Emissions testing using the On-Board Diagnostic system (OBD) testing is the quickest and most efficient of all emissions tests. Most vehicles from model years 1996 and newer have standardized computer systems that monitor the electronic sensors of engines and emission control systems while the vehicle is operating to ensure that they are working correctly. The OBD test evaluates emissions during normal operation conditions. It can lead to earlier detection of vehicle problems, helping motorists to avoid costly repairs and improve vehicle emissions. If your vehicle's Malfunction Indicator Light (MIL), commonly referred to as the 'Check Engine' light, remains lighted during your vehicle's emissions test, your vehicle will automatically fail its emissions test. See below for more information.

The inspector will connect a communication cable from the emissions testing equipment to your vehicle's Diagnostic Link Connector (DLC). Through the cable, the emissions testing equipment will request specific communication protocols from your vehicle, allowing the testing equipment to verify whether your vehicle emissions control system complies. Information is retrieved from your vehicle; nothing is uploaded or installed on your vehicle.

Vehicles subject to OBD testing include 1999 and newer gasoline-powered and 1999 and newer diesel-powered vehicles with the Gross Vehicle Weight Rating (GVWR) less than 8,501 pounds.

The following medium-duty vehicles (trucks, vans, larger SUVs) with the GVWR between 8,501 pounds to 10,000 pounds are also subject to OBD testing:

• 2007 or newer diesel-powered vehicles
• 2008 or newer non-diesel vehicles (gasoline, compressed natural gas (CNG), liquid propane gas (LPG), ethanol, and methanol)

In addition to this test, all vehicles are subject to the visual verification of the catalytic converter; if one or more catalytic converters are missing from your vehicle, the emissions test will fail.

What is the purpose of the Malfunction Indicator Light (MIL)?

The Malfunction Indicator Light (MIL), commonly known as the 'Check Engine' light, turns on to alert the driver to poor performance, high emissions, or poor fuel economy. The MIL is usually represented by the symbols here:

During normal operation, the MIL will turn on for a few seconds at ignition and turn off once the engine is running. However, if the light remains on, the On-Board Diagnostics (OBD) system has detected a problem(s) with the engine, transmission, or emissions control system. Please don't ignore this! If the MIL remains on when the vehicle undergoes an emission test, the vehicle will FAIL.

What does it mean when my vehicles MIL remains on?

When a vehicle's MIL remains on, the OBD system has detected an emissions problem. The OBD captures the issue and stores a record of it in the vehicle's computer memory, which a qualified technician can then use to diagnosis the problem. Sometimes the OBD system will automatically turn the MIL off. For example, if a gas cap is not properly tightened after refueling, the OBD system may detect vapor leakage and turn on the MIL. Once the gas cap is tightened, the MIL will go off.

What should I do when the Malfunction Indicator Light (MIL) remains on?

You should have the vehicle serviced as soon as possible by a qualified auto mechanic, such as a Certified Emissions Repair Technician (CERT). Timely diagnosis may save you money because:

• Small problems can be identified before they become major expenses.
• A faulty component may be covered by the vehicle's emission warranty.
• Test results allow a CERT to accurately pinpoint problems, avoiding costly diagnostic fees.

What does it mean when my vehicle's MIL is flashing?

A flashing MIL means your vehicle's engine has a significant issue and may cause the catalytic converter to overheat and start a fire. You should stop driving the vehicle and have it diagnosed by a qualified technician as soon as possible.

What are the most common reasons why a vehicle may fail the OBD test?

• MIL remains on while vehicle is running.
• OBD monitors are not ready (this can happen when the battery has been disconnected).
• OBD connector is missing, damaged, or inaccessible.
• MIL does not operate or OBD system is not functioning correctly.
• There is a communication failure between the OBD and the emissions analyzer.

Why did my vehicle fail its OBD emissions test for readiness?

If your vehicle previously required an emissions repair, your vehicle must be driven in a certain drive trace to reset the monitors. Ask your Certified Emissions Repair Technician or whoever serviced your repairs for information about your vehicle's drive trace. Failure of the readiness test could also mean your vehicle's OBD system was not ready to examine the emissions control system because the required number of monitors had not completed their analysis yet.

What does 'not ready' mean?

In most cases, it means a disconnected battery or an "off" Malfunction Indicator Light (MIL). Please note: Disconnecting your battery, replacing a defective or low voltage battery, or clearing the OBD computer prior to testing will result in failing an emissions test.

If my vehicle failed for readiness, what should I do next?

Normally, a week of combined highway and city driving (drive-cycles) will reset the OBD monitors and allow an OBD test to be completed.

• Drive the vehicle at least three separate times, turning it on/off.
  • One drive-cycle should be local in-town driving.
  • One drive-cycle should be highway driving: and
  • One drive-cycle should be after the vehicle was unused for a period (e.g., overnight).
• Have the gas tank 1/4 to 3/4 full
• Drive smoothly and avoid rapid acceleration.

Also keep in mind:

• If the vehicle fails for "unset readiness" at retest the required number of monitors must be ready.
• If the vehicle initially failed for a catalyst-related Diagnostic Trouble Code, the catalyst monitor must be "ready" when retested.
• If problems continue, some car manufacturers may be able to reset the monitors with their diagnostic computer.
• You may have a CERT perform a drive-cycle conditioning to get your vehicle ready for an OBD test. Ask if there is a cost for this service.

The Pre-Condition Two-Speed Idle (PCTSI) Test is designed for 2007 and older non-diesel vehicles with the GVWR of 8,501 pounds to 10,000 pounds. The PCTSI measures tailpipe emissions (hydrocarbons/carbon monoxide) at cruise engine speed and then at an idle speed. In addition to this test, all vehicles are subject to the visual verification of the catalytic converter; if one or more catalytic converters are missing from your vehicle, the emissions test will fail.

The Gas Cap Pressure Leak Test is required on vehicles that receive the Pre-Condition Two-Speed Idle (PCTSI) test. This test ensures the gas cap(s) on your vehicle are capable of properly sealing the gas tank, which allows the vehicle's evaporative emission system to properly dispose of fuel vapors. Note, if your vehicle requires the PCTSI test and the gas cap(s) are missing from your vehicle, it will be rejected from emissions testing. Malfunctioning gas caps will also cause your vehicle to fail the emissions test.

The Modified Snap Acceleration (MSA) test is for older diesel-powered vehicles. The emissions analyzer will utilize an opacity meter to measure the density of smoke emitted from the vehicle's tailpipe.

Vehicles subject to the Modified Snap Acceleration (MSA) include:

• 1999 through 2006 diesel-powered vehicles with the GVWR between 8,501 pounds to 10,000 pounds.

Please note that NOT all Test Centers can accommodate this emissions test. Please contact the motorist hotline at (877) 469-2884 or click here to find the closest Full-Service Test Center.

In addition to this test, all vehicles are subject to the visual verification of the catalytic converter; if one or more catalytic converters are missing from your vehicle, the emissions test will fail.