Computer Control Module

Greetings. My car is a 92 DOHC MultiPort Fuel Injection, Automatic with 200,000 KMs. Until recently this car was extremely reliable and very fuel efficient. Burned about a Litre of Oil every 1,000 KMs. Then the Trouble started. First, the Engine began to Overheat. The Cooling Fan would not start. I replaced the Coolant Temp Sensor unit. The Cooling Fan still does not start. Had to run a separate power line to the Cooling Fan with a manual switch in order to at least be able to drive the car. This is inconvenient, but not a show stopper as far as I care. But it gets worse. The Car has begun to run EXTREMELY rich. Fuel consumption is unbearable. Black Smoke in great volumes. The Service Engine Soon light used to come on sometimes when the engine used to over heat, but it has not come on at all recently. I tried reading the Computer Trouble Codes with no luck. There are no codes stored in the CCM. All I get is the Code 12, Diagnostic Mode Enabled. Nothing else. The car hesitates under acceleration, wants to stall when leaving a red light, and will over heat if I don't use the manual Cooling Fan switch I installed. Service Engine Soon light does not come on when Overheating. I Checked the EGR Valve, it does not stick, and holds a vacuum well. The PVC Valve was changed. The Injectors are all clicking smoothly without any interruptionos at any RPM. This has been a great car so far, in spite of the High Mileage. It was well taken care of obviously, and has been very good until only very recently. What can happen to mess up a car so badly so quickly. What could the problem be? Can the Computer Control Module itself be to blame? How can you check that item for reliability? Any comments would be greatly appreciated. Thank You.