One of the most dreaded waits in the city is the emission testing for vehicles. This morning I had to face the wait. First I get there an hour early and wait till the bay door open then, a four car position that had me next in line.  While waiting I noticed the driver of one of the cars being tested, get out and the tech did something to the vehicle. I said to myself, that driver must have forgotten his controls, you know, lights on/off, wiper on/off that stuff. Well, when I get under the radar I do the lights on/off, brake on then gas, when the tech asked me to step out the vehicle so he could start the On-Board Diagnostics, or OBD.

I said, uh? The tech hooked a terminal to my truck and wha-la, emissions done. WDF! On-Board Diagnostics is a generic term referring to a vehicle’s self-diagnostic and reporting capability. OBD systems give the vehicle owner or a repair technician access to state of health information for various vehicle sub-systems. The amount of diagnostic information available via OBD has varied widely since the introduction in the early 1980s of on-board vehicle computers, which made OBD possible. Modern OBD implementations use a standardized fast digital communications port to provide realtime data in addition to a standardized series of diagnostic trouble codes, or DTCs, which allow one to rapidly identify and remedy malfunctions within the vehicle. This was great because of the long lines known for Memphis’s Emission Stations.

There are different types of gas analyzers used in emissions tests. The Four Gas Analyzer is an equipment used to monitor the four gases that are needed to be monitored according to the EPA emissions standards. These gases are Carbon Monoxide (CO), Hydrocarbons (HC), Carbon Dioxide (CO2), and Oxygen (O2). The Gas Analyzer is inserted in the tail pipe of the vehicle to measure the levels of emission of these gases. The Four Gas Analyzer uses a sampling probe that tests a tiny part of the exhaust gases. There are also Diesel Smoke Analyzers which are more suitable for vehicles running on diesel. This analyzer also uses the infrared technique with Bluetooth communication. Some analyzers have combined functionalities that combine the ability to measure gas and smoke. Almost all of the Gas Analyzers have LCD screen and computer system to display the data gathered from the test. I didn’t see none of THAT!

The intent of OBD-I was to encourage auto manufacturers to design reliable emission control systems that remain effective for the vehicle’s “useful life”. OBD-II is an improvement over OBD-I in both capability and standardization. The OBD-II standard specifies the type of diagnostic connector and its pinout, the electrical signaling protocols available, and the messaging format. It also provides a candidate list of vehicle parameters to monitor along with how to encode the data for each. The OBD-II specification provides for a standardized hardware interface—the female 16-pin (2×8) J1962 connector. Unlike the OBD-I connector, which was sometimes found under the hood of the vehicle, the OBD-II connector is required to be within 2 feet (0.61 m) of the steering wheel (unless an exemption is applied for by the manufacturer, in which case it is still somewhere within reach of the driver). SAE J1962 defines the pinout of the connector as:

  1. Manufacturer discretion. GM: J2411 GMLAN/SWC/Single-Wire CAN.
  2. Bus positive Line of SAE-J1850 PWM and SAE-1850 VPW
  3. Ford DCL(+) Argentina, Brazil (pre OBD-II) 1997-2000, Usa, Europe, etc. Chrysler CCD Bus(+)
  4. Chassis ground
  5. Signal ground
  6. CAN high (ISO 15765-4 and SAE-J2284)
  7. K line of ISO 9141-2 and ISO 14230-4
  8. Bus negative Line of SAE-J1850 PWM only (not SAE-1850 VPW)
  9. Ford DCL(-) Argentina, Brazil (pre OBD-II) 1997-2000, Usa, Europe, etc. Chrysler CCD Bus(-)
  10. CAN low (ISO 15765-4 and SAE-J2284)
  11. L line of ISO 9141-2 and ISO 14230-4
  12. Battery voltage

The assignment of unspecified pins is left to the vehicle manufacturer’s discretion.

There are five signalling protocols currently in use with the OBD-II interface. Any given vehicle will likely only implement one of the protocols. Often it is possible to make an educated guess about the protocol in use based on which pins are present on the J1962 connector:

  • SAE J1850 PWM (pulse-width modulation – 41.6 kB/sec, standard of the Ford Motor Company)
    • pin 2: Bus+
    • pin 10: Bus–
    • High voltage is +5 V
    • Message length is restricted to 12 bytes, including CRC
    • Employs a multi-master arbitration scheme called ‘Carrier Sense Multiple Access with Non-Destructive Arbitration’ (CSMA/NDA)
  • SAE J1850 VPW (variable pulse width – 10.4/41.6 kB/sec, standard of General Motors)
    • pin 2: Bus+
    • Bus idles low
    • High voltage is +7 V
    • Decision point is +3.5 V
    • Message length is restricted to 12 bytes, including CRC
    • Employs CSMA/NDA
  • ISO 9141-2. This protocol has an asynchronous serial data rate of 10.4 kBaud. It is somewhat similar to RS-232, but that the signal levels are different, and that communications happens on a single, bidirectional line without extra handshake signals. ISO 9141-2 is primarily used in Chrysler, European, and Asian vehicles.
    • pin 7: K-line
    • pin 15: L-line (optional)
    • UART signaling (though not RS-232 voltage levels)
    • K-line idles high
    • High voltage is Vbatt
    • Message length is restricted to 12 bytes, including CRC
  • ISO 14230 KWP2000 (Keyword Protocol 2000)
    • pin 7: K-line
    • pin 15: L-line (optional)
    • Physical layer identical to ISO 9141-2
    • Data rate 1.2 to 10.4 kBaud
    • Message may contain up to 255 bytes in the data field
  • ISO 15765 CAN (250 kBit/s or 500 kBit/s). The CAN protocol is a popular standard outside of the US automotive industry and is making significant in-roads into the OBD-II market share. By 2008, all vehicles sold in the US will be required to implement CAN, thus eliminating the ambiguity of the existing five signalling protocols.
    • pin 6: CAN High
    • pin 14: CAN Low

All OBDII pinouts use the same connector but different pins are utilized with the exception of pin 4 (battery ground) and pin 16 (battery positive).

November 16th Memphis Chief Administrative Officer George Little said the new testing, mandated under the Environmental Protection Agency’s Clean Air Act regulations, will give more accurate emissions readings and speed up inspections. The city started onboard diagnostic testing of vehicles that are model years 1996 or newer as they go through annual inspection. Tailpipe emissions testing will still be used on vehicles from 1995 and before. During 2008, 32,568 of the 414,616 vehicles inspected, or 7.8 percent, were three years old or less.

Generally, the equipment used for vehicle emissions testing varies but the overall performance and duty is the same. In a smog check program, there are generally 3 types of vehicle inspections. The Acceleration Simulation or ASM is a test done to check for the emission of dangerous gases like carbon monoxide. On-Board Diagnostics or OBD is the computer system in a vehicle the monitors the performance and status of the ignition system and emission control equipment. TSI or Two-Speed Idle is used to test vehicles model year 1995 and older for emission gases.

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  1. Hello ! I’m new on this forum, hope to talk to you soon 🙂
    I love cars and tuning, and you ?

  2. ptz camera says:

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  3. Heitenunk says:

    do i take a car insurance business to small claims court? The insurer business turned down my claim, (I would take the responsible driver to small claims though I’ve no address to serve them or send a requirement letter). One other driver was at fault but his particular insurance company says there is actually a difference in your statements to make sure they have to use the word of their insured vs. my word. I believe they acted in undesirable faith and did not accomplish a proper investigation would this even be a valid claim in small claims court? I have to take the to blame drivers insurance carrier (not my own) to small claims for that damages to my car.

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