Many issues prevent a car from idling, but in a vehicle with an onboard computer, the issue is often in the computer. A diagnostic scan tool provides error codes to determine the issue. On vehicles with carburetors, the issue is often with the accelerator pump.
All cars manufactured as of 2015 contain at least one onboard computer. The computer monitors and adjusts the engine constantly to ensure it runs at top efficiency. However, if the computer receives bad information from one of the sensors, it cannot make the proper adjustment. Most of the time, this causes the check engine light to illuminate. When the light illuminates, the computer stores a diagnostic code that is useful in understanding the issue. The mechanic attaches the tool to the port to retrieve the code and compares it with a chart for diagnosis of the issue. In some instances, it may be necessary to run further diagnostic tests to determine the idle problem.
While most manufacturers currently use fuel injection to deliver fuel to the cylinders, older vehicles use a carburetor to mix fuel with air. The carburetor requires an accelerator pump to increase the fuel it feeds to the engine and increase speed. Wear causes failure of this pump or electrical circuitry in the carburetor. To restore the idle, the carburetor requires repair or replacement.