Alright here's my problem. I just rebuilt my engine and have been running it for about 500 miles. When I put the engine back in my car I noticed I had one sensor on the engine that had no wire and no where to plug in to the EFI wiring harness. Yes it is the head temperature sensor. Now I don't know if my dad mistakingly replaced a plug that was originally there with an actual head temp sensor, but I dont think there was ever a head temp sensor there. I dont think my year car has head temp sensors. Its a 91 GXE Automatic... So what did I do to find out? I went to a nissan dealer and they let me borrow and make copies of a 91 Maxima service manual. Well I looked and looked and talked to a technition about my head temp sensor and how I did not know where it pluged in at. Il be @*##! if I could find anywhere in that 91 service manual, anything about a head temp sensor. Acording to the manual it did not even exist on a 91 Max. I looked at the EFI wiring harness diagram and It only showed an engine temp sensor which I do have hooked up correctly and is located on the water pump pipe. So I thought my problem was solved, there was no such thing as a head temp sensor on a 91 Max and I could drive off happy with no worries. Well not realy because now after I have been driving my car about 500 miles whenever I go to put my car in park it either idles to fast or too slow and sounds like its going to flat line.... So I just did a ECU diagnostics and you know what it said...
Head Temperature sensor
Exhaust Gas Sensor

yes thats right, il be @*%$ if I could find where the head temp sensor is to plug in, its not in the manual at Nissan so why the hell is my ecu saying there's a problem.
The exhaust gas sensor is not a problem to me I will just replace it, but why is it saying head temp sensor if they do not exist on a 91???

Somebody help im going crazy