From 2008, California requires CAN. Older cars may use CAN or one of three other OBD2 protocols.
Between the adapter and the vehicle sits a wired protocol. Which one a car uses depends on its age and market. This guide names the four families, shows what California’s rule requires from 2008, and lists the protocols an ELM327 chip can speak.
The plain answer
An OBD2 scan has two links. The first is the wireless link between your phone and the adapter. The second is a wired protocol between the adapter and the car. Four protocol families exist for the wired side: SAE J1850, ISO 9141-2, ISO 14230-4 and ISO 15765-4 CAN. In California’s rule, CAN is allowed on model year 2003 and later vehicles and required on 2008 and later vehicles. The other three may not be used from 2008.
Cars from 2008 on speak CAN. Older cars may speak CAN or one of the other three. A good adapter tries them until the car answers.
What California’s rule says
| Protocol | Model-year rule in 13 CCR §1968.2(g)(3) |
|---|---|
| SAE J1850 | May not be used on any 2008 or subsequent model year vehicle. |
| ISO 9141-2 | May not be used on any 2008 or subsequent model year vehicle. |
| ISO 14230-4 | May not be used on any 2008 or subsequent model year vehicle. |
| ISO 15765-4 (CAN) | CAN “shall be allowed on any 2003 and subsequent model year vehicle and required on all 2008 and subsequent model year vehicles.” Required emission-related messages use a 500 kbps baud rate. |
The U.S. EPA accepts systems built to California’s requirements. Other countries set their own rules, so a car sold outside the United States can differ. The rule also says that any pin in the connector that supplies electrical power must be fused and may not exceed 20 volts DC, whatever the nominal vehicle voltage.
What an ELM327 chip can speak
The ELM Electronics data sheet says the ELM327 supports 12 protocols and can search for the right one automatically. Its protocol list is the reason one adapter can serve cars of many ages.
| ELM327 number | Protocol | Speed and notes |
|---|---|---|
| 0 | Automatic | The chip searches for a protocol the car answers. |
| 1 | SAE J1850 PWM | 41.6 Kbaud |
| 2 | SAE J1850 VPW | 10.4 Kbaud |
| 3 | ISO 9141-2 | 5 baud initialization, 10.4 Kbaud |
| 4 | ISO 14230-4 KWP | 5 baud initialization, 10.4 Kbaud |
| 5 | ISO 14230-4 KWP | Fast initialization, 10.4 Kbaud |
| 6 and 7 | ISO 15765-4 CAN | 11-bit and 29-bit IDs at 500 Kbaud |
| 8 and 9 | ISO 15765-4 CAN | 11-bit and 29-bit IDs at 250 Kbaud |
| A | SAE J1939 CAN | 29-bit ID, 250 Kbaud default, adjustable |
| B and C | User-defined CAN | Defaults of 125 Kbaud and 50 Kbaud, adjustable |
The data sheet counts protocols 1 to 9 as its nine OBD protocols. Protocol A is SAE J1939, a heavy-duty network standard, and B and C are open slots that a user configures. The data sheet describes the chip. A finished adapter also depends on its own board and firmware, so adapters that share a chip name can behave differently.
Why the protocol matters for a scan
- Start-up. Some protocols need a timed wake-up sequence. The chip runs it, and a failure looks like no connection even though Bluetooth is fine.
- Several controllers. A CAN network can have many modules, and more than one may answer a request. Software must keep the replies apart, as our PID guide explains.
- Speed. Newer cars exchange data faster, which helps live gauges but does not make an unsupported value appear.
- Heavy-duty vehicles. California’s heavy-duty rule allows a connector conforming to SAE J1962 or SAE J1939-13. A truck with a different connector may need a different cable, and a passenger-car adapter does not cover it.
If a scan will not connect
- Confirm the adapter is seated and the ignition is on while the car is parked.
- Check the car’s market and year against our compatibility guide.
- Reconnect the phone link and try again, as the adapter may need to re-search.
- Try a second known-good adapter if you can. A failing adapter and an unsupported car look alike from the app.
- Ask a qualified shop if the car still refuses to answer.
Axlyne’s boundary
Axlyne talks to a compatible ELM327 adapter and lets the adapter negotiate the wired protocol with the car. It requests a reviewed set of standard generic OBD2 data and does not claim heavy-duty, manufacturer-enhanced or special-purpose network coverage. Support varies by vehicle, adapter and phone.
Official sources
- California Code of Regulations, Title 13, §1968.2, section (g)(2) and (g)(3)
- California Code of Regulations, Title 13, §1971.1, heavy-duty on-board diagnostics
- ELM Electronics, ELM327 data sheet
- SAE International, J1978-1 OBD2 scan-tool first-generation protocols
- SAE International, J1979 diagnostic test modes
- U.S. EPA, 40 CFR 86.1806-17, onboard diagnostics for 2017 and later vehicles