Idle control with 93 jeep GC 6 cyc engine

[William Patterson]

1993 Jeep GC with the 6 cylinder engine. Had problems with passing smog. Replace the O2 sensor and then the Cat before it then passed with flying colors. The Cat was completely empty inside! I then starting getting a fast idle when starting….about 1500 rpm when idle should be 750. I did also replace the idle motor that controls the air fuel mixture into the throttle body which also controls the idle speed. That seemed to fix it but now it is back.
Does not do it all the time and eventually slows down to normal after running awhile when driving. Also the engine seems to not idle as smooth as it always has. Engine Codes since the check engine light also comes on are: 12, 21 and ending code 55. That says the O2 sensor is not flucuating which means that new sensor must be faulty?? The 12 code says something about no battery to the PCM in the last 50 trys or some strange thing.
Does any of this give you a clue as to what is wrong? The battery and the connection to the PCM all seem tight and fine. The only other thing I can think of is that the new O2 sensor has gone bad. Any ideas?

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[Jason White]

Check for vacuum leaks especially between the throttle body and cylinder head. Also check the ECT (engine coolant temp) sensor and be sure it is operating properly. Some vehicles the engine temp sensor for the gauge on the dash and the one for the ECM is different.

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