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Ready or Not

OBDII Emissions Testing

Available Topics

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.

Transmission-related Diagnostic Trouble Code (DTC).

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.

What is OBDII?
On Board Diagnostics-Second Generation (OBDII), is a computerized system on 1996 & 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 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.

We do allow two (2) non-continuous monitors to be Not Ready and still pass the test, but three (3) or more Not Ready's and the vehicle will fail.

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.

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 occures 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

Reasons your vehicle May have failed the OBDII test
•Catalytic Converter failure,
•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.

If your vehicle happens to have a transmission Diagnostic Trouble Code (DTC) that requires $$$’s to fix, please contact your local DPS Waiver Station for a thorough description of the Waivers that are available. Transmission DTC repairs can possibly range anywhere from $1500-$3000 to repair and we are handling these types of issues on a case by case basis and a waiver will be issued if diagnosis shows the extent and cost of repair. Some transmission problems may be repaired cheaper, for example, a transmission speed sensor problem, but if diagnosis proves that the cost of repair is expensive, please contact your local DPS Waiver Station to set up an appointment and please make sure to bring your repair receipt that shows the cost of repairs to correct the transmission DTC. The diagnosis will have to be performed by either a Recognized Emissions Repair Facility (RERF) or your dealership, because the waiver station will only accept a diagnosis/repair receipt from one of these facilities in order to issue you a waiver for this type of issue.

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.

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.



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Last modified: June 13, 2007