CAL ID: the name of the program
The calibration ID is the name the carmaker gives the program on a module, such as the engine computer (California BAR). Cars with several modules, or modules with several software parts, report several CAL IDs.
CVN: a code calculated from the program
The calibration verification number is calculated by the module from its own software. BAR’s engineers describe it as a number set by carmakers to verify the integrity of the vehicle software (BAR, 2019). A modified program produces a new CVN that differs from the original (BAR).
Who requires it
California’s OBD II regulation requires 2005 and later cars to calculate a CVN for their emissions-related modules (13 CCR 1968.2). In 2018 GM had dealers reprogram 2018 Equinox and Traverse engine modules that didn’t report the required CVN (GM service update 18126). European OBD rules don’t require a CVN (ICCT, 2016), so cars built for other markets may not report one.
How to read them
Both come over the OBD-II port in the “vehicle information” service, Mode 09: PID 04 returns the CAL IDs and PID 06 the CVNs, in the same order (Mode 09 in practice). Any ELM327-type adapter can ask for them. Our check does it from the browser and lists every pair.
CVN mismatch: what it means
If a car reports the same CAL ID as other cars but a different CVN, it isn’t running the same program. That’s what California’s smog check looks for: a car whose software doesn’t match the factory or an approved configuration fails (BAR).
A match is weaker evidence. A car flashed back to stock reports the factory CVN again, and some tuning tools adjust changed software so it reports the original CVN, a step often called a CVN fix (tuningblog).
Where the factory values come from
Carmakers don’t publish them. California’s smog program keeps its own list, which isn’t public. We build ours from checks: for each CAL ID, the CVN most other cars report is treated as the factory one. How we decide.