OBD-II (On-Board Diagnostics) Wiring Diagram
This is a free printable obd diagram: download the diagram as SVG or open it and print to paper or PDF.
Trace every pin of the standardised OBD-II 16-pin J1962 diagnostic link connector — grounds, CAN bus, protocol pins, and battery feed — before connecting a scan tool or building a custom interface.
OBD-II is a federally mandated diagnostic standard applicable to all petrol/gasoline passenger vehicles sold in the USA from 1996, Canada from 1998, Europe (EOBD) from 2001 for petrol and 2004 for diesel, and Australia from 2006. The diagnostic link connector (DLC) is standardised under SAE J1962. It is a 16-pin, D-shaped female connector located within 1 metre of the driver, accessible without tools. This standardisation means the physical connector and the mandatory pin assignments are identical across every compliant vehicle regardless of manufacturer — a significant point often overlooked by hobbyists who treat each vehicle as having a unique pinout.
The mandatory pin assignments under SAE J1962 are: pin 4 — chassis ground; pin 5 — signal ground; pin 16 — battery positive (+12 V, unswitched). These three pins are present and correctly assigned on every OBD-II compliant vehicle without exception. Pin 16 is live at all times regardless of ignition state, which means the DLC is always powered and a connected scan tool can drain the vehicle battery if left connected for extended periods without a vehicle communications gateway.
Communication protocol pins are manufacturer-assigned within the OBD-II framework, but CAN bus (ISO 15765-4) has become the dominant protocol and is mandatory on US-market vehicles from model year 2008. CAN high occupies pin 6 and CAN low occupies pin 14. When both pins 6 and 14 are present and functional, the vehicle communicates via CAN.
Other protocol pins include: pin 7 (ISO 9141-2 / ISO 14230 K-line, used on many European and Asian vehicles prior to CAN mandate); pin 15 (ISO 9141-2 L-line, less common); pins 2 and 10 (SAE J1850 VPW, used historically on older GM vehicles); pin 2 (SAE J1850 PWM bus positive, used historically on older Ford vehicles). Pins 1, 3, 8, 9, 11, 12, and 13 are manufacturer-defined and may carry OEM-specific protocols, additional CAN networks (medium-speed CAN, LIN), or may be unused.
OBD-II scan tools draw their operating power from pin 16 and return through pins 4 and 5. Never apply external power to any other pins without understanding the specific vehicle's wiring architecture.
How to wire obd diagram
- Locate the DLC and inspect for damage The J1962 DLC is located within 1 metre of the driver, typically under the dashboard to the left of the steering column, accessible without tools per the SAE J1962 specification. Inspect the connector for bent pins, corrosion, or debris. Debris in the DLC is surprisingly common and causes scan tool no-communication faults.
- Verify mandatory pin voltages With a multimeter set to DC voltage, probe pin 16 against chassis ground — expect 12+ V at all times. Then measure pins 4 and 5 against battery negative — both should show less than 0.2 V (near zero resistance to battery negative). Any elevated voltage on the ground pins indicates a poor earth connection that will cause communication errors.
- Check CAN bus termination resistance With the ignition off and the scan tool disconnected, measure resistance between pins 6 (CAN-H) and 14 (CAN-L). A correctly functioning high-speed CAN bus will measure approximately 60 Ω (two 120 Ω termination resistors at opposite ends of the bus in parallel). A reading of 120 Ω suggests one termination resistor has failed. A reading near 0 Ω indicates a short between CAN-H and CAN-L.
- Connect scan tool and establish communication Insert the scan tool connector into the DLC firmly. Turn the ignition on (engine not necessarily running). Allow the scan tool to auto-detect the communication protocol. If auto-detection fails, manually select CAN (ISO 15765-4) for vehicles 2008 and newer, or try K-line (ISO 14230) for older European vehicles.
- Read and record diagnostic trouble codes Navigate to the DTC (diagnostic trouble code) read function. Record all stored codes including pending codes, which indicate a fault detected but not yet confirmed across the required drive cycles. Pending codes are often missed when only checking confirmed codes.
- Interpret live data streams Access live data (Parameter IDs, PIDs) to view real-time sensor values. Mandatory OBD-II PIDs include engine RPM (PID 0x0C), vehicle speed (PID 0x0D), engine coolant temperature (PID 0x05), and oxygen sensor voltages. These are supported on all compliant vehicles; extended PIDs are manufacturer-specific.
Specifications
| Connector standard | SAE J1962 — 16-pin, D-shaped, trapezoidal female (vehicle side) |
|---|---|
| Pin 4 | Chassis ground (mandatory, all OBD-II compliant vehicles) |
| Pin 5 | Signal ground (mandatory, all OBD-II compliant vehicles) |
| Pin 6 | CAN bus High (CAN-H) — mandatory on US vehicles from MY 2008 |
| Pin 14 | CAN bus Low (CAN-L) — mandatory on US vehicles from MY 2008 |
| Pin 16 | Battery positive +12 V, unswitched (mandatory, all OBD-II compliant vehicles) |
| CAN bus termination resistance (healthy bus) | Approximately 60 Ω between pins 6 and 14 (two 120 Ω resistors in parallel) |
| High-speed CAN bus speed (ISO 15765-4) | 500 kbit/s (most common); 250 kbit/s on some powertrain and chassis buses |
Safety warnings
- Never apply external voltage to any OBD-II connector pin other than the ground pins — incorrect voltage on data communication pins can destroy ECU transceivers and modules, causing extensive and costly damage.
- Pin 16 is live at all times regardless of ignition state. Shorting pin 16 to any ground pin will immediately blow the DLC supply fuse and may damage the vehicle wiring. Always use insulated probes when measuring.
- Do not clear diagnostic trouble codes without first recording them and understanding their cause — clearing codes erases the fault history and resets readiness monitors, which may cause a vehicle to fail an emissions inspection and erases diagnostic data needed for accurate repair.
- This is a general reference for the SAE J1962 standard. Manufacturer-specific pins and protocols vary. Consult the vehicle manufacturer's wiring diagram before probing pins other than 4, 5, 6, 14, and 16.
- Some vehicles with advanced security systems (immobilisers, stolen vehicle tracking) may log DLC access events. Check whether this applies to the vehicle before performing diagnostics for resale or fleet purposes.
Tools needed
- Digital multimeter (voltage and resistance measurement)
- OBD-II scan tool or code reader
- Automotive wiring diagram for the specific vehicle
- Back-probe pins or piercing probes (for live circuit measurements)
- Electrical contact cleaner (for corroded DLC pins)
- Small pick tool (for removing debris from DLC)
Common mistakes
- Assuming all 16 pins are standardised — only pins 4, 5, and 16 are mandatory on every vehicle; all other pins depend on manufacturer and protocol.
- Failing to check pin 16 fuse integrity when a scan tool powers on partially or not at all — the fuse is often in an auxiliary fuse panel, not the main fuse box.
- Measuring CAN bus resistance with the ignition on — doing so may yield a false reading due to active transceiver pull-up/pull-down; measure with ignition off and scan tool disconnected.
- Treating a no-communication fault as an ECU failure before verifying power, grounds, and CAN bus integrity at the DLC — ECU failures are far less common than connector, wiring, or fuse faults.
- Leaving a high-current OBD dongle connected permanently, resulting in a flat battery after several days of non-use.
Troubleshooting
- Scan tool does not power on when connected
- Cause: Blown fuse on DLC supply circuit (pin 16), open-circuit ground, or corroded DLC pins Fix: Measure voltage at pin 16 against chassis ground. If absent, locate and replace the DLC supply fuse (typically labelled 'DLC', 'OBD', or 'Data Link' in the fuse panel legend). If voltage is present, inspect the scan tool's connector for bent or corroded pins.
- Scan tool powers on but shows 'No Communication' or 'Link Error'
- Cause: CAN bus fault, wrong protocol selected, or high-resistance ground on pins 4 or 5 Fix: With ignition off and scan tool disconnected, measure resistance between pins 6 and 14 — expect approximately 60 Ω for a healthy CAN bus. Measure pin 4 and pin 5 to battery negative — expect less than 0.2 Ω. Elevated ground resistance is a common cause; inspect chassis ground connections at the ECU and battery.
- CAN bus resistance measures 120 Ω instead of 60 Ω
- Cause: One of the two 120 Ω CAN bus termination resistors has failed open Fix: A single termination resistor failure (120 Ω reading) allows CAN communication to continue on many vehicles but causes signal integrity issues, especially at higher bus speeds or on longer harness runs. Locate the failed termination resistor — one is typically inside the ECU, the other may be in a separate module or at the DLC end of the harness.
Frequently asked questions
Is the OBD-II connector pinout truly standardised across all vehicles?
The mandatory pins are standardised: pin 4 (chassis ground), pin 5 (signal ground), and pin 16 (+12 V battery) are identical on every SAE J1962-compliant vehicle. CAN bus on pins 6 (CAN-H) and 14 (CAN-L) is mandatory on US vehicles from model year 2008. Manufacturer-specific pins (1, 3, 8, 9, 11, 12, 13) vary by manufacturer and vehicle.
What voltage should I measure at pin 16 of the OBD-II connector?
Pin 16 is an unswitched battery positive supply, so you should measure system battery voltage at all times — approximately 12.4–12.8 V with the engine off, 13.8–14.5 V with the engine running and charging system active. No voltage at pin 16 indicates a blown fuse on the DLC supply circuit (a common fault after battery replacement).
What is the difference between pins 4 and 5 (the two ground pins)?
Pin 4 is chassis ground — connected directly to the vehicle body/chassis earthing point. Pin 5 is signal ground — a cleaner reference earth for electronic communication signals, typically connected to the ECU ground network rather than the chassis directly. Both must have low resistance to battery negative for reliable scan tool communication.
Why is my scan tool powering on but not communicating with the vehicle?
The tool is receiving power from pin 16 but failing to establish a communication link. Most commonly caused by: wrong protocol selection, CAN bus wiring fault (pins 6 and 14), corroded DLC pins, or a blown fuse on the ECU power circuit. Measure resistance between pins 6 and 14 with the ignition on — a healthy CAN bus typically shows 60 Ω (two 120 Ω termination resistors in parallel).
Can I leave a scan tool or OBD dongle plugged in permanently?
Pin 16 is live at all times. A permanently connected device draws current continuously, which can drain the battery if the vehicle is not driven regularly. Quality OBD dongles go into a low-power sleep mode, drawing less than 3 mA, which is generally acceptable. Cheap devices may draw 50–100 mA continuously, flattening a battery within a few days.