This is a very common problem. The two fans at the front should always run together, first at slow speeds with 6v and then if the temp stays high, they will switch to 12v and run fast.
Your non working fan has either a blown motor, or more likely, a bad relay.The other fan then compensates by always blowing on fast speed when the temp sensor actuates it.
The fans are turned on at approx 92 degrees for slow speed and 97 ish for high speed.The actual temps will be marked on the temp sensor which is screwed into the top right hand side of the rad (as viewed from the front of the car).
I had this exact problem and it turned out to be a corroded relay.On my 306 these were located in a triangular plastic box behind the front plastic grill in full view of road spray,rain,salt etc.No wonder they rusted! a bad bit of design really.
Something to get you started.
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