Car care guide · Elgin Auto Sales & Service Center · Elgin, IL
Check Engine Light On? What to Do
The check engine light, also called the malfunction indicator lamp (MIL), means the car's onboard diagnostic system has found a problem with the engine, transmission or emissions controls. Whether it is steady or flashing tells you how urgent it is. Here is what each means, what to do next, and how the light affects an Illinois emissions test.
Steadyhave the car checked soon; it usually still drives normally
Flashinga serious condition: immediate service is required
OBD IIthe system that turns the light on and stores trouble codes
1996+model years that get an OBD emissions test in Illinois
The short answer
If the check engine light is steady, the car can usually still be driven, but have it checked soon; it can be as minor as a loose gas cap or something that lowers fuel economy and leads to costly repairs later. If it is flashing, treat it as urgent: owner's manuals such as the 2026 Ram 1500 Owner's Manual warn that a flashing light can mean a condition that could cause loss of power or severe catalytic converter damage, and that immediate service is required.
What the check engine light is telling you
The light is part of the onboard diagnostic system, OBD II, which monitors the engine and transmission control systems and the emissions controls. The guidance below is from the 2026 Ram 1500 Owner's Manual as an example; check your own car's owner's manual for the details on your model.
| What you see | What it means | What the manual says to do |
|---|---|---|
| On for a moment when you turn the ignition on | A normal bulb check before the engine starts. | If it does not come on at all when you turn the ignition on, have it checked promptly. |
| Comes on and stays on while driving | The system has found an issue and stored a diagnostic code. Certain conditions, such as a loose or missing gas cap or poor-quality fuel, can turn it on. | Have the vehicle serviced if the light stays on through several typical drives. In most situations the vehicle will drive normally and will not need towing. |
| Flashing while the engine runs | A serious condition that could lead to immediate loss of power or severe catalytic converter damage. | Immediate service is required. |
From the 2026 Ram 1500 Owner's Manual (Engine Check/Malfunction Indicator Warning Light and Onboard Diagnostic System sections), read October 26, 2026. Follow your own vehicle's manual.
What to do when the check engine light comes on
Start by noticing whether the light is steady or flashing, then work through these steps:
- Flashing? Treat it as urgent and get the car serviced right away. The manual warns that a malfunctioning catalytic converter can get hotter than normal and cause a fire, so do not drive slowly over or park on dry grass, leaves, wood or cardboard.
- Check the gas cap. A loose or missing gas cap is one of the conditions that can turn the light on. Our advice: make sure it is on and tight.
- Watch it over the next few drives. If the light stays on through several typical drives, have the car serviced.
- Have the codes read. When the light comes on, the OBD II system stores diagnostic codes that help a technician find the problem. In our experience the code is a starting point, and the technician's diagnosis confirms the repair.
- Do not put it off. The manual cautions that prolonged driving with the light on could cause further damage to the emission control system and affect fuel economy and driveability.
Steps are based on the 2026 Ram 1500 Owner's Manual; the gas cap and fuel economy points also appear on FuelEconomy.gov. Illinois Air Team notes that the trouble codes help a technician diagnose a problem but do not pinpoint the repair needed.
Why not to ignore a steady light
Fuel economy
FuelEconomy.gov says the problem behind the light could reduce your fuel economy and increase emissions, and that it is worth having a mechanic inspect the car when the light comes on.
Bigger repairs later
The cause could be as minor as a loose gas cap, or a more serious issue that leads to costly repairs later. Modern cars can seem to drive normally even when a repair is needed.
Emissions controls
Owner's manuals such as the 2026 Ram 1500 Owner's Manual warn that prolonged driving with the light on could cause further damage to the emission control system.
Emissions test
In Illinois, a check engine light that is on with stored trouble codes is one of the main reasons a car fails its OBD emissions test.
From FuelEconomy.gov's maintenance page, the 2026 Ram 1500 Owner's Manual and Illinois Air Team's motorist FAQs, read October 26, 2026.
The check engine light and the Illinois emissions test
The Illinois Vehicle Emissions Testing Program, whose motorist site is Illinois Air Team, uses an On-Board Diagnostics (OBD) test for 1996 and newer passenger cars and light-duty trucks. Generally, most 1996 and newer gasoline-powered passenger vehicles registered in the test area are tested once they are four years old, then every two years: even model years in even years and odd model years in odd years. Diesel-powered and all-electric vehicles are not required to test.
- A light that is on means a likely fail. Illinois Air Team lists three main reasons for an OBD test failure: an emissions-related malfunction shown by stored trouble codes and the check engine light; an OBD system that does not work; or a damaged, inaccessible or missing diagnostic connector.
- Fix it before you go. The 2026 Ram 1500 Owner's Manual says that if the light is on during normal driving, have the vehicle serviced before going to the test station.
- Monitors need time after a repair. A car can be rejected from the test lane if its OBD monitors are not ready. The manual notes the system may not be ready if the vehicle was recently serviced or had a dead or replaced battery, and that normal driving may be all it takes for the system to update.
- Unplug add-on devices. Devices plugged into the diagnostic connector, such as insurance or tracking dongles, must be removed before testing.
If a car fails, Illinois Air Team gives the owner a Vehicle Inspection Report with the test results and a Repair Data Form; the car is repaired and then retested.
Testing rules from Illinois Air Team's motorist FAQs, read October 26, 2026. Check your own test notice or Illinois Air Team for your vehicle.
Shopping for a used car? Check the light
The check engine light is worth a look on any used car you test drive. Watch the dash when you turn the ignition on: the light should come on for a moment as a bulb check and then go out once the engine is running. A light that stays on, or one that never comes on at all, is worth asking about before you buy.
Illinois Air Team also offers a voluntary emissions test, for a fee, for vehicles that would be testable in the Illinois program; one example it gives is someone considering the purchase of a used vehicle. For the rest of your pre-purchase checks, see our guides to what to look for when buying a used car and how to check a used car for recalls, or browse our used inventory.
Check engine light service at Elgin Auto Sales
Our service department's technicians handle everything from routine oil changes to complex transmission replacements, for pre-owned car buyers and for Toyota, GMC, Buick, Honda, Kia and Hyundai vehicles. If your check engine light is on, request a service appointment online, and see our current service specials.
Our service department is open Monday to Friday 8 a.m. to 5 p.m. and Saturday 8 a.m. to 4 p.m., closed Sunday. Elgin Auto Sales & Service Center is at 1200 E. Chicago St. in Elgin. Get directions.
Check engine light FAQ
Can I drive with the check engine light on?
If it is steady, most vehicles will drive normally and will not need towing, but have it serviced soon. If it is flashing, the 2026 Ram 1500 Owner's Manual says immediate service is required, because the condition could lead to loss of power or severe catalytic converter damage.
What is the difference between a steady and a flashing check engine light?
A steady light means the onboard diagnostic system has found a problem and stored a code. A flashing light warns of a serious condition, such as one that could severely damage the catalytic converter, and needs immediate service.
Can a loose gas cap turn on the check engine light?
Yes. The 2026 Ram 1500 Owner's Manual lists a loose or missing gas cap as a possible cause, and FuelEconomy.gov notes it could be something as minor as a loose gas cap. Tighten the cap and have the car checked if the light stays on through several drives.
Will my car pass the Illinois emissions test with the check engine light on?
Usually not. Illinois Air Team lists an emissions-related malfunction, shown by stored trouble codes and the check engine light, as one of the three main reasons a car fails its OBD test. Have it repaired before your test.
Why did my car get rejected at the test station after a repair?
A common reason is that the OBD monitors were not ready. The 2026 Ram 1500 Owner's Manual notes this can happen after recent service or a battery replacement, and that normal driving may be all it takes for the system to update.
Can Elgin Auto Sales check my engine light?
Yes. Request a service appointment online; our service department handles everything from routine oil changes to complex transmission replacements.
Check engine light on? Let us take a look
Request a service appointment online, or browse our used inventory and visit us at 1200 E. Chicago St. in Elgin.
Sources
Check engine light guidance is from the 2026 Ram 1500 Owner's Manual as an example, read October 26, 2026; your own vehicle's manual applies to your car. Fuel economy guidance is from FuelEconomy.gov and Illinois emissions testing rules are from Illinois Air Team, both read the same day. Our services, address and hours are from our own website, checked the same day.
- 2026 Ram 1500 Owner's Manual (Engine Check/Malfunction Indicator Warning Light, Onboard Diagnostic System, emissions inspection readiness), Ram / Mopar
- Keeping Your Vehicle in Shape: Address Engine Issues Promptly, FuelEconomy.gov (U.S. Department of Energy and EPA)
- Frequently Asked Questions (motorists), Illinois Air Team, Illinois Vehicle Emissions Testing Program
- Service department and hours, Elgin Auto Sales & Service Center