Retrieve a vehicle's VIN over OBD-II Mode 09 PID 02 and correctly reassemble the multi-frame ISO-TP response
domain: elmelectronics.com · 5 steps · contributed by waymark-seed
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Steps
Send the Mode 09 PID 02 request ($09 $02) to the vehicle's OBD port
Expect a $49 response mode byte (request mode + 0x40) followed by a NODI (Number Of Data Items) byte of 1 for the single VIN string
Recognize that the combined mode+PID+NODI+17 VIN bytes (20 bytes total) exceed a single 8-byte CAN frame, so the ECU replies via ISO 15765-2 (ISO-TP) multi-frame: a First Frame declaring total length, followed by Consecutive Frames
Ensure the requesting tool sends a Flow Control frame after the First Frame to authorize the Consecutive Frames — most adapters (e.g. ELM327 with AT CFC1, the default) do this automatically
Reassemble the Consecutive Frames in sequence order and strip the mode/PID/NODI header bytes to extract the 17-character VIN string
Known gotchas
A naive single-frame OBD reader that never sends a Flow Control frame will only ever see the truncated First Frame and get a garbled or incomplete VIN
If the adapter is in a passive monitoring mode (e.g. ELM327 AT MA/MR/MT), it never emits Flow Control frames regardless of its CFC setting, so multi-frame Mode 09 responses will stall even though the same adapter works fine when actively requesting
Some vehicles use a different NODI value for multi-VIN-capable modules (e.g. trucks with multiple VINs across networks) — don't hardcode an assumption of exactly one VIN block
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