Diagnostics · Updated November 5, 2026
Check Engine Light On: The Most Common Causes
Summary: The check engine light is the one that scares people most and, at the same time, the most misunderstood. When it comes on, your car's computer detected something out of the ordinary and stored a fault code. It can be something as simple as a loose gas cap or as serious as a damaged catalytic converter. What is certain: the light does not tell you what it is, only that the car needs to be scanned. Here we explain the causes we see most at the shop in Toyota and Honda cars 10 to 15 years old.
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01Signs it is happening to you
- The check engine light is steady on the dashboard (fault recorded, the car usually keeps running).
- The light is flashing: this is different and more urgent; it means an active misfire and you need to slow down and get it checked soon.
- The car uses more gas than before.
- The exhaust smells stronger or the engine feels uneven at idle.
- It did not pass the emissions test or your VEIP is coming up soon with the light on.
02Why does it happen?
The most common causes are these. The first is the oxygen sensor: it measures the exhaust gases to adjust the mixture, and over the years it becomes slow or stops working, raising gas consumption. The second is code P0420, which indicates that the catalytic converter is no longer cleaning the gases as it should; sometimes the converter is truly damaged and sometimes the root cause is a sensor or a misfire that gradually poisoned it. The third is the EVAP system, the one that controls gasoline vapors: a gas cap that is not closed properly or has a worn-out seal, a stuck purge valve or a cracked small hose turns on the light. Misfires from worn spark plugs or coils, and dirty mass airflow sensors, are also frequent.
03What happens if you ignore it?
An ignored light turns into a chain of problems. A bad oxygen sensor increases consumption and overloads the catalytic converter until it is damaged, and that is a major repair. An unattended misfire sends unburned gasoline into the exhaust and destroys the converter from the inside. In addition, in Maryland, with the check engine light on your car does not pass VEIP, and you can end up with your registration suspended for not complying with emissions.
04When should you go to the shop?
If the light is flashing, do not keep driving as if nothing were wrong: reduce your speed and come in as soon as possible. If the light is steady and the car feels normal, it is not an emergency, but schedule the check that same week. At the shop we scan the codes, but we do not stop there: we verify with tests what the real cause is, because the code points to the system, not always the exact part.
Did your check engine light come on? Message us on WhatsApp at (202) 985-9208 and we will scan it to tell you what it has. We are Eben-Ezer Auto Repair, in Brentwood, MD, and we explain everything in Spanish.
