str8 from an ebay guide:
These items have sprung up like weeds in any automotive parts search: Speed Mod Chips. They claim to boost HP by 10-20 (more recent ones claim 70+ - unlikely) and not void any warranty.
EDIT: Similar devices are being sold as "gas savers". Just as with these purported "chips", they are nothing more than resistors and will do little to nothing to help improve fuel economy or performance. Only a properly maintained and tuned engine, properly inflated tires, good quality fuel, and sensible driving habits will improve your vehicle's fuel economy.
WRONG
These aren't "chips" at all, only a resistor. Many of these "chips" are plugged into the IAT, or Intake Air Temperature Sensor connector. The IAT is a Thermistor that changes a reference voltage to the engine's computer (usually 5 volts). This voltage is translated onto a "temperature" and the computer will increase or decrease fuel delivery according to this temperature. Cold air is more dense and requires more fuel, hot air requires the opposite.
This "chip" fools the computer into thinking the intake air is colder than it really is and consequently feeds more fuel into the combustion chamber. While many sellers state that the chip keeps fuel delivery within a certain "curve", this information means nothing. The modification essentially forces the engine to run rich all the time. The idea is more fuel equals more power. Unfortunately, rich running conditions will actually rob the engine of power, as the air/fuel ratio is far from ideal and will make combustion more difficult.
Today's computer controlled engines rely on a battery of sensors to help it run properly. If one sensor does not operate properly, the "runability" of the engine is adversely affected. In the case of a constant rich condition, the oxygen sensors will recognize that there is unburned fuel in the exhaust (due to poor combustion from the bad air/fuel ratio). As a result the PCM will command the engine to run lean in an attempt to correct the condition. (if the mod chip doesn't cause the engine to run OVERLY rich, the computer can negate all effects of the "chip" by LEANING the fuel ratio, making the mod useless!). If the rich condition cannot be corrected by leaning the ratio, guess what? You get a big, fat CHECK ENGINE light! (They claim the mod won't trigger that, either!) And if your vehicle is under warranty and the dealership finds that stupid little resistor where your IAT should be connected, you can KISS YOUR WARRANTY GOOD-BYE!
EDIT: Any "gas saver" that tricks the engine into running leaner can also be negated by O2 sensors that detect a lean condition. The PCM can and will attempt to richen the mixture in an attempt to achieve that ideal air/fuel ratio. If the PCM cannot correct the condition within certain parameters then the check engine light will be triggered and lean codes set.
If your vehicle is out of warranty and you ignore the light, more severe damage can occur. The catalytic converter cannot cope with excessive amounts of unburned fuel and after a while of running rich and dumping raw fuel into the catalyst, the converter will be permanently damaged. Raw fuel and poor combustion will also foul the plugs and foul plugs have high resistance. High resistance can damage the ignition system! Lastly, raw fuel can wash down and glaze the cylinders, and it can wash out the oil ring on the piston which will negatively affect piston lubrication and will cost compression.
The bottom line is this: The "mod chips" DO NOT boost horsepower - the graphs they show are fake. They DO set the Check Engine Light. They DO void the manufacturer's warranty. And they CAN cause severe engine damage. Anybody considering this cheap performance enhancing option should think again and invest in a real, professionally programmed performance enhancing chip that REALLY adjusts fuel and timing curves. These pro chips cost more money for a reason: they work. These cheap eBay "mod chips" DO NOT.
Source:
http://reviews.ebay.com/ebaymotors/...ot-Gas-Savers-quot_W0QQugidZ10000000001478993