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I have heard both usages. I guess that they are the same thing but wasn't exactly sure.

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Technically no.

"Codes" are errors that are set when a problem is detected, typically affecting emissions.

"incomplete monitors" are tests that have not fully run to verify the validity of certain systems - perhaps HO2S, EVAP, catalyst, etc.

An incomplete monitor is not saying there's a problem; it's just indicating that the ECU has not been driven through a complete drive cycle to verify the validity of the tests. It doesn't pass an emissions inspection, but it doesn't fail either.

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  • However, AFAICT, an incomplete monitor on a used car is a big red flag right? It most likely means that the owner has reset stuff to prevent codes from showing up during the test drive or while scanning the OBD 2 port.
    – Utku
    Sep 24, 2019 at 17:34
  • Possibly, but not always. There are two types of codes, pending and permanent. You can "scan" for pending codes, even if there is no CEL or SES. The same effect (incomplete monitors) could be caused by replacing a dead battery.
    – SteveRacer
    Sep 30, 2019 at 23:53

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