Car care guide · Ford of Peoria · Peoria, IL
Check Engine Light: What It Means and What to Do
The check engine light, also called the malfunction indicator lamp, is how your vehicle's on-board computer tells you it has found a problem. It may be as small as a loose gas cap or something that needs prompt repair. This guide explains a steady light versus a flashing one, what to do next, and how the light affects the Illinois emissions test, using guidance from state emissions programs and fueleconomy.gov.
What should I do when the check engine light comes on?
Notice whether it is steady or flashing. A steady light means the on-board diagnostics system found a problem and service is needed soon, though you do not have to stop immediately. A flashing light means a severe engine misfire: reduce speed and get service as soon as possible. Check that the gas cap is tight, and have a technician read the stored trouble codes.
What the check engine light means
Federal regulations set requirements for on-board diagnostics (OBD) systems. The OBD system watches emissions-related parts and systems, and when it detects a fault that may cause high engine or evaporative emissions, it stores a diagnostic trouble code and turns on the malfunction indicator lamp: the check engine light. Depending on the vehicle, it reads "Check Engine" or "Service Engine Soon," or shows an engine symbol, and it is usually yellow.
The Illinois Air Team says this light is reserved for emissions problems and that other conditions use other warning lamps. The OBD system also monitors powertrain parts that affect emissions; the Illinois Air Team says that if the light is on for a transmission code, that code is emissions related.
A vehicle can seem to drive normally while the light is on. Fueleconomy.gov notes that modern vehicles may seem fine even when a repair is needed, which sometimes leads drivers to ignore the light, and that the cause could reduce fuel economy, increase emissions or lead to costly repairs later.
From the Illinois Air Team (Illinois EPA vehicle emissions program) FAQ, Oregon Department of Environmental Quality OBD page and fueleconomy.gov (U.S. DOE/EPA) Keeping Your Vehicle in Shape page, read November 9, 2026.
Steady vs. flashing check engine light
How the light behaves tells you how urgent the problem is.
| Light | What it means | What to do |
|---|---|---|
| Steady (stays on) | The OBD system detected a problem and stored a trouble code; service is needed. | You do not need to stop immediately, but get service as soon as possible. |
| Flashing or blinking | A rather severe engine misfire that can quickly damage emissions parts, especially the catalytic converter. | Reduce speed and get service as soon as possible. Oregon DEQ says driving is discouraged until it is repaired. |
| On, then off by itself | The OBD system rechecked the problem and no longer found it. | This is the OBD system working as designed, for example after a loose gas cap is tightened. |
From the Illinois Air Team (Illinois EPA vehicle emissions program) FAQ and Oregon Department of Environmental Quality OBD page, read November 9, 2026. Check your owner's manual for information specific to your vehicle.
What to do when the check engine light comes on
These steps follow guidance from the Illinois Air Team, Oregon DEQ and fueleconomy.gov.
1. Steady or flashing?
If the light is flashing, reduce speed and get service as soon as possible: severe misfire can damage the catalytic converter in a short time.
2. Check the gas cap
If you just refueled, make sure the cap is tight. A loose cap lets fuel vapor escape and can turn on the light; once it is tightened, the light should go off within a few days.
3. Read the owner's manual
Both the Illinois Air Team and Oregon DEQ point drivers to the owner's manual for manufacturer-specific information about the light.
4. Get the codes read
A service technician can quickly retrieve the stored diagnostic trouble codes from the vehicle's computer. The codes point to the component or system that needs diagnosis and repair.
5. Do not just clear it
Turning the light off manually without fixing the cause resets the readiness monitors; if the problem is still there, the light comes back on.
6. Ask about warranty
Federal law requires warranty coverage for emissions control systems, and many automakers extend it, so the repair may be covered by warranty.
From the Illinois Air Team (Illinois EPA vehicle emissions program) FAQ, Oregon Department of Environmental Quality OBD page and fueleconomy.gov (U.S. DOE/EPA), read November 9, 2026.
When does the check engine light turn off?
The OBD system is designed to turn the light off by itself when the problem is gone. The Illinois Air Team says that if the system evaluates a component or system three consecutive times (with some exceptions) and no longer detects the problem, the light turns off automatically, so drivers may see it come on and then go off.
The loose gas cap is the classic example: the OBD system detects the vapor leak, turns the light on, and once the cap is tightened the light should go off within a few days. That is the system working as designed, not a fault.
A technician can also turn the light off manually. But if that is done without correcting the cause, the OBD system resets its readiness monitors and starts its checks over, and if it detects the problem again the light comes back on.
From the Illinois Air Team (Illinois EPA vehicle emissions program) FAQ, read November 9, 2026.
The check engine light and the Illinois emissions test
Illinois EPA runs vehicle emissions inspections for the Chicago and Metro-East St. Louis areas. If your vehicle is registered where testing applies, the test for most 1996 and newer vehicles is an on-board diagnostics (OBD) check: a scan tool is connected to the vehicle's computer to read the OBD system status (readiness checks), whether the check engine light is commanded on, and any stored trouble codes.
The Illinois Air Team says a vehicle fails the OBD test if the check engine light is commanded on and trouble codes are present. It also fails if the OBD system is inoperative or the diagnostic connector is damaged, inaccessible or missing.
Turning the light off with a scan tool right before the test does not work. If the light is cleared without fixing the problem, the OBD system resets its readiness monitors, and a vehicle whose monitors have not finished their checks is rejected as "not ready." If the system finishes its checks and finds the problem again, the light comes back on and the vehicle fails. See our Illinois emissions testing guide for who is tested and how often.
From the Illinois Air Team About Vehicle Emissions Testing and FAQ pages and the Illinois EPA Vehicle Emissions Testing page, read November 9, 2026.
Check engine light service at Ford of Peoria
If your check engine light is on, schedule a visit with the Ford of Peoria service team and mention whether the light is steady or flashing. The Works® at Ford of Peoria includes a synthetic blend oil change, tire rotation and pressure check, brake inspection, Multi-Point Inspection, fluid top-off, battery test, and a filter, belts and hoses check.
Schedule service online or check our service specials. Ford of Peoria is at 2211 W. Pioneer Pkwy. in Peoria. Get directions.
Ford of Peoria service hours
| Days | Hours |
|---|---|
| Mon-Fri | 7 a.m.-5:30 p.m. |
| Sat | 8 a.m.-noon |
| Sun | closed |
From Ford of Peoria's website, November 9, 2026. Holiday hours can differ.
Check engine light FAQ
What does the check engine light mean?
It means the vehicle's on-board diagnostics (OBD) system has found a problem that may raise emissions and stored a diagnostic trouble code. The Illinois Air Team says the light shows that a problem has been detected and vehicle service is needed.
Can I keep driving with the check engine light on?
If it is on steadily, the Illinois Air Team says the light is meant to tell you that service is needed, not that you must stop the vehicle immediately; get service as soon as possible. If it is flashing, reduce speed and get service as soon as possible.
What does a flashing check engine light mean?
A blinking or flashing light indicates a rather severe level of engine misfire, according to the Illinois Air Team. Severe misfire over even a short period can seriously damage emissions parts, especially the catalytic converter, which is typically the most expensive to replace. Oregon DEQ says vehicle operation is discouraged until the malfunction is repaired.
Can a loose gas cap turn on the check engine light?
Yes. If the gas cap is not tightened after refueling, the OBD system can detect the vapor leak and turn the light on. The Illinois Air Team says that once the cap is tightened, the light should go off within a few days.
Will the check engine light turn off by itself?
It can. The Illinois Air Team says that if the OBD system checks a component or system three consecutive times (with some exceptions) and no longer finds the problem, the light turns off automatically.
Should I clear the code with a scan tool?
Clearing the light does not fix the cause. If it is turned off without fixing the problem, the OBD system resets its readiness monitors, and if it finds the problem again the light comes back on.
Does the check engine light affect gas mileage?
It can. Fueleconomy.gov says the cause could be as minor as a loose gas cap or a more serious issue that could reduce fuel economy, increase emissions and lead to costly repairs later, so have the vehicle inspected.
Will my car fail the Illinois emissions test with the check engine light on?
Yes. The Illinois Air Team says a vehicle fails the OBD test if the check engine light is commanded on and diagnostic trouble codes are present. Clearing the light just before the test does not help: the readiness monitors reset, and the vehicle is rejected as not ready or fails when the problem is found again.
What does The Works at Ford of Peoria include?
The Works® at Ford of Peoria includes a synthetic blend oil change, tire rotation and pressure check, brake inspection, Multi-Point Inspection, fluid top-off, battery test, and a filter, belts and hoses check.
Check engine light on?
Schedule service with the Ford of Peoria team, or read our Illinois emissions testing guide.
Sources
What the check engine light means, steady versus flashing, the gas cap, how the light turns off and readiness monitors are from the Illinois Air Team FAQ (the Illinois EPA vehicle emissions program) and the Oregon Department of Environmental Quality OBD page; the fuel economy note is from fueleconomy.gov, administered for the U.S. Department of Energy and the EPA. Illinois emissions test details are from the state program pages listed. All read November 9, 2026. Address, hours and The Works are from Ford of Peoria's own website.
- Frequently Asked Questions (what the MIL means, steady vs. flashing, gas cap, how the light turns off, readiness monitors, warranty), Illinois Air Team (Illinois EPA vehicle emissions program), read November 9, 2026
- On-Board Diagnostics (OBD) test FAQ (check engine light, flashing light, gas cap, trouble codes), Oregon Department of Environmental Quality, read November 9, 2026
- Keeping Your Vehicle in Shape (Address Engine Issues Promptly), fueleconomy.gov (U.S. DOE/EPA), read November 9, 2026
- About Vehicle Emissions Testing (how the OBD test works, pass and fail conditions), Illinois Air Team (Illinois EPA vehicle emissions program), read November 9, 2026
- Vehicle Emissions Testing (Chicago and Metro-East St. Louis areas), Illinois EPA, read November 9, 2026
- General maintenance: The Works, Ford of Peoria, November 9, 2026
- Address and service hours (structured data), Ford of Peoria, November 9, 2026