How to Pass OBD ii Emissions Test?

How to Pass OBD ii Emissions Test?

Many states in the country need an emissions check before renewing your car’s registration. Emissions testing also referred to as smog check, ensures your vehicle meets the Environmental Protection Agency EPA emissions standards.

Note that how your vehicle is tested depends on the requirements laid down by the state or local government. The year of vehicle manufacture and model plays a role in how often you test your car using an OBD scanner.

There are usually two types of emissions tests. These include onboard diagnostics and tailpipes. The emissions test that your car will undertake will depend on your state’s requirements and your vehicle’s age.

Conducting an OBD2 test is super easy compared to doing a tailpipe test. With a professional automotive diagnostic scanner, you have to plug it into the vehicle’s diagnostic port. This enables the technician to communicate with the onboard computer to flag any emissions-related issues. Besides, OBD2 tests are conducted on only cars manufactured from 1996 and newer vehicles.

On the contrary, a tailpipe test needs to put an exhaust gas analyzer into the vehicle’s tailpipe. You need to tighten the gas cap properly. The analyzer’s work is to measure the levels of hydrocarbons (HC) and carbon monoxide (CO), both of which are pollutants and safe carbon dioxide and oxygen.

What does an official OBD2 emissions test consist of?

An official OBD2 emissions test consists of three parts, which include:

An inspector examines to find out if the MIL light comes on when the key is turned on. Hence, if the light fails to come on, then the vehicle fails the bulb inspection.

A diagnostic san tool is plugged into the diagnostic port, and the system checks for monitor readiness. If more than the recommended number of monitors are not ready, the car is rejected and requested to return later after being driven well to set the readiness flags.

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Besides, the diagnostic code reader also examines the MIL light status and downloads all error codes that may still be present. If the MIL light is turned on and there are some OBD2 trouble codes present, the car fails the test and must be repaired. Additionally, the vehicle will fail if the DLC has been messed with, is missing, or fails to deliver any information.

The final system check involves the scan tool being utilized to instruct the MIL lamp to verify it is taking instructions from the onboard computer. When the OBD2 light is on, or a car has failed an OBD2 emissions test, your first duty is to examine the issue. If that’s the case, plug into the OBD II system, retrieve any stored codes, and look at any OBD ii system data that might help you find out what’s happening with the fuel mixture. For an accurate result, you can use the Autel Maxisys Pro MS908P that has an oxygen sensor.

Tips for passing an emissions test

Knowing the tips and tricks to use to pass emissions tests is very crucial. But it’s also important to note that the demands for passing a tailpipe test are different from passing an OBD II test.

How to pass the tailpipe test

If your state or local government requires your car to pass a tailpipe, here’s what you need to do to fix an emission problem:

  • Warm-up your engine

Before having your car tested, drive the vehicle for at least 20 minutes so that it can warm up. Moreover, you should plan an appointment with the testing center so that you can be admitted while your car’s engine is still warm. This is vital because both the engine and catalytic converter will be at optimum temperature for testing.

  • Ensure your vehicle is up-to-date on routine service

Never ignore the car’s manufacturer’s recommended maintenance schedule, as it keeps your vehicle operating at its best. The schedule service is vital as it features filter replacements, fluid changes, and tune-ups that can impact an emissions test’s outcomes.

Related article: Easy Ways to Understand OBD2 scanner

  • Restore any present engine-related issues

If your car’s engine is stalling, running rough, or showing any other issues, you will have to fix the challenges before having an emissions test. This is because engine-related problems typically lead to an increase in tailpipe emissions.  

  • Ensure the check engine light is turned off

Your vehicle will never pass an emissions test with the check engine light on. Therefore, if the light is on, you will have to tackle the problem that made the check engine light come on.

  • Retest your vehicle for free at a smog test location

Taking your vehicle through another round of emission testing leads to more expenses. That’s why it’s advisable to visit a shop that provides retesting for free if your car doesn’t pass on your first attempt.

How to Pass OBD ii Emissions Test?

Some states require passing both the tailpipe emissions test and OBD II emissions. But how to test an O2 sensor with a scanner? If your state only needs your car to pass an OBD II emissions test, here’s what you should do:

  • Ensure the check engine light is turned off

Like the tailpipe emissions testing, your car will not pass an emissions test if your “check engine light” is on. Ensure to rectify all the challenges that may be causing the CEL light to turn on.

  • Ensure that all tires keep the vehicle stable and straightforward to control

This is because the pressure of the tire affects the dynamometer when gathering exhaust. Exceptional tires keep your vehicle moving well during the smog test.

  • Always change your oil for a smooth operating engine

This is an excellent habit that will help to keep your car healthy. This is because contaminated oil removes the engine’s clean breath and makes your vehicle fail an emission test.

  • Make sure all the system monitors have run.

The vehicle’s computer runs self-tests referred to as “monitors” on emissions-related systems. When conducting an emissions test, a smog technician will examine that all of the monitors work correctly.

The monitors are reset every time the battery is disconnected. Besides, clearing diagnostic trouble codes from your car’s computer’s memory also resets the monitors. If none of these has been conducted on your vehicle, you may have to ensure the monitors have run before emissions testing. This is because if they don’t run, your vehicle will have a “not ready” outcome instead of passing or failing.

You can inspect whether the monitors have run by utilizing an advanced OTC 3211 code reader. If you do not have a scanner, you should drive your vehicle at different speeds for a prolonged period. This will help you to have a better opportunity of being ready for emissions.

What makes an OBD2 emissions test fail?

  • Emissions equipment failure
  • Engine and transmissions issues
  • Air or fuel delivery challenges in the fuel tank
  • Ignition system problems
  • Module and sensor problems


Environmental Protection Agency (EPA) sets an emission test standard that you must need to follow. With the above guide, knowing how to pass an OBD2 emissions test shouldn’t be a problem. Note that having a reliable automotive diagnostic scanner will help a lot in passing emissions tests. Also, always ensure to go for scheduled maintenance according to your car’s manufacturer.

Passing an emissions test shouldn’t only help have your car registered, but it should also help you maintain a clean environment.

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