Open sources label pin 16 battery positive and pins 4 and 5 grounds, and the data pins follow the car’s protocol.
The OBD2 connector has 16 pins, and open sources name nine of them. This guide shows which pins those sources label for power, ground and data, which protocols use which pins, and where the sources stop.
The plain answer
The OBD2 connector has 16 pins in two rows of eight. Open sources label pin 16 battery positive, pin 5 signal ground and pin 4 chassis ground. The data pins follow the car’s protocol. SAE J1850 uses pin 2 and, in its PWM form, pin 10. ISO 9141-2 and ISO 14230-4 use pin 7 and, on some cars, pin 15. Most modern cars connect CAN to pins 6 and 14.
Pin 16 is labeled battery positive. Pins 4 and 5 are labeled grounds. On most modern cars, CAN runs on pins 6 and 14.
California’s rule for 2004 and later vehicles, 13 CCR §1968.2, requires the “Type A” connector from SAE J1962. The rule names no pin numbers, and SAE J1962 holds the assignments. This guide lists the pins that an open source names and makes no claim about the rest. Our connector-location guide covers where the connector sits.
California’s connector rule
Section (g)(2) of 13 CCR §1968.2 covers the connector. The section’s title limits it to 2004 and subsequent model-year passenger cars, light-duty trucks and medium-duty vehicles and engines. EPA’s rule for model year 2017 and later vehicles points to the version of §1968.2 that California approved on July 31, 2013, and lets EPA approve certification to a later California version. The table follows the text on Cornell Law School’s site, whose history notes list a last amendment filed on November 30, 2022. An earlier edition may differ in detail.
| Topic | Rule text | Section |
|---|---|---|
| Connector type | A standard data link connector conforming to the “Type A” specifications of SAE J1962, April 2002 version, for vehicles outside the phase-in. | (g)(2.1) |
| Newer vehicles | For 30 percent of 2019, 60 percent of 2020 and 100 percent of 2021 and later model-year vehicles, the “Type A” specifications and location in the September 2015 version of SAE J1962. A manufacturer may use an alternate phase-in schedule of equivalent compliance volume, but 100 percent of 2021 and later vehicles must comply. | (g)(2.2) and (g)(2.2.3) |
| Power pins | “Any pins in the connector that provide electrical power shall be properly fused”, and they “may not exceed 20.0 Volts DC regardless of the nominal vehicle system or battery voltage”. | (g)(2.3) |
| Second connector | Manufacturers may not equip vehicles with additional diagnostic connectors in the driver’s side foot-well region, in the area bound by the driver’s side of the vehicle and the driver’s side edge of the center console (or the vehicle centerline when there is no center console), if an additional connector can be mated with SAE J1962 Type A external test equipment. | (g)(2.4) |
| Pin numbers | None. The only pin reference is the sentence about pins that provide electrical power. | (g)(2.3) |
| Earlier OBD II rule | “A standard data link connector in a standard location in each vehicle based on SAE specifications shall be incorporated.” That rule incorporates SAE J1962 of January 1995. | 13 CCR §1968.1, (k)(4.0) |
SAE’s catalog page for the July 2016 revision of J1962 says the document is “technically equivalent to ISO/DIS 15031-3: December 14, 2001” and specifies “connector contact allocation”. We read that page and none of the standard itself.
The pins open sources name
ELM Electronics, which makes the ELM327 chip, labels the J1962 pins in its reference circuit (Figure 9) and in a modified version of that circuit (Figure 15). California’s Bureau of Automotive Repair (BAR) names the grounds in its 2012 specification for smog-test readers. The table combines both documents.
| Pin | Name in the source | Source |
|---|---|---|
| 2 | J1850 Bus+ | ELM327 data sheet, Figures 9 and 15 |
| 4 | Chassis ground | BAR specification, version 2.5 of October 22, 2012 |
| 5 | Signal ground | ELM327 data sheet, Figures 9 and 15; BAR specification, version 2.5 |
| 6 | CAN-H | ELM327 data sheet, Figures 9 and 15 |
| 7 | ISO-K | ELM327 data sheet, Figures 9 and 15 |
| 10 | J1850 Bus- | ELM327 data sheet, Figures 9 and 15 |
| 14 | CAN-L | ELM327 data sheet, Figures 9 and 15 |
| 15 | ISO-L | ELM327 data sheet, Figures 9 and 15 |
| 16 | Battery Positive | ELM327 data sheet, Figures 9 and 15 |
| 1, 3, 8, 9, 11, 12, 13 | No name in any open source we read | We make no claim about these pins |
The names have limits. The ELM327 data sheet says that most modern vehicles connect a CAN network to pins 6 and 14. It adds that at one time “the use of these pins was left to the vehicle manufacturer, and a number of different systems were connected to them.” ELM Electronics notes that a few vehicles have been reported to have no pin 5, and it tells builders to use pin 4 on those.
Which protocols use which pins
The ELM327 data sheet’s reference circuit connects each protocol to specific pins, and its text names some of them. The assignments come from one maker’s design.
| Protocol | Pins | What the data sheet says |
|---|---|---|
| SAE J1850 PWM | 2 and 10 | The Bus- output serves “J1850 PWM applications”, and the PWM input “must convert a differential input into a single-ended one”. |
| SAE J1850 VPW | 2 | In Figure 9, the VPW input listens to the Bus+ line through a resistor divider. |
| ISO 9141-2 and ISO 14230-4 | 7, and 15 on some cars | “The ISO data is both sent and received on the ISO-K line.” Many vehicles do not need the L line “for initiation, but some do”. |
| ISO 15765-4 (CAN) | 6 and 14 | “Most modern vehicles have a CAN network connected to pins 6 and 14 of the OBD connector.” |
California ties these protocols to model years. SAE J1850, ISO 9141-2 and ISO 14230-4 “may not be used on any 2008 or subsequent model year vehicle”, and ISO 15765-4 is “required on all 2008 and subsequent model year vehicles”. A car built to that rule from 2008 on sends its required emission-related messages over CAN, which the data sheet connects to pins 6 and 14. Our protocols guide has the full year table.
Power and ground
ELM Electronics’ typical circuit takes its power from the vehicle through pins 16 and 5. A protecting diode and a filter capacitor sit between those pins and a five volt regulator.
California’s rule requires a fuse on any power pin and caps it at 20.0 volts DC. BAR’s 2023 specification requires its smog-test readers to work with 10.0 to 32.0 volts DC on pin 16, with pin 5 as the ground. The same specification says a reader may not require power on pin 16 and must carry another power source for the case where pin 16 has none.
ELM Electronics tells builders that the diagnostic connector offers no switched ignition voltage for ignition monitoring, so a separate wire is needed. It says most vehicles will not respond without the ignition key in the ON position, so it tells users to turn the ignition on without starting the engine.
Pin 5 is the signal ground. BAR’s 2023 specification tells its readers to use pin 5 for signal ground when the car connects it to ground, and to carry another ground when pin 5 has none. Pin 4 is the chassis ground in BAR’s 2012 specification, which says a reader may not require pin 4 to be grounded in order to communicate.
We found no open source that says why the standard puts battery power on pin 16. The documents above show how tools use the pin.
If an adapter will not power up or connect
- Park the car and keep the engine off.
- Make sure the adapter is properly connected to the vehicle and powered, as ELM Electronics advises.
- Turn the ignition on without starting the engine. ELM Electronics says most vehicles will not respond without the key in the ON position.
- Check the car’s year against the protocol years above. Our compatibility guide lists the OBD2 start dates we could verify.
- If the adapter gets no answer, compare your car with the reasons BAR names for a car that fails to communicate in a smog test, among them “a damaged OBD connector or wiring, an aftermarket stereo, a transmission computer response instead of engine computer response, and invalid data from the vehicle”. Our protocols guide lists further checks.
What this guide does not cover
- Heavy-duty vehicles. California’s rule for 2010 and later heavy-duty engines requires a data link connector “conforming to SAE J1962 or SAE J1939-13 specifications”. CARB’s draft heavy-duty inspection specification describes “the standard 16-pin type connector” for J1979 tools and a standard 9-pin connector, black or green by engine year, for J1939 tools. A passenger-car adapter may not fit a truck built to the second option.
- Type B. The passenger-car rule requires Type A. BAR’s 2023 specification mentions a “Type A or Type B vehicle connector”. We could not read the standard’s definition of the two types.
- Older cars. ELM Electronics notes that before OBDII several vehicles, “most notably those made by General Motors”, used “a very similar connector (mostly)” for factory communications and typically used a protocol its ELM327 chip does not support. Cars from that era may not follow this pin list.
- Your own car. The names above come from ELM Electronics and BAR documents. The ELM327 data sheet warns that some non-CAN vehicles may use pins 6 and 14 for other functions.
Axlyne’s boundary
Axlyne records supported standard generic OBD2 responses through a separate compatible ELM327 adapter that plugs into this connector. The adapter handles the pins and the wired protocol, and Axlyne talks to the adapter. Support varies by vehicle, adapter and phone. Axlyne does not claim coverage of manufacturer-specific or enhanced data, heavy-duty connectors or special-purpose networks. Set up while the vehicle is parked.
Official sources
- California Code of Regulations, Title 13, §1968.2, sections (g)(2) and (g)(3)
- California Code of Regulations, Title 13, §1968.1, section (k)(4.0)
- California Code of Regulations, Title 13, §1971.1, heavy-duty on-board diagnostics
- U.S. EPA, 40 CFR 86.1806-17, onboard diagnostics for 2017 and later vehicles
- SAE International, J1962 diagnostic connector (catalog page)
- ELM Electronics, ELM327 data sheet
- California Bureau of Automotive Repair, Data Acquisition Device Specification, October 2023
- California Bureau of Automotive Repair, OBD Inspection System Data Acquisition Device Specification, version 2.5, October 22, 2012
- California Bureau of Automotive Repair, On-Board Diagnostic Test Reference
- California Air Resources Board, Heavy-Duty Inspection and Maintenance Program, OBD Data Collection and Submission Specification (draft)