Why a Check Engine Light Will Cause You to Fail an Emissions Test in Lake County, IN

For vehicles model years 1996 and newer, the onboard diagnostic (OBD) system continuously monitors emissions-control systems and stores diagnostic trouble codes (DTCs) when it detects problems. If the OBD determines a condition could cause emissions to exceed federal standards, the Malfunction Indicator Light (MIL), commonly called the Check Engine light, will illuminate.

Federal rules restrict the use of that warning lamp to indicate emissions-related problems, so an illuminated Check Engine light is treated as an emissions failure by inspectors—which is never good.

How an Illuminated Light Leads to a Failed Emissions Test

During an emissions inspection, technicians connect to the vehicle’s OBD system to read stored DTCs and check the status of readiness monitors. A vehicle with one or more DTCs classified as emissions-related will receive a fail result. In practice, an illuminated Check Engine light signals to the inspection equipment and the inspector that emissions control systems are compromised; that alone constitutes a failure. The light also must function properly—if the indicator lamp itself is inoperative, that can also cause a failure.

Key Items Inspectors Check

• Presence of emissions-related diagnostic trouble codes (DTCs)
• Proper operation of the Check Engine light and related indicator systems
• Completion status of OBD readiness monitors, which indicate whether the vehicle’s computer has verified system performance

Readiness Monitors, Rejects, and Retests

Even when the Check Engine light is not illuminated, a vehicle can be rejected or fail if the OBD readiness monitors are incomplete. These monitors are software checks that require certain driving conditions and time to complete. Some monitors, such as the evaporative emissions (EVAP) monitor, may be more difficult to calibrate and may be considered incomplete under specific circumstances.

Still, most incomplete monitors will result in a rejection. Clearing codes improperly—such as by resetting the vehicle’s computer without completing the necessary drive cycles—can also cause a reject or fail on subsequent testing. Inspectors typically require that the vehicle’s own systems clear codes through proper operation and that the computer complete its readiness checks before a retest is performed.

What the Inspection Report Shows

When a vehicle fails, the Vehicle Inspection Report will list the cause of the failure and identify the systems or components that triggered the result. That report serves as official documentation required for repair facilities and follow-up testing. A properly completed inspection report from a certified facility will indicate whether a retest is eligible and, if so, what steps, if any, are necessary to clear the failure designation.

Impact on Lake County Vehicle Owners

Failing an emissions test due to a Check Engine light can delay registration, result in additional diagnostic and repair visits, and create administrative requirements for retest documentation. Because the test assesses whether emissions-control systems are operating within federal limits, an illuminated Check Engine light is one of the most common and straightforward reasons for a testing failure in Lake County. Vehicle owners should be aware that both active trouble codes and incomplete readiness monitors are treated seriously by inspectors.

Practical Considerations

• An illuminated Check Engine light is a primary and immediate cause of an emissions test failure.
• Both stored emissions-related trouble codes and OBD monitor completion status are evaluated; either can cause failure.
• After repairs performed by a certified facility, allow the vehicle’s OBD system time to complete readiness monitors before returning for a retest.

Because the Check Engine light is the OBD system’s indicator that emissions-related components are not performing within required standards, an illuminated light—or a malfunctioning indicator—will result in an emissions test failure in Lake County, IN.



Understanding how the OBD system, DTCs, and readiness monitors influence test outcomes helps vehicle owners navigate the inspection and repair process and ensures a smoother path to passing a retest.

Is Your Check Engine Light On in Merrillville, IN?

If you discover your Check Engine light is on in Merrillville, Crown Point, Lowell, or Hobart, IN, contact us at Exhaust Masters. We are a 5-star-rated, full-service auto shop with a long-standing presence in the community. Exhaust Masters has been diagnosing Check Engine lights since 1974. Contact us today for any of your automotive needs. Our goal is to ensure you pass the emissions test the first time. Click here to read our Google reviews!