Basically the cylinder balance test turns off injectors one at a time and then the EEC checks the rpm drop. After all the cylinders have cycled through it checks all of the rpm levels and if one cylinder or more make no significant decrease in rpm then it flashes the cylinder number on the code reader to which cylinder it thinks should be checked.
To start the test you have to go WOT after the cylinder ID test flashes on your code scanner. I'm not sure this test works for all years but it did for mine
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