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How the check works

Every engine computer keeps two codes about its own software. We read them over the OBD port and compare them with other cars running the same software.

Two codes every module keeps

The CAL ID (calibration ID) is the name of the program the carmaker put on the module. The CVN (calibration verification number) is a code the module calculates from that program. California’s Bureau of Automotive Repair puts it simply: a modified program produces a new CVN that differs from the original (BAR).

California’s OBD II rules require 2005 and later cars to calculate a CVN for their emissions-related modules (13 CCR 1968.2). Carmakers take this seriously: in 2018 GM had dealers reprogram 2018 Equinox and Traverse engine modules that didn’t report the required CVN (GM service update 18126).

Any OBD-II adapter can ask for both codes. They come back as one pair per software part, and a car often has several modules that answer.

How we decide

Carmakers don’t publish lists of factory CVNs, and California’s own database isn’t public. So we use other cars. For every CAL ID, the CVN that most other cars report is treated as the factory one.

  • Each car counts once, using its latest check. We tell cars apart with a scrambled form of the VIN, never the VIN itself.
  • Only checks read with an adapter count. Numbers typed in by hand are compared, never counted.
  • Cars their owners tell us are tuned are never used as a reference.
  • We only judge a CAL ID once at least 3 other cars have reported it and at least 80% of them agree on one CVN.

That gives four possible answers for each software part:

  • Same as the other cars: no sign of tuning found.
  • Different from the other cars: the program isn’t the one they run.
  • Not enough data yet: fewer than 3 other cars with this CAL ID so far.
  • Other cars disagree: no single CVN is common enough to call it the factory one.

Why a match isn’t proof

  • A car flashed back to stock software reports the factory CVN again.
  • Some tuning tools adjust the program so it reports the original CVN, often called a CVN fix (tuningblog).
  • Carmakers can still see some of this. Dodge says a reflash leaves a permanent mark even after going back to stock (Motor Biscuit), and that record isn’t something an OBD adapter can read.

That is why the best answer we give is “no sign of tuning found”, not “not tuned”.

What else we read

  • The VIN, to show the make, model and year from NHTSA’s public decoder.
  • The check-engine light, and stored, pending and permanent fault codes. Permanent codes stay until the car’s own test passes again.
  • Readiness tests: which emissions self-tests have finished since the codes were last cleared.
  • Warm-ups, distance and engine time since the codes were last cleared. A small number means someone cleared them recently.

Is it safe for the car?

The check only sends “show me” requests. It never clears codes or writes anything to the car. Do it parked, with the ignition on. Never run it while driving.

Your data

We keep the CAL IDs, CVNs, make, model, year and the fault-code summary, so the next car can be compared with this one. We don’t keep the VIN. Privacy details.