Check Engine Light:  Why It’s on and What to Do

The check engine is the most common warning light that appears on the dashboard.  Well over half of this site’s content is related to solving the various reasons why the check engine light comes on.

The good news is that the check engine light indicates that there are Diagnostic Trouble Codes (DTCs) stored in your vehicle’s computer, which is a very good thing.  They can really help save time determining what is wrong with your vehicle.

Why is My Check Engine Light On

The check engine light is often referred to as the Malfunction Indicator Lamp (MIL) or Service Engine Soon light.  All three terms are 100% interchangeable.

Table of Contents

What Are Trouble Codes?

Pulling Trouble Codes

Trouble codes are a number sequence of letters and numbers that correspond with a standardized list of problems that can occur in a vehicle. Some of them are manufacturer specific, while most have the same meaning no matter who made the vehicle.

Common OBD II Codes 

The most common of all trouble codes is P0420 (it indicates the catalytic converter isn’t scrubbing pollutants efficiently).

Here are some of the other most common codes:  

  • P0442– Small EVAP leak (most commonly fixed by tightening a loose gas cap or replacing it)
  • P0300– Random misfire (bad ignition coil(s), spark plugs, or spark plug wires).
  • P0128– Engine running too cool (almost always caused by a thermostat that’s stuck open)
  • P0172- Air Fuel mixture too rich (there are many causes, including oxygen sensors, leaking injectors, or too much fuel pressure)

The most common codes are related to the emissions system.  Some of them are as easy to fix as replacing the fuel cap.  Others require serious diagnostic tests from a repair shop.

How to Pull Trouble Codes


You can either pull the trouble codes from your PCM (Powertrain Control Module), or you can bring your vehicle to a parts store or shop to have it scanned.

Pull Them Yourself  

You can put a scan tool into the OBD II port and pull codes out of the diagnostic system yourself.  Any vehicle made for the 1996 model year and up uses the same protocol.

You can usually find the OBD II port underneath the steering wheel. Follow the prompts on your scanner.

Take Your Vehicle to a Shop or Parts Store

These guys will be happy to scan your vehicle

If you don’t have a scan tool, you can take your vehicle to the local parts store.  Most of them are happy to pull the codes for you for free

A lot of auto shops also offer free scan service, but you may find pressured into authorizing whatever repairs they suggest on the spot.

Once you have the codes in hand, you can look them up and determine what’s wrong with your vehicle.

How to Reset the Check Engine Light

There are three ways to reset the check engine light.  We recommend trying to identify the problem and fixing it before resetting the check engine light.

1. Fix and Wait

The best way to clear the check engine light is to pull the trouble code, identify and fix the issue and then drive around until the module that threw the code recognizes that the problem is fixed.  Sometimes it can take a few drive cycles.  Other times, it’ll happen as soon as you fire the vehicle up.  Some folks prefer to clear the code and see if it comes back on

2. Use an OBDII Scanner

You can use an OBDII scanner to turn off the check engine light.  The catch is, that if you don’t fix the problem, the light will turn on again.

This is usually done by connecting a scan tool to your vehicle’s onboard computer. Once you’ve identified and fixed the issue, wait for three or four days of normal driving to see if the Check Engine Light turns back on. If it does not come back on after this period, then you can assume that the issue has been resolved and the light has been naturally reset.

3. Disconnect the Battery

If you’ve identified and fixed the issue that caused your Check Engine Light to come on, resetting the light is as simple as disconnecting and reconnecting the vehicle’s battery. This can be done by unhooking both cables from the battery terminals (negative cable first) and waiting 30 seconds before reconnecting them.

Once the battery is reconnected, your Check Engine Light should be reset and should no longer be illuminated. If the light does come back on again after you reset it, then it means that the issue has not been properly fixed and needs further attention.

Common Questions

Will a Vehicle Fail Inspection With the Check Engine Light On

If you are looking to pass emissions and the check engine light it is an automatic fail in almost every State in the Union.  It doesn’t matter why the check engine light is on, it only matters that a code is stored.  Here’s a list of the emissions regulations of each State.

You can’t reset a check engine light to circumvent emissions requirements.  When you reconnect the battery, the OBD II system is going to need several drive cycles to qualify to take an emissions test.  

Can You Drive With the Check Engine Light On?

Whether you can drive a vehicle with the check engine light on depends on the code(s) that triggered the check engine light.  

You certainly can’t drive the vehicle with the check engine light flashing.  A flashing check engine light indicates tha there is a serious problem with the engine.

Why is My Check Engine Light Flashing?

A flashing Check Engine Light typically indicates a serious issue with the vehicle and should not be ignored.  It usually indicates a catastrophic misfire that can dump raw fuel into the exhaust system, which can damage the catalytic converter and cause backfiring.  Do not drive your car or truck with a flashing check engine light.