1. Locate the Data Link Connector (DLC). In a 1999 Acura RL, it is usually found under the dashboard on the driver's side.
2. Connect a scan tool or code reader to the DLC.
3. Turn on the ignition without starting the engine.
4. Using the scan tool or code reader, select the option to read or clear trouble codes.
5. The scan tool will display any trouble codes stored in the vehicle's computer system.
6. Select the option to clear the trouble codes.
7. Turn off the ignition and wait a few seconds.
8. Start the engine and check if the check engine light has turned off.
Note: If the check engine light comes back on after resetting, it indicates that there is an ongoing issue with the vehicle that needs to be addressed. It is recommended to have the vehicle inspected by a qualified mechanic or technician to diagnose and fix the underlying problem.