Whats the difference between check engine light and service engine soon?
The check engine light indicates a fault detected by the engine’s computer, often tied to emissions; the service engine soon light signals a maintenance reminder or service due. Brand by brand, the exact icon and behavior can vary, so consult your owner’s manual or a qualified technician if you’re unsure. This article explains how to tell them apart, what each light typically means, and how to respond.
What they indicate
Check Engine Light (CEL)
The Check Engine Light, also known as the Malfunction Indicator Lamp (MIL), signals that the vehicle’s onboard computer has detected a fault somewhere in the engine, fuel, ignition, or emission control systems. A Diagnostic Trouble Code (DTC) is stored to help identify the problem. The issue can range from a loose gas cap to a misfire, a faulty oxygen sensor, or a failing catalytic converter. If the light is steady, it usually indicates a fault that should be diagnosed soon; if it is blinking, it typically means a severe misfire that can damage the catalytic converter and requires immediate attention.
Service Engine Soon (SES)
The Service Engine Soon light is a maintenance reminder indicating that routine service or a scheduled maintenance item is due. It is not always triggered by a current engine fault. Common triggers include oil changes, air filter replacements, tire rotations, fluid inspections, or other recommended maintenance tasks. The exact symbol can vary by manufacturer, often appearing as a wrench icon or a text message such as “Service” or “Maintenance Due.”
Key differences at a glance
Below is a concise comparison to help distinguish the two lights at a glance.
- Purpose: CEL signals a fault in the engine or emissions system; SES signals maintenance due or service recommended.
- Codes vs reminders: CEL typically stores Diagnostic Trouble Codes (DTCs); SES generally does not indicate an emitted-code fault.
- Common causes: CEL covers faults like misfires or sensor failures; SES covers scheduled maintenance items like oil changes or filter replacements.
- Appearance: CEL is usually an engine icon or the words “Check Engine”; SES often uses a wrench icon or a maintenance message, depending on the brand.
- Urgency: CEL generally requires diagnostic attention and repair; SES is a maintenance task that should be completed, but is not necessarily an immediate engine fault.
Understanding these distinctions helps you decide whether you should schedule immediate diagnostics or plan routine maintenance. In some vehicles, the lines blur and a single light may handle both roles, so it’s important to check the owner’s manual for your specific model.
Common triggers and maintenance implications
The following list outlines typical triggers for each light and what they imply for your car’s upkeep.
- Check Engine Light (CEL) triggers: Misfires, worn spark plugs, faulty oxygen sensors, mass airflow sensor issues, vacuum leaks, damaged catalytic converter, faulty fuel pressure regulator, or a loose gas cap. Any of these can illuminate the CEL and require diagnosis with an OBD-II scanner.
- Service Engine Soon (SES) triggers: Maintenance milestones reached or due, such as oil and filter changes, air filter replacement, brake inspection, tire rotation, fluid checks, or other routine inspections per the manufacturer’s schedule. SES signals are reminders, not direct engine faults.
Keep in mind that some vehicles may show maintenance reminders in addition to or instead of a traditional CEL, depending on the maker’s maintenance minder or reminder system. When in doubt, consult your owner’s manual or a trusted technician to verify what needs attention.
What to do when a light comes on
Steps to take when either warning light appears can help you avoid unnecessary risk or damage to the vehicle.
- Note whether the light is steady or flashing. A flashing CEL can indicate a severe misfire and may require immediate attention or towing; a steady CEL is a fault that should be diagnosed soon.
- For a CEL: use an OBD-II scanner to retrieve stored or pending DTCs, or visit a mechanic to read codes and assess the fault area (fuel, ignition, emissions, sensors, etc.).
- For an SES: review the owner’s manual for the maintenance schedule and confirm which tasks are due; schedule the indicated maintenance (oil change, filter replacement, tire rotation, inspections, etc.).
- Check quick items you can safely address: ensure the gas cap is tightened properly, check for obvious leaks or visual problems, and verify that the vehicle is running smoothly before driving long distances.
- If repairs are needed or the codes point to a critical subsystem, seek professional service promptly. If you notice the engine running rough, smelling unusual, or overheating, pull over safely and seek assistance.
After service, the SES reminder can reset if the maintenance items have been completed; the CEL may reset automatically after the fault is resolved and codes are cleared by a scanner or a technician. Different brands have different resetting behaviors, so follow the manual or technician instructions.
Brand variations and practical tips
Brand-specific nuances
There is no universal standard for icons or timing across all makes and models. Some brands use a wrench icon or a “Maintenance Due” message for SES, while others may show distinct CEL or MIL indicators. Some vehicles combine maintenance reminders with the check-engine light, while others clearly separate them. Always refer to the owner’s manual for your specific model to understand what each light means and the recommended actions.
Summary
The check engine light and the service engine soon light serve different roles in vehicle diagnostics and maintenance. The CEL points to a detected fault in the engine, emissions, or related systems and typically requires diagnostic work and repair. The SES serves as a maintenance reminder, signaling that scheduled service is due or recommended. Because iconography and rules vary by manufacturer, verify meanings in the manual and consult a qualified technician if lines are unclear or the lights behave unexpectedly. Staying current with maintenance and addressing faults promptly helps protect exhaust emissions, fuel efficiency, and overall reliability.
