Where on the engine is it? Diagram

No, it's not a sensor. The engine computer controls idle speed by an "actuator" called an IAC -idle air control -motor or valve. It sits right on the throttle body where the air hose, the big boot, connects to the top of the engine. It controls the idle air passage-smaller air passages besides the larger throttle plate opening- according to computer command.
Probably your IAC motor or valve has a problem. A good cleaning often helps. Remove it and scrape off gently the deposits. Don't push the end piece, the pintle, back and forth, and don't spray cleaner up towards the little motor. Check the air passages where it mounts on the throttle body and clean as best you can. If it needed cleaning, take off the big boot and use a sensor safe cleaner, like throttle body cleaner, and old rags and old toothbrushes, and clean out the inside of the throttle body and the big butterfly valve, the throttle valve, inside. Note that when it is at rest, no gas pedal giving it gas, the throttle valve is almost completely closed. Those separate air passages in your idle control system, what the IAC motor controls, is the only way for the engine to get enough air to keep running. If they clog up, or the valve or motor doesn't work anymore, anyway the computer sees a wrong result from engine rpm sensors, and what it is commanding from the IAC motor, so it alerts you the problem is in the...in the idle cntrl system. Have at it, friend. It's either do it yourself or pay labor costs for a pro.

have you checked the idle control motor

This is often caused by a dirty throttle valve and Idle speed control motor, please do the following service to cure this issue.

Get a can of Throttle valve cleaner (it is made by CRC) from the local auto parts store, spray it into the air intake of the running engine until you have used about half the can, you will need to hold the idle up while you do this, it will try to stall, after this shut the engine down and disconnect the Negative (-) battery cable for 5 minutes. The intake is now clean, if you continue to have the idle speeds problems you may need to seek the help of a professional.

your problem may be a bad iac ( idle air control ) valve. also check or clean egr valve passage way to engine. or the egr itself is stuck open. ( exhaust gas recirculation ) valve.

the water pump is behind the plastic covers on the right side of engine requires removing the timing belt to replace water pump

check vacuum lines

If you recently replaced the battery, you may need to reset the idle air control valve setting. This is done by removing the NEGATIVE battery wire from the battery while the vehicle is HOT. Then you must wait several hours for the vehicle engine to become cold. Next, once could, reconnect the negative cable. Start the vehicle, place your foot on the brake and imediately put the vehicle in drive. Stay in that situation until the vehicle engine bcomes hot to normal temperature range. This will be known by hearing the radiator cooling fan turning on. You must NOT use the airconditioner during this cause it will be on during the event. Once accomlplished place in park and allow the engine to cool, engine off. This resets the engine computer to tell the idle/load speed control valve to add air and resuling in additional fuel to the engine under various torque loads.

you might need a new idle air control module, dont adjust your idle

The Idle Air Control (IAC) motor, attached to the side of the throttle body or check out auto zone.com manual it show the iac
http://www.autozone.com/autozone/repairinfo/repairguide/repairGuideContent.jsp?chapterTitle=Components+%26amp%3B+Systems&partName=Engine+Performance+%26+Emission+Controls&pageId=0900c1528021e78d&subChapterTitle=Idle+Air+Control+Motor&partId=0900c1528021e705

Does it knock while the engine is at idle?