As others have said, getting the cat fully warmed is key.

In my experience, the O2 sensor has been a top culprit for bad scores on injected cars.

I have also found significantly different results from one testing station to the next, FWIW.

Once got a bad score when I KNEW the motor was in top shape. I had recently done a substantial rebuild, and gone over everything. Went up the street, and got an excellent score.
The state recently launched an investigation into the accuracy of the tests.

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1986 300E 5-Speed 240k mi.