Short on time? Here are the highlights:
- The Service Engine Soon light appears when a vehicle requires routine maintenance or has minor issues like a loose gas cap or bad oxygen sensor.
- In comparison, a Check Engine light comes on when there’s a more urgent problem concerning the engine, transmission, or exhaust system.
- The safest response to an SES light is promptly finding and addressing the cause. Driving 50 to 100 miles should be fine, but any more puts your car at risk.
- If the Service Engine Soon light stays on after fixing the cause, you can reset it by cycling the key on/off or following a manufacturer-specific process.
I’ll now start by explaining what the “Service Engine Soon” light means and what you should do.
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Service Engine Soon Light: Explained
A car’s “Service Engine Soon” (SES) light—not to be confused with the “Check Engine” light—is designed to alert drivers when regular maintenance is needed.
You may also see the SES light for minor issues involving the gas cap, ignition system, exhaust, etc.
What causes the Service Engine Soon light to come on?
The most common reasons include:
- Loose/broken gas cap
- Bad oxygen sensor
- Faulty Ignition Coil
- Mass airflow sensor failure
- Routine service needed
- Low fluid levels
- Bad spark plugs
- Catalytic converter issue
This guide provides more details on each of these causes. Moreover, we’ll also cover how to reset the Service Engine Soon light, something you can do yourself after addressing the cause.
Finally, we’ll answer some frequently asked questions about the Service Engine Soon light, including:
- Can I drive with the Service Engine Soon light on?
- How much does it cost to fix a Service Engine Soon light?
- Why is my Service Engine Soon light on after an oil change?
- Will a Service Engine Soon light reset itself?
- How serious is a Service Engine Soon light?
But first, let’s review the difference between the Service Engine Soon and Check Engine lights.
Service Engine Soon vs. Check Engine: What’s The Difference?
Typically, the “Check Engine” light appears when there’s an unexpected, more urgent problem with your vehicle’s engine, transmission, or exhaust system.
In comparison, a “Service Engine Soon” light is designed to illuminate when your car requires routine, regular maintenance. You may also see the Service Engine Soon light may appear if a sensor fails or detects an imbalance in the intake and emissions systems, among others.
Note: The role of the Service Engine Soon and Check Engine lights differ between automakers, as does what sets them off. The safest response to either light appearing (or any dashboard indicator) is promptly addressing the cause.
Common Reasons The Service Engine Soon Light Appears
1. Loose/Broken Gas Cap
Not only does your car’s gas cap prevent debris from entering the gas tank, but it also keeps vapors from escaping.
If the cap is loose or broken, the electronic control unit (ECU) will sense a loss of pressure in the tank, potentially prompting the SES light.
2. Bad Oxygen Sensor
Most cars have two or more oxygen sensors measuring the amount of unburnt oxygen at various points in the exhaust system.
A bad oxygen sensor won’t send data readings to the vehicle’s ECU. This can affect combustion intervals and engine timing and, possibly, illuminate the Service Engine Soon light.
3. Faulty Ignition Coil
Your car’s ignition coils boost the battery’s low electrical current until it’s powerful enough to ignite the engine’s air-fuel mixture.
Besides misfires, poor fuel mileage, and backfiring, a bad ignition coil may also display an SES light on your dash.
4. Mass Airflow Sensor Failure
A mass airflow (MAF) sensor measures how much air enters your vehicle’s engine. The ECU uses this data to determine how much fuel is needed for a balanced air-fuel ratio.
Other than prompting a Service Engine Soon light, you may also experience a rough idle, hesitation while accelerating, black exhaust smoke, and more if the MAF sensor fails.
5. Routine Service Needed
An SES light commonly appears when your car needs routine servicing. This doesn’t mean it shows every 90 days when you need a new air filter.
No, the SES light displays when a minor issue is detected, like decreased airflow from a clogged air filter or a loss of crankcase pressure from a bad PCV valve.
6. Low Fluid Levels
If your car is running low on motor oil, coolant, wiper fluid, or other fluids, it’s very likely the SES indicator lights up.
Fluid loss may just mean you need to add more, but there can also be a leak. Regardless, your car needs fluids to function, as even a lack of wiper fluid can lead to costly repairs.
7. Bad Spark Plugs
Spark plugs ignite your car engine’s air-fuel mixture, creating the power that turns the wheels. If one or more spark plugs fail, symptoms like misfires, poor gas mileage, or a rough idle will soon present.
Yes, the SES will also likely display, alerting you that it’s time to replace your car’s spark plugs.
8. Catalytic Converter Issue
You may also see the Service Engine Soon light display if there’s a problem with your catalytic converter, which helps reduce the toxicity of the fumes exiting the tailpipe.
A faulty catalytic converter may not be able to properly filter harmful compounds, leading to increased emissions, poor engine performance, a sulfuric smell, and more.
Other reasons for the SES light include contaminated gas (stick with Top-Tier gas for the best results), wiring issues, a clogged diesel particulate filter, and worn brake pads, among others.
No matter the cause, the best response is to address the issue before it worsens—the very reason for the SES light.
How To Fix A Service Engine Soon Light
The first step after having a Service Engine Soon light appear is to find the cause. Start by checking all major fluids, like the motor oil, coolant, etc., and the gas cap.
If fluids are low or the cap is loose, you may be able to resolve the SES light by adding more or tightening it. If nothing stands out, you’ll need to take your car to a repair shop or scan it using an OBD II tool.
After addressing the cause of the SES light, the indicator should disappear after cycling the key on/off 10 or more times.
To cycle the key, insert it into the ignition and turn it to the on position (don’t start the engine), wait two seconds, turn it off, and repeat.
The exact process for resetting the Service Engine Soon light depends on your vehicle’s make and model but can usually be found in the owner’s manual. Note that some models may need specific diagnostic tools only found at a repair shop.
If you really want to reset the SES light, disconnecting/reconnecting the battery “should” make it disappear. However, if the cause has not been resolved, the SES indicator will likely reappear shortly.
Service Engine Soon Light: FAQ
An SES light does not usually indicate an immediate problem, so in most cases, it’s safe to drive for 50 to 100 miles without harming your vehicle.
A flashing Service Engine Soon light is a different matter, in which case you should pull over when safe and turn the engine off.
Services you might pay for after seeing an SES light include an oil change ($40 to $100), a new air filter ($20 to $30), and a coolant flush ($100 to $150), to name a few.
Ultimately, following a routine service schedule should prevent the SES light from showing, something I estimate will cost around $600 to $700 annually.
Seeing a Service Engine Soon light after changing your car’s oil is common. The most probable reason is that the oil dipstick is not secured.
This, along with a loose oil cap, can not only lose oil but create a vacuum leak, prompting the SES light.
Yes, a Service Engine Soon light will reset itself, but only after the cause is resolved. Generally, an SES light will reset after the vehicle’s key has been cycled on/off 10 to 20 times.
If you reset the SES light without fixing what caused the issue, it will likely reappear.
A Service Engine Soon light is not usually serious, but it indicates something needs attention soon. Ignoring an SES light may cause it to escalate to something that is serious. Your best bet is to take the vehicle to a shop within 50 to 100 miles.
Service Engine Soon Light: A Good Reminder To Follow A Routine Maintenance Schedule
Following a routine maintenance schedule is the best way to improve your vehicle’s longevity and reduce the likelihood of the Service Engine Soon light appearing.
What’s involved in a routine service schedule? Learn more in our detailed guide.