The following is a straight cut and paste from http://www.fuelsaving.info/chipping.htm
"Special mention should be made here of the devices that are connected to the engine ECU's air temperature sensor plug. These are sometimes referred to as "performance chips" by the people who sell them, though in fact they are nothing of the sort. These devices are just ordinary resistors, worth about 2p, which "work" by fooling the ECU into thinking that the intake air is much colder than it really is. These devices have two main effects:
1) Richer fuelling. On engines that use a manifold pressure sensor (not an air meter) to determine the air flow into the engine, air density is a critical parameter. Colder air is more dense, so at a particular manifold pressure there is more mass of air flowing into the engine. If the ECU thinks the air is cold, it will inject more fuel to cope and so the mixture will be richer. Unfortunately, while it is quite true that a rich mixture (around lambda 0.9) gives more power than a stoichiometric mixture (lambda 1), all engines are set to run pretty close to this optimum mixture at full load anyway. Hence adding more fuel is unlikely to be of benefit - what is likely to happen is that your fuel consumption will be terrible since the excess fuel is simply wasted.
If your engine uses an air flow meter, then air temperature has very little effect on the injected fuel quantity so the device will do almost nothing to the mixture. Even on systems with manifold pressure sensors, the lambda sensor will try and correct back to lambda = 1 and so the effect on fuelling may be small.
2) More advanced ignition timing. When the intake air is cold, the engine is less likely to "knock" . Therefore you can use more ignition advance, which would normally provide more power. (As a general rule, on most engines, the ignition timing is set somewhat retarded from optimum because of the danger of knocking). So, if you fit a device like this that fools the engine into thinking the air is very cold, it will (in most cases) apply more ignition advance, which will probably produce more power. But, it gives a very much greater risk of catastrophic knock, which can effectively destroy the engine! Normally there is a certain amount of safety reserve built in and so generally you would get away with it, but the risk is very high.
So one of these "chips" may give you a slight increase in performance, but may also seriously damage your engine. And on any modern vehicle you are quite likely to trigger the Check Engine Light / Malfunction Indicator Lamp, as the ECU will realise that the air temperature signal is implausible. Avoid!"
eBay Performance Chip Q & A
