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Flip the "Stop/Run" switch to Run, then hold the Odometer selector then turn the ignition on. When the needles sweep release and you should see "STSSP" one the Odometer, hold the odometer selector switch for 5 seconds to select which system to display the codes on and 5 more seconds to clear the codes for the system. Use the odometer select switch to step through each page or each system.
High Idle generally indicates that the ECM thinks that the engine is cool and is getting it up to operating temp. Check the attached page from my service manual to see what I'm talking about.