From July 19, 2021, California will start testing for non-approved ECU tunes during its biennial Smog Check program.
First spotted by Car Bibles (via Road & Track), the rules state that any software not provided by the car's manufacturer or approved through a California Air Resources Board executive order will fail the test. Details of the test procedure haven't been mentioned, though it will likely be via a check of the car's OBD II port.
All hope isn't lost if you live in California and have indeed tuned your car. If you can have the original software the car came with plugged back in, you can use it to pass the test.
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