On-Board Diagnostics Testing

drawing of auto mechanic working on car

What To Know

If your vehicle is a 1996 or newer model, it comes equipped with an On-Board Diagnostics System (OBD II). This system is designed to catch emissions problems before they become a major concern.

  • OBD alerts you to conditions that waste fuel, shorten engine life and result in potentially expensive repairs.
  • OBD lets you know your vehicle is contributing to unhealthful air by emitting excess pollutants.

A vehicle will not pass an emissions test when the check engine light is on. If the battery has been disconnected or the trouble codes erased, you will have to drive the vehicle under varying conditions for up to one week before it will pass. Remove any device that monitors your driving habits before a test and replace it afterwards.

Be as bright as this dashboard light!

service engine light graphic gif

Take your vehicle to a qualified OBD II repair shop when you see the Check Engine light. A flashing light indicates a severe problem which could damage the catalytic converter.

Emission Tests with OBD II

OBD cable under dashboard

Your yearly smog check on 1996 and newer vehicles is done through OBD II, rather than the older two-speed or "tail pipe" test. OBD II is more accurate than the two-speed test and less costly than the dynamometer tests used in some other states. The test itself is also much faster. Stations can often complete an OBD II test in five minutes or less.

The emissions technician will scan your Vehicle Identification Number on the driver's door using a bar code reader. The technician will then plug a cable from the analyzer into your vehicle's OBD computer. There is no other connection or probe. Not only does this quickly indicate whether your vehicle passes, it also helps the technician pinpoint any malfunctions.

Nevada emission inspections stations are equipped with advanced analyzers that send emissions test results to the DMV in real time. Simple screens walk the operator through the test. Many stations also offer registration renewals once you have completed the test.

OBD II is the lowest cost, most accurate inspection for newer vehicles. Vehicles 1995 and earlier will continue to be tested under the two-speed method.

Do's and Don'ts

DO take your vehicle in for service right away if the “Check Engine” or “Service Engine Soon” light comes on and stays lit. This light should not be ignored. Even though the vehicle might seem to be running just fine, there is a problem which has potential to increase emission levels.

One item you can check easily is the gas cap. A loose or broken gas cap will cause the Check Engine light to come on. The light go off by itself once the gas cap is properly tightened.

Sometimes when a Check Engine Light comes on the vehicle’s engine computer will go into a “back-up” or “limp-home” mode. This allows the vehicle to continue operating until repairs can be made. Back-up or limp-home modes are not the most emission or fuel efficient way for the vehicle to operate. The problem needs to be repaired.

DO check with your local dealer to find out if your vehicle is covered by warranty or recall if a problem occurs with your vehicle’s OBD-II system. Expensive emission components could be covered under warranty for up to 8 yrs or 80,000 miles, whichever occurs first.

DO get your annual emission test early. Remember, the test is valid for 90 days on renewals and private party transactions. If the emission test finds a problem with your vehicle, you will have some time to take care of the problem before your registration expires.

DON’T drive your vehicle if the “Check Engine” or “Service Engine Soon” light is flashing. A flashing light means that a problem exists that may cause damage to expensive emission control systems. So, get your vehicle to a repair facility immediately. A ruined catalytic converter can cost thousands of dollars to replace.

DON’T take your vehicle in for an emission test right away if you just replaced a battery, or if the existing battery had gone dead, was disconnected or has been cranking slowly. If a battery drops below five volts the vehicle’s computer memory will be lost. Your vehicle will not pass the test and will be “Rejected”. The vehicle will need to be driven under varying conditions for up to one week in order for the OBD-II system to run all of its on-board diagnostic tests.

If your license plates are expired, or will expire within the week, go to your nearest DMV office and request a movement permit. It is very rare to find a vehicle that will not be ready for emission testing after a week’s normal driving. In the event you have any questions, please contact the Department of Motor Vehicles Emission Control Lab in your area. In Las Vegas, please call (702) 486-4981. In Reno, please call (775) 684-3580.

DON’T clear any Data Trouble Codes using a scanner, hoping to pass the emission test. If you erase the Data Trouble Codes and then take the vehicle in for emission testing right away, it will be “Rejected”. Clearing the Data Trouble Codes erases all of the OBD-II system's memory settings. The vehicle will need to be driven under varying conditions for up to one week. Additionally, erasing the codes may hide problems and make it more difficult to diagnose and fix your vehicle.

DON'T take the vehicle in for a test if you have a device that monitors your driving habits plugged in to the connector below the dashboard. You must remove the device and replace it after the test. Emission stations are required to fail the vehicle if such a device is plugged in.

Frequently Asked Questions

See also the U.S. EPA State and Local Transportation Resources.

What is OBD?

The On Board Diagnostics system is composed of a computer that monitors the emissions system and key engine components. It can usually detect a malfunction or deterioration in these components before the driver becomes aware of the problem. The U.S. EPA has required auto makers to install OBD systems on all cars and light-duty trucks made since 1996.

OBD systems are designed to alert the driver when a component in the engine management or emissions systems begins to deteriorate or malfunction. Early detection of minor problems, followed by timely repair, can often prevent more costly damage to components such as the catalytic converter.

How does OBD work?

When the OBD computer detects a problem, it stores a Diagnostics Trouble Code and may illuminate the "Check Engine" or "Service Engine" light on the vehicle's dashboard. This light cannot be turned off until the necessary repairs are completed or the condition no longer exists.

When you take your car in for diagnosis or for an annual emission inspection, the repair technician retrieves the trouble codes from the computer using a "Scan Tool."  By using this information, a properly trained repair technician can quickly and accurately fix any problem.

Under certain conditions, the dashboard light will blink or flash. This indicates a severe problem. The driver should reduce speed and seek service as soon as possible. Malfunctions which cause a flashing light can seriously damage emission control system components in a short period of time, especially the catalytic converter.

What does OBD have to do with air pollution?

Motor vehicles are the largest source of air pollutants in North America. Although modern vehicles are getting cleaner due to improved engines and emission controls, emissions are low only when everything is in proper working order.

When an engine is not at maximum efficiency, performance is lost, fuel is wasted and air pollutant emissions increase. OBD can detect emission component problems before there is an obvious problem with the car or truck.

How do I know there is a problem with my car or truck?

OBD systems are designed to alert drivers when something in the engine or emission control system begins to deteriorate or fail. When the OBD system determines that a problem exists a code is stored in the computer memory. The computer may illuminate a dashboard light indicating "Service Engine Soon" or "Check Engine."

This light informs the driver that there is an emission-related engine problem. If the light is on, there is a real problem relating to emission control that needs to be fixed. If the light is flashing the driver needs to take the car to a repair technician as soon as possible.

How do I turn the engine light off?

The repair technician will turn off the dashboard light after the necessary repairs have been made. There are also situations under which the vehicle’s OBD system can turn off the dashboard light automatically if the conditions that caused the problem are no longer present.

Is OBD going to cost me more money?

No. It may actually save you money. OBD is designed to aid the driver as well as the environment by alerting the driver early to deterioration and failure of parts that affect engine function and/or pollutant emissions.

Early detection of engine or emission system failures followed by a timely repair will prevent more costly repairs down the road. It can also help lower repair costs because the test is more specific and can help reduce troubleshooting time.

What happens after on OBD test?

Vehicles that pass the OBD inspection will be able to proceed with vehicle registration. All emissions test results are transmitted electronically to the DMV. We encourage you to renew by Internet, at a kiosk or at emissions stations that offer this service.

If a car or truck fails the OBD test, the owner will be provided with a test report that specifies the reason for failure. Vehicles that fail the test must be repaired in order to register the car or truck.

Who can fix an OBD problem?

Only a qualified trained service technician equipped with the correct diagnostic and repair equipment can perform OBD-related repairs. Vehicle owners should ask their vehicle repair facility if the technicians have received proper training and have access to the necessary equipment to properly service OBD-equipped vehicles.

Are OBD related repairs covered by my warranty?

Federal law requires that the emissions control systems on 1996 and newer vehicles be warranted for two years or 24,000 miles (which ever comes first). Many auto manufacturers provide extended coverage as well. Federal law also requires that the on-board computer and the catalytic converter on 1996 and newer vehicles be covered for 8 years or 80,000 miles. Consult your vehicle owners manual or warranty for coverage.