Service Engine Soon Light

As a bulb and system check, SERVICE ENGINE SOON light will glow when ignition switch is turned to ON position and engine is not running. When engine is started, light should go out. If not, a malfunction has been detected in the computerized engine control system or SERVICE ENGINE SOON light circuit is faulty. To verify proper operation of SERVICE ENGINE SOON light on gasoline vehicles, see DIAGNOSTIC CIRCUIT CHECK in BASIC TESTING article. To verify proper operation of SERVICE ENGINE SOON light and retrieve trouble codes on diesel vehicles, see DIAGNOSTIC CIRCUIT CHECK chart in TESTS W/CODES for diesel article.