Hi guys I have an Audi A6 and one of my fans is not working when I turn on the air conditioning the fan behind the radiator turns on but the fan on the side of the passenger doesn't. So when I'm in the city my car overheats a lil but when Im in the freeway it doesn't. (with A/C on)

I recently replaced the ECT sensor I really hope someone can help me with this problem cause its getting hot outside and I have a new born so I need the A/C

Thank you!