Got this from a tuning doc I use
IAT
The IAT is less sensitive to cold start issues. You can add more temp to this signal than you can to the CTS. Just keep in mind that you are not only lowering your lean-out limits, you are also retarding your ignition timing. If you put a timing light on the engine as you adjust IAT values, you won’t see the timing change. The timing changes under load. Hotter air is more prone to detonation. This is why the ECU retards the timing.
If you are tuning on the hottest day of the year, you may find out just how high of a signal you can generate before setting codes. Typically it is in the 240° F range. If you are tuning in the middle of February, then you can offset the signal from your base cold reading and things will be fine for now. Come June or August, this setting may be high enough to trip codes. Allow for this when tuning.
the MAP sensor sees a given load, the TPS sees a corresponding throttle angle, the CTS sees a normal operating temperature, and the O2 sensor is saying the engine is too rich, the ECU will comply…to a point. When the ECU has leaned out the AFR (Air/Fuel Ratio) beyond what the programming claims is an acceptable range, the ECU will go into Open Loop and ignore the O2 sensor. It then reverts to Look-Up tables for its source of information. At this point, mileage will invariably go down, and often a trouble code is set.
Consider the conditions needed for the ECU to accept lean fuel commands. If the engine is warmer than it actually is, the ECU will accept leaner. If the engine is under less of a load, the ECU will want to deliver less fuel. If the incoming air is hotter, the ECU will accept lean commands more readily. If MAF (Mass Air Flow) sensor equipped, less air entering the engine will require less fuel.