Hello all. Working on a 2004 3.4l Alero. A friend's car, so I don't know if it had had any problems with overheating or driveability prior to this incident. It overheated into the red zone, the temp would go down at highway speeds, then go back up at idle. Parked, checked the reservoir for fluid level, and the coolant looked horrible. Determined that the transmission cooler seal had failed, and replaced the radiator. After filling, flushing, filling, and bleeding, thermostat appears to be opening as expected as engine gets up to temp. However, only one cooling fan is working. Turning on the a/c causes only fan #1 to come on. I thought they should both turn with a/c on? Hooked the non-working fan directly to battery, and it ran fine. Pulled out coolant temp sensor & checked resistance. Reading went up in hot water, down in cold (crude form of diagnosis, I know, lol). All fuses and relays checked out fine. What next, and thanks in advance for any help.