Problem with temp guage on '99 cavalier : attn Ian

My mother has been having a problem with temp guage on ' 99 cavalier with 2.2 4 cyl. When the motor got up to running temp and you would go 60 mph or
faster the temp guage would start bouncing from normal temp to cold. Today my father and I changed the coolant temp sensor behind the alt. Cranked the car and temp guage went to middle of gauge. When the fan came on the needle went cold and the fan cut off in a couple of minutes and the needle went back to normal. The fan came back on and needle went back to cold and stayed and the fan never cut off. We opend the bleeder screw with the heater on air came out and some anti-freeze and the needle never came off of cold. Does anyone else have anymore ideas or have had the same symptoms. thanks
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Several things to look at. Is the sensor you got the right one for the car? Put the old sensor back and see if it acts the same as it did or like the new one. Check the new sensor with a meter on the stove. Check wiring closely as connectors could be corroded and wire shorted/broke/chewed by rats.

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