TxDPS - FAQs: OBDII Emissions Testing

FAQs: OBDII Emissions Testing

OBDII Testing

Understanding OBDII Failures

Reasons your vehicle may have failed the OBDII test.
Understanding the Ready/NOT Readycodes.
Transmission-related Diagnostic Trouble Code (DTC).
What causes a malfunction or DTC?

What is OBDII?
On Board Diagnostics-Second Generation (OBDII) is a computerized system on 1996 and newer model year vehicles that monitors emissions-related components and systems for proper functionality. The OBDII utilizes an on board computer to test and monitor all the emissions-related components and systems of the vehicle.

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.

What systems are checked during the OBDII test?
The On-Board Diagnostics (OBDII) emissions test for 1996 & newer vehicles consists of the following:

Check 1 - Malfunction Indicator Lamp (MIL) or Service Engine Soon Light, Check Engine Light, Check Engine Symbol check
This is considered the bulb check portion of the test. The light must come on when the ignition is turned to the ON position and the light must go OFF when the engine is started. The inspector answers these two bulb check questions as either a PASS or FAIL. If the light is not working, the vehicle will FAIL the test, because the purpose of the light is to alert the driver of the vehicle that there is a problem with the OBDII system (exceeding emissions) and the vehicle needs to be diagnosed and repaired.

Check 2 - MIL Command Status and Diagnostic Trouble Codes (DTC's) check
If the MIL command status is ON, then the vehicle's Powertrain Control Module (PCM) is commanding the MIL to turn ON, because a problem has been detected within the OBDII system and a DTC has been set.
For example, an engine misfire DTC - PO301. If the MIL command status is ON with a DTC set, the vehicle will fail. If the OBDII system is operating properly, the MIL command status will be OFF.

Check 3 - OBDII (Non -Continuous) Readiness Monitors check
These monitors are like self checks of each emissions component (EGR, O2 sensor, etc) within the OBDII system. These non-continuous monitors are considered Ready until either DTC's have been cleared with a scan tool or if battery power has been disconnected, then the monitors will reset back to Not Ready.

In order to get the monitors set back to Ready, the vehicle must be driven through a drive cycle, which is a combination of city/highway driving for a set period of time. This drive cycle relearns the OBDII system and rechecks all of the emissions component's individual systems. As each system completes its own self test, the monitor will set back to Ready.
Effective October 15, 2008:

For vehicles year model 2001 and newer, we allow one (1) non-continuous monitor to be Not Ready and still pass the test, but two (2) or more Not Ready's will cause the vehicle to fail.

For vehicles year model 1996 – 2000, we allow two (2) non-continuous monitors to be Not Ready and still pass the test, but three (3) or more Not Ready's will cause the vehicle to fail.
There are three (3) continuous monitors that are always Ready: 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 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 is illustrated in a "Ready or Not" poster displayed in all of the emissions testing stations as well as the Recognized Emissions Repair Facilities (RERF's).

What are the advantages of OBDII testing?
OBDII testing is quicker than previous emissions tests. This test determines whether there is a malfunction and/or deterioration of the devices that control the exhaust-emissions level. It can lower repair costs by detecting and storing a code specific to the problem. The OBDII test should readily identify vehicle problems, thereby reducing repair time and costs. This will, in turn, contribute to lower vehicle exhaust emissions.

Reasons your vehicle may have failed the OBDII test

Catalytic Converter failure,
Faulty gas cap,
Fuel metering out of specification,
Incorrect ignition timing,
Inoperative injection system and/or thermostatic air cleaner,
Mis-routed vacuum lines, and
Faulty Positive Crankcase Ventilation (PCV) System or Exhaust Gas Recirculation (EGR) System.

Transmission-related Diagnostic Trouble Code (DTC)

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.

Transmission DTC repairs can range from $1500 to $3000, although some types may be cheaper (e.g., a transmission speed sensor problem). A waiver may be issued if diagnosis shows the extent and cost of repair to be unreasonable. Contact your DPS Waiver Station to set up an appointment. Make sure to bring your repair receipt showing the cost of repairs required to correct the transmission DTC. Only diagnosis and repair estimates from a Recognized Emissions Repair Facility (RERF) or a dealership will be accepted to qualify for this type of waiver.

NOTE: A transmission problem that has been detected by the On-board Diagnostics system (OBDII) 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. Transmission DTC’s can cause the vehicle to exceed emissions standards. The cost of repair may exceed the $600 minimum repair cost to receive an Individual Vehicle Waiver and still may not be enough to correct this problem. The transmission must be removed from the vehicle in order to be repaired, contributing to the high cost of repair. With a transmission DTC, you may not notice a drivability problem unless the transmission slippage gets worse. Sometimes servicing the transmission (replacing fluid and filter) 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 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 check all electrical circuits, etc. to properly pinpoint and diagnose the problem.

Remember: The transmission is part of the vehicle's power train, so a transmission problem will turn on the MIL and can cause the vehicle to exceed emissions standards.

What causes a malfunction or DTC?
The OBDII system test determines whether there is a malfunction and/or deterioration of the components that control the vehicle exhaust-emissions levels. If this occurs, a DTC, or Diagnostic Trouble Code, is set in the PCM. A failure occurs when there is a malfunction with the OBDII equipment including the on board computer and related wiring, or when an emissions related component has failed, causing the vehicle's exhaust emissions to be 1 1/2 times the allowable emissions for the vehicle, as determined by the manufacturer.