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If you are going to replace the IAC you do not need to replace the ECM as well, however, you WILL need to perform the Idle Air Volume Learn procedure.
DESCRIPTION "Idle Air Volume Learning" is an operation to learn the idle air volume that keeps each engine within the specific range. It must be performed under any of the following conditions:
PRE-CONDITIONING Before performing "Idle Air Volume Learning", make sure that all of the following conditions are satisfied.Learning will be cancelled if any of the following conditions are missed for even a moment.