The EPA explains that every 1996 and newer car and light truck has an on-board diagnostics (OBD-II) system that watches the engine, transmission and emissions controls and turns on the “Check Engine” light when it detects a problem. The light should flash briefly when you turn the ignition on, then stay off. A loose or missing gas cap is one of the faults OBD can detect.
Common causes
The stored trouble code points to the system; this table shows common culprits behind the light.
| Cause | Clues | DIY or shop |
|---|---|---|
| Loose, damaged or missing gas cap | Light came on soon after refueling; evaporative system code | DIY: tighten or replace the cap, then drive a few trips |
| Engine misfire | Rough idle, shaking, flashing light; misfire codes | Plugs and coils often DIY; stop if flashing |
| Oxygen sensor | Worse fuel economy; O2 sensor code | DIY on some cars with a sensor socket; shop on others |
| Catalytic converter efficiency | Catalyst code, often after untreated misfires | Shop |
| Mass airflow or vacuum leak | Hesitation, lean codes, hissing | DIY inspection; repairs vary |
| Weak battery or charging voltage | Several unrelated codes at once after a no-start | Free battery and charging test |
Based on EPA OBD guidance and common repair experience. A code identifies a system, not always the failed part.
Free code scans at AutoZone and Advance
AutoZone’s Fix Finder service checks your check engine light and gives you a free report. Advance Auto Parts offers free engine code scanning on most OBD II vehicles at most stores and will refer you to a technician if more diagnosis is needed. Both are listed on the stores’ official service pages; services vary by location.
| AutoZone | Advance Auto Parts | |
|---|---|---|
| Check engine light scan | Free Fix Finder service: AutoZone checks your check engine light and gives you a free report. Source | Free engine code scanning on most OBD II vehicles at most stores. Source |
From each store's official website, checked Nov 2, 2026. Programs and policies change and some services are not offered at every location or in every state; confirm with your local store.
A code reader is also inexpensive to buy if you want to check codes at home.
What to do, step by step
- If the light is flashing, slow down, avoid hard acceleration and heavy loads, and get the car checked as soon as possible, as the EPA advises.
- If it is solid, check the gas cap at your next stop. The EPA says to have the car serviced if the light does not go out after a few trips.
- Get the code read, write it down, and look up whether it points to a part you can replace yourself.
- Check NHTSA’s recall lookup: some engine and emissions faults are covered by recalls or extended warranties.
Safety: if the light comes on with a red oil pressure light, overheating, or loss of power, pull over safely and stop the engine. Clearing a code without fixing the cause only hides the problem.
Frequently asked questions
If it is solid and the car drives normally, you can usually drive to get it checked soon. If it is flashing, the EPA says a severe problem such as a catalyst-damaging misfire is occurring: minimize driving and get it checked as soon as possible.Can I drive with the check engine light on?
Yes. The EPA lists a loose or missing gas cap as a fault OBD can detect. Tighten it; if the light stays on after a few trips, get it checked.Will a loose gas cap turn on the check engine light?
AutoZone’s Fix Finder service and Advance Auto Parts’ free code scanning are both listed on their official service pages, at most locations.Where can I get my check engine light read for free?
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Sources
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