What is OBDII Testing

It’s a common question for the average auto owner, and one that usually comes up when you notice a check engine light or check engine trouble code on your dashboard. Answering the question of what is OBDII testing is fairly easy, but it’s not always an easy problem to resolve.

OBDII is a computer system implemented in 1996 to help monitor the emissions and general health of your automobile. Using the sensors located in your engine bay, this computer system can monitor values and what is happening in your engine and transmission. When it detects a problem, such as a lack of signal or value that’s outside of the manufacturer’s predetermined ranges, it will trigger a Malfunction Indicator Lamp (MIL) or check engine light.


Today I’ll be helping you answer the question of “What is OBDII Testing” and hopefully assisting you in passing your emissions test or turning off your check engine light.

What does the DTC or Malfunction Indicator Lamp mean?

The check engine light on your dash shows you that something in your emissions or drivetrain system has failed. Whether from deterioration or damage to a sensor or the wiring harness a malfunction is stored in the Engine Control Unit (ECU).


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Based on the OBDII DTC in question, you must diagnose and troubleshoot your vehicle to turn off the MIL lamp and restore your vehicle to normal operation.

How can I tell when I have a DTC?

Your MIL or CEL lamp will be lit on your dashboard or instrument display panel. When an error occurs your dash or instrument panel will display a light or warning to inform you of the situation.


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This may be shown as a SERVICE ENGINE or CHECK ENGINE light. Using a scan tool, you must connect to the diagnostic system of your vehicle to read the stored codes.

Possible causes of a failed emissions test


  • Failed or bad Catalytic Converter
  • Failed or bad gas cap
  • Improper ignition timing
  • Bad fuel
  • Broken vacuum lines or damaged emissions control
  • Bad or failed PCV system

What’s involved in OBDII Testing ?


Test 1 – MIL – SES light – By turning the key to the “ON” position, your emissions technician is looking for your Malfunction Indicator Lamp (MIL) or Service Engine Soon Light, Check Engine Light, or Check Engine Symbol check to turn on briefly. This light must then turn off after the start up test is conducted. If the light remains on, this is a FAIL condition for your emissions OBDII testing.

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Test 2 – DTC Check – By plugging into your diagnostic port your emissions technician will be checking for a stored MIL or DTC status in your ECU. This informs the technician of any previous issues, and allows him to see what the code is.

Test 3 – OBDII Readiness – If you disconnecting your battery to clear your OBDII trouble codes, this part of OBDII testing will be a big hurdle for you. The Readiness monitors in your vehicle inform the smog or emissions tech that the vehicle is operating within normal specifications. Several checks are made and usually these readiness monitors can take some time before clearing.

You may or may not have to drive and complete several driving cycles to get your OBDII Readiness to pass. Because this system uses so many different kinds of checks and tests, it can take time as the ECU relearns the OBDII system and rechecks all of the emissions component’s individual systems. When your OBDII Readiness is set to READY, you can pass your emissions test and OBDII testing procedures.

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Emissions related OBDII Testing Failures


Usually sensor or input sensor related, these kinds of failures can occur when your engine’s mass air flow sensor becomes old or damaged, fuel injectors or crank and cam position sensors become old or damaged. With the proper diagnosis this repair should not be overly costly. Be sure to take your vehicle to an experienced mechanic that knows how to avoid misdiagnosing your OBDII Testing problems.

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Drivetrain related OBDII Testing Failures


Unfortunately drivetrain related OBDII testing problems can be very expensive. Because the nature of the OBDII diagnostic system entails testing and regulating your automatic transmission, many times a drivetrain related OBDII DTC code means a rebuild of your transmission.

Some times you may have a smaller problem, like an error with your vehicle speed sensor (VSS) or Transmission Control Solenoid. If you are having problems obtaining an accurate quote for repair, don’t be afraid to contact your state’s list of Recognized Emissions Repair Facilities (RERF) for more details.

While the transmission and drivetrain do not directly impact your engine’s emissions, driving with a slipping transmission, torque converter or torque converter clutch (TCC) can affect your emissions greatly. With a transmission related OBDII DTC check engine code, you may not notice a drivability problem unless the transmission slippage gets worse.

You may want to try the easy fixes for your transmission related OBDII trouble code, such as flushing your transmission of fluid and replacing it. While addressing your drivetrain related OBDII check engine code, you may also want to double check your ECU for additional DTC codes. Many times other emissions related engine codes can adversely affect your powertrain, and mistakenly trip a transmission or torque converter code.

Have any questions about OBDII testing? Leave us a comment or submit your OBDII problem and let us know!

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