I don't know the specific part values, or where exactly they are installed. What the resistor (or possibly diode) would do is modify the sensor reading on the sensor circuit that it is installed on. The computer would see the modified reading and adjust it's operation accordingly. The most common use I could think of would be to have the computer see a lower air temp that is actually available, and make adjustments that would provide a degree of performance that wouldn't normally be avaiable at the real outside temperature. I am sure there are other examples, and better explanations.
Overall, the result may be cheap improvements on apparent performance. In the long run, the changes may create other problems in the way other circuits are responding to the false info.
In most cases, the provider would encase the component in epoxy to hide the actual value of the part used. 1/4 watt 20% tolerance resistors used to be 2 for $.25 at Radio Shack. Switching diodes such as the 1N914 or 1N4148 could be purchased 10 for a dollar. Their cost is less than a pack of Juicy Fruit, but the knowledge of where to put them is what the selller is marketing.
Remember, you have no recourse with the seller if the low cost mod causes your servicing dealer to deny a waranty claim for repairs that they determine may have been caused by the sensor reporting inaccurate info and the PCM making adjustments according to that inaccurate info.
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