You have a faulty ECT sensor. I have had that happen to me once when using an off brand ECT. I never had any of the other ones I have installed give me any problems but I now only install factory Volvo ones. When the ECT takes a crap the F.I. ECU will run the cooling fan at half speed continuously to prevent an overheat situation, it is part of the default programming for this fault. The 9186486 ECT shows that it is applicable to the 960, 850, and all of the S/V/C and XC70 models.
The one that I had came in with the typical ect code and green crap leaking out of the front of the sensor. I replaced the sensor only to have the light come on the next day. When looking at it the next day I could make the fan come on and off by wiggling the connection right at the sensor, an ohm meter showed that it was going open when wiggled, I replaced it with a factory one and all is good again.
Mark
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