Code 51 AC signal on; IDL switch open (part of the TPS). This code will only appear if the IDL switch is open (ie: you're stepping on the throttle in diag mode or the TPS is not properly calibrated) or the A/C system is on or says it is. When the cause of the code is corrected it will automatically remove itself, no need to reset the ECU.
There's nothing amazing about these codes being present. Check electrical connections and wiring and then refer to the TSRM for data on how to test the sensors.
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