What is an OBD-II PID?
A PID is a number that asks the engine control unit for one value. In service 01 the ECU answers with 41, the PID and one to four data bytes, named A, B, C and D. A formula turns the bytes into a value.
| PID | Formula | Unit |
|---|---|---|
| 04 · Calculated engine load | 100 x A / 255 | % |
| 05 · Engine coolant temperature | A - 40 | °C |
| 0C · Engine speed | (256 x A + B) / 4 | rpm |
| 0D · Vehicle speed | A | km/h |
| 10 · Mass air flow rate | (256 x A + B) / 100 | g/s |
| 2F · Fuel tank level input | 100 x A / 255 | % |
| 42 · Control module voltage | (256 x A + B) / 1000 | V |
| 5E · Engine fuel rate | (256 x A + B) / 20 | L/h |
| A6 · Odometer | (A x 2^24 + B x 2^16 + C x 2^8 + D) / 10 | km |
How do you read a CAN answer?
On CAN, the answer comes from ID 7E8 for the engine. The first data byte is the length, then 41, the PID and the data. The rest is padding.
7E8 04 41 0C 1A F8 AA AA AA
A = 0x1A = 26, B = 0xF8 = 248
rpm = (256 x 26 + 248) / 4 = 6904 / 41726 rpm
How is a request built?
A single-frame request has the length 02, the service 01 and the PID, padded to 8 bytes. Sent to 7DF, every ECU that supports the PID answers.
7DF 02 01 0D CC CC CC CC CC
answer: 7E8 03 41 0D 3C ... -> 0x3C = 60 km/h
Which J1962 pins matter?
| Pin | Role |
|---|---|
| 16 | Battery positive, 12 V on type A, 24 V on type B |
| 4 | Chassis ground |
| 5 | Signal ground |
| 6 | CAN high, ISO 15765-4 |
| 14 | CAN low, ISO 15765-4 |
| 7 | K-line, ISO 9141-2 and ISO 14230-4 |
| 15 | L-line, ISO 9141-2 and ISO 14230-4 |
| 2 | SAE J1850 bus positive |
| 10 | SAE J1850 PWM bus negative |
| 1, 3, 8, 9, 11, 12, 13 | Manufacturer discretion |
Assumptions and limits
- Only well-documented service 01 PIDs are included. Manufacturer PIDs and other services are not decoded.
- Not every vehicle answers every PID. Ask PID 00 first to see what the ECU supports.
- Multi-frame answers, such as the VIN, are not reassembled here.
- Pin roles are from the public summary of the standard. Your vehicle wiring diagram has the final word.
- The page does not describe what any Tracom model reads from the OBD port.
Sources
- OBD-II PIDs: public summary of SAE J1979 service 01 formulas
- On-board diagnostics: J1962 connector and pin roles
- SAE J1979, E/E diagnostic test modes: official page, current revision May 2025
- SAE J1962, diagnostic connector: official page, July 2016 revision
Last checked 3 November 2026. The official SAE pages confirm what each standard covers. The formulas and pin roles sit in the paid text, so the first two links are secondary summaries: verify against the current official text before you rely on a figure.