Warning light
Check engine light on with no codes
Start by confirming the warning symbol and which systems your reader actually scanned.
Identify the exact lamp
Find the symbol or message in the owner’s manual. A maintenance reminder, transmission warning or stability-control lamp can require a different procedure from an emissions scan.
Check communication and scan coverage
If the tool says it cannot connect, record the error. If it connects, check whether it reports that the MIL is commanded on and what code states the current menu reads. A basic reader may only cover emissions-related data.
Bring the clues to a better scan
Record the vehicle and engine, warning behavior and the scan tool’s exact message. An all-module scan can help when the original reader could not reach the relevant module. Treat any flashing light or severe drivability symptom as urgent.