There is no idle sensor on the 2000 Chevrolet Impala, according to my online resource.
But, there is the following that senses rpm of the engine and thus the idle speed when the Throttle Position Sensor is at 5% or the accelerator pedal is not depressed:
Crankshaft Position Sensor
Engine speed is a very important input to the Powertrain Control Module (PCM). Crankshaft speed and position are the basis for many calculations made by the computer. Crankshaft position values are transmitted to the computer by pickup coils also known as Permanent Magnet (P/M) generators, Hall Effect sensors or optical sensors. The Crankshaft Position Sensor (CKP) also known as engine speed sensor is typically located in proximity to the crankshaft.
In addition, the PCM uses the CKP sensor in conjunction with the camshaft position sensor to calculate and perform misfire diagnostics.
- A No Start / Intermittent Start condition - Can be caused by a faulty crankshaft position sensor due to loose connections, bad grounds, high resistance in the circuit, or opens in the circuit.
- Engine Misfire Diagnostic Trouble Codes
Engine Mechanical conditions can also create the same or similar symptoms. (Don't overlook basic mechanical integrity).