The 2001 Mercury Grand Marquis uses a power train control module to monitor and control the engine and transmission. When the module detects a fault in any of the sensors on the power train, it generates a diagnostic trouble code. Once a code is stored, the "Check Engine" light turns on to advise you of the fault. After retrieving the codes, troubleshooting and repairing the fault, you can reset the light. If you understand the fundamentals of automotive repair, you can accomplish this task in about 20 minutes.
Sit in the Grand Marquis's driver seat with the scan tool. Plug the scan tool's datalink cable into the Mercury's diagnostic port. The multi-pin port is a black, rectangular connection located under the driver's side of the dashboard; it resembles a computer monitor port.2
Turn the Grand Marquis's ignition cylinder to the "Run" position. Follow the operating instructions for your scan tool to run a quick test on the power train control module. With most scan tools, this consists of following a series of prompts on the tool's display screen, which you navigate with the interface buttons on the tool.3
Erase the diagnostic trouble codes when prompted to do so by your scan tool. Most scan tools will give you a prompt that reads something like "Clear codes: Yes or No," and you can select "Yes" with the interface buttons.4
Complete the quick test per your scan tool's instructions. Disconnect the scan tool's datalink cable from the Grand Marquis's diagnostic port by hand. Turn the Mercury off.