OBDII Emissions Testing
How does OBDII testing work?
What systems are checked during the OBDII test?
What causes a malfunction or DTC?
Reasons your vehicle may have failed the OBDII test.
Understanding the Ready/NOT Ready codes.
What are the advantages of OBDII testing?
Links to other OBDII technical resources.
View our OBDII Poster (pdf version) to help you understand how the test works.
This system is so sophisticated, it can detect malfunctioning components and systems before more serious failures occur and even before the driver of the vehicle becomes aware of a problem. The OBDII enables a vehicle owner to make cost-effective repairs before more costly damage is done to the vehicle.
OBDII testing uses a scan tool that plugs into the vehicle's computer and determines that the emission system and components are working properly. The test downloads stored information from the vehicle's computer to identify emission systems or components that are not working properly.
When an emissions control
malfunction is detected, a dashboard light illuminates stating "Check
Engine" or "Service Engine Soon." If the OBDII system detects a
problem, a corresponding diagnostic trouble code (DTC) is stored in the
computer's memory. Using an OBDII scan tool, a repair technician can quickly
retrieve diagnostic codes from the vehicle computer and make necessary repairs
before a more serious problem develops.
In order to get the monitors set back to Ready, the vehicle must be driven through a drive cycle, which is basically a combination of city/highway driving for a set period of time. This drive cycle basically relearns the OBDII system plus rechecks all of the emissions component's individual systems and as each system completes its own self test, the monitor will set back to Ready.
If a problem still exists in any of the individual systems, a DTC will set again once the monitor changes to Ready. This is why we check the readiness monitor status as part of the OBDII emissions test, because if a vehicle has a MIL ON with a DTC set and the owner disconnects the battery to clear the light to try to pass the test, all the non-continuous monitors reset back to Not Ready and the vehicle now fails for Not Ready.
There are 3 continuous monitors that are always Ready and these are the Misfire, Fuel System, and Comprehensive Components. If you look at your Vehicle Inspection Report (VIR), you will see the emissions monitors listed and their Ready/Not Ready status. If any one of the non-continuous monitors say N/A (Not Available) or N/S (Not Supported), then there is no monitor for that system.
This information was formatted into a "Ready or Not" poster that was designed by DPS with input from the Texas Commission on Enviromental Quality (TCEQ). This poster is in all of the emissions testing stations as well as the Recognized Emissions Repair Facilities (RERF's) and it has been beneficial to the OBDII testing program here in Texas.
causes a malfunction or DTC?
your vehicle May have failed the OBDII test
If your vehicle has a
transmission-related Diagnostic Trouble Code (DTC), please read the important
steps below and contact the Waiver
Station in your area for more information.
NOTE: A transmission problem that has been detected by the On-board Diagnostics Second Generation (OBDII) system will turn on the Malfunction Indicator Lamp (MIL) and set a DTC. The problem could be caused by a number of things including a bad torque converter clutch (TCC) or shift solenoid switch, etc, and transmission DTC’s can cause the vehicle to exceed emissions. The cost of repairs in most cases can exceed the $600 repair cost to receive an Individual Vehicle Waiver, so that’s why these vehicles are issued a waiver, because even if you spent $600, it wouldn’t be enough to correct this problem. In most cases, the transmission must be removed from the vehicle in order to be repaired and that’s why the cost is expensive. With a transmission DTC, you may not notice a driveability problem unless the transmission slippage, etc, gets worse. Sometimes, just be servicing (replacing the fluid & filter) the transmission will take care of the problem and sometimes not. Also, you want to make sure that there are no engine-related DTC's that are keeping the MIL ON, because an engine misfire problem will set a DTC and can cause transmission shifting problems. In other words, you may feel a shifting problem, when it may just be an engine misfire. The RERF or dealer will properly diagnose the problem and will check all electrical circuits, etc, to pinpoint the problem.
Remember: The transmission is part of the vehicle's powertrain, so a transmission problem will turn on the MIL and can cause the vehicle to exceed emissions standards.
are the advantages of OBDII testing?
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