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Definitely sounds like a problem with the temperature sensor or the circuit to it or possibly the oxygen sensor not heating up. If the car does not see that it's warming up, it will stay in fast idle. If it can't see the engine warming up, it will stay in open loop mode which has a fast idle. If you wanted to throw parts at it, I would replace the coolant temperature sensor and possibly the o2 sensor. But better, it should be diagnosed first by connecting a monitor to it and data logging the sensors to see which one has failed or is slow to respond.
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