Check Engine Light and OBD

When the pesky red, orange, or amber “check engine” light comes on, it usually elicits mild panic or a few choice curse words. It is annoying but actually necessary. In this article, we will cover what the check engine light is, how you can use an OBD reader to get an idea of the problem, and some simple things to check before rushing off to the mechanic.

The Purpose of the Check Engine Light

The actual name of the check engine light is the Malfunction Indicator Lamp or MIL. But no one calls it that. Everyone knows it as that annoying, glowing engine icon. But the question is, why do cars even have it?

As emissions and fuel consumption regulations became tighter and stricter in the late 80’s and 90’s, car manufacturers had to come up with better systems to control both fuel intake and exhaust. Onboard computers were added to more precisely control fuel injection and monitor exhaust. These onboard computer systems use sensors (like oxygen sensors) that are hard to diagnose if they go bad. You can’t just look at them and tell if they are bad or not.

That’s where the check engine light comes in. If one of these sensors or other electronic components goes bad, the check engine light turns on as a warning that “something” is wrong. It’s something, but not very helpful in itself. So, car manufactures came up with what they called onboard diagnostics. Around 1996, the system was standardized across car makers, and onboard diagnostics or OBD II was born.


Before the OBD II standardization, each manufacturer had its own set of codes. I remember having to use a paperclip to connect to jump two leads on my 1984 Chevy Chevette to read the codes. The check engine light would flash a kind of morse code you had to remember and look up in the manual.  Enter the standard of OBD II.

With OBD II, a standardized OBD II port was designed to allow access to the computer or engine control unit (ECU).  Now you can easily get the codes you need to help you figure out what might be wrong.

Diagnostic Trouble Codes (DTC)

To read the OBD II codes, you simply attach a relatively inexpensive (about $35) OBD reader to the OBD port.  A diagnostic trouble code will appear if there are any issues. You can easily Google the code number to find out what it means for your particular car.

Knowing the potential problems before taking the car to a mechanic can give you peace of mind by validating what they say the problem is.  But before you take the car to the mechanic, you might want to check a few simple potential issues first.

Simple Fixes

Because these codes are often triggered by air, fuel, and exhaust sensors, the actual problem may be easy to fix. First, always check your gas cap. That’s right, a loose gas cap can trigger a check engine light! Next, check the air filter. If it’s really dirty, try replacing it. The lack of air flow can trigger the check engine light too. Finally, visually inspect wiring to see if there are any nicks or cuts. Sometimes sensors are fine, but the wires to them break or get cut.


When the check engine light comes on, don’t ignore it. It could be a serious problem. If you want, use an OBD II reader to get the codes and look them up. You can also try some of the simple fixes mentioned above. Whatever you do, respond to the check engine light and don’t just cover it with some black tape. That’s just called denial.

Written by Gary Pradel