'91 Euro Cooling fan prob

My friend has a '91 Lumina Euro & the cooling fan doesn't come on. He has checked the fan motor, relay, & coolant temp sensor. If he checks for codes stored in the computer, using a jumper wire it comes on. I have a '98 cavy with a 2.0L engine that is doing the same thing. Nethier one of us has been able to figure this out. Could it be the computer it's self??

In the case of the lumina of the fan comes on in diagnostic mode then the ECM is fine. The relay and fuse can also be ruled out if it works in diagnostic mode. This leaves the coolant temp sensor. Does it ever overheat? I mean like spray antifreeze out of the over flow bottle?

He replaced the coolant temp sensor, made no difference. Since it started doing this he hasn't driven it enough for it to overheat(only drives short distances now). He says it does run hotter though.