5 speed transmission engine will rev up stay reved up while driving & in neutral on 2005 Subaru Forester

I have to turn it off to make it stop the rpms going past 4000

Asked by for the 2005 Subaru Forester
Are there any codes available to help with.
Well I have the check engine light on with the cruise control blinking and the code read P0420.The mechanic told me the front left and front center converter was going out and the code for that was the 420. The engine reving in neutral just started a few days ago with no new warning signs
the convertor code shouldn't be the cause of the high RPM, but in the real world I have seen a lot of things effecting something you wouldn't think of until you run data a the floe chart. Is yours throttle by cable ,or by wire.
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