car with fewer sensors- however, I doubt it would even start since there are apt to be many other issues than just the different number of O2 sensors.
Voltage low, voltage gone then reappearing: sounds like a connection issue. I'd have him check the bulkhead connector inside the driver's fenderwell. Ford did not put any connection grease on the pins to protect the connection from corrosion, and the connector is not completely weather proof. As far as I know nobody has ever had a problem with the bulkhead connector, so it's not for sure, but it does fit part of your issues. I'm assuming, of course, that your mechanic physically checked the wiring from the O2 sensors to the bulkhead connector. An O2 sensor creates it's own voltage of less than or equal to 1v through a chemical reaction to oxygen in the exhaust, and it doesn't take much resistance, corrosion, or loose connection to stop less than 1v.
I hope that helps, and please keep us posted if he figures it out.