Almost all modern vehicles employ oxygen sensors to tell the vehicle's computer if the air/fuel mixture is too rich or too lean. The computer uses the information from the 02 sensor to determine if more or less fuel should be added to the mix in order to maintain the correct proportion.
Gas vehicle engines (as opposed to diesel engines) are designed to operate at an air/fuel ratio of 14.7 to 1. When these proportions are being supplied to the engine, a certain amount of oxygen will be detected in the exhaust by the 02 sensor, and this information is fed into the vehicle's computer. If more oxygen is sensed, the computer thinks the mixture is too lean (not enough fuel), and adds fuel to the mix. Likewise, if less oxygen is sensed, the computer thinks the mixture is too rich (too much fuel) and cuts back on the fuel fed to the engine.
There's a big problem with this scenario as soon as you start adding a system that increases the efficiency of the petroleum combustion. There are many different systems that can do this, with HHO being the most popular. What happens is that for any given air/fuel ratio, burned more efficiently, the oxygen content in the exhaust will rise. The oxygen content rises as the fuel is burned more efficiently for a number of reasons. Chief amongst these are a) less fuel is being used to produce an equivalent amount of engine revolutions, and b) less oxygen is being consumed to create carbon monoxide in the exhaust. The bottom line is there is more oxygen in the exhaust as the fuel burning efficiency is increased.
Note:Many people new to the HHO industry think that the "O" part of the "HHO" is what is causing the problem. In actual fact, the amount of oxygen that is being added to the intake in a supplemental hydrogen system is so small that it is virtually undetectable by the sensors. This has nothing to do with the need for sensor adjustment. There are thousands of times more oxygen being drawn into the intake from the outside air. Consider a system that produces 2 LPM of HHO, which is more than any car needs. 1/3 of that volume will be oxygen, or .66 LPM of oxygen. An average V8 engine will draw in about 2,000 LPM of oxygen from the outside air. So adding a fraction of 1 LPM to 2000 LPM will have no effect on the sensor's signal. However, the hydrogen will cause the same petroleum fuel to now turn the engine 35% more times. This causes the oxygen from the intake to increase by 35%, from 2000 LPM to 2700 LPM. This is what is causing the problem. And that is why any other type of system that improves fuel combustion efficiency has the exact same problem.
So, now that we have spent time and money to install an HHO system, and we are getting a more efficient fuel burn, what does the vehicle's computer do? It dumps gas into the mix in an attempt to get an oxygen reading in the exhaust equal to it's earlier, inefficient setup. This will then tend to negate the fuel savings of our new system, and in some cases will actually cause an increase in fuel consumption.
The handling for this situation is simple. The signal coming from the 02 sensor needs to be adjusted to compensate for the increased fuel efficiency being achieved by your HHO system. Basically the added oxygen in the exhaust fools the computer into thinking the mixture is too lean, causing it to (incorrectly) richen the mix. We need to un-fool the computer so it stops doing that. We do this by making it think there is less oxygen in the exhaust than there actually is. Devices exist that will adjust the signals sent from the vehicle's sensors to compensate for the extra oxygen. These devices can recover your fuel mileage gains when using HHO or other fuel combustion efficiency enhancements.
EFIE: Electronic Fuel Injection Enhancer
EFIE stands for Electronic Fuel Injection Enhancer. It was developed for fuel injected vehicles, and was found to be necessary in order for other fuel efficiency devices to work. This includes virtually all cars and trucks today. The only engines that don't require an EFIE or similar device are the old engines without a computer.
Photos of EFIE Devices
There are a number of different types of types of oxygen sensor and EFIEs. When you are ready, you can find out more about the different kinds by reading Types of Oxygen Sensors.
I should also mention that there are other types of device that attempt to achieve the same goal. The most common is a MAP Enhancer that works on the MAP sensor. "MAP" stands for "Manifold Absolute Pressure", and therefore the sensor is designed to tell the computer the pressure in the manifold. MAP Enhancers are usually marketed as MAP/MAF Enhancers, which means they also work on MAF sensors. "MAF" stands for "Mass Air Flow" and measures the amount of air coming in to the intake. By modifying these sensors you can sometimes fool the computer into delivering less gas. However, achieving this result using an EFIE is preferred as it works directly on the signal that is incorrect in the first place (the oxygen sensor). Adjusting the MAP or MAF sensor is attempting to add an error in one sensor that will hopefully offset the error in the O2 sensor.
Likewise there are devices for adjusting the signal from your temperature sensors. There are usually temp sensors for the intake air and for the coolant. I don't recommend using any of these devices. I have experimented with all of them. Using them in conjunction with an EFIE, and by themselves. I have found the best results are obtained by an EFIE, and no other sensor modification.
None of the devices described in this article are fuel efficiency devices by themselves. If all you did was add an EFIE to your car, with no other fuel efficiency system, you might gain a few mpg, but you also might not. But the engine won't be able to run as it was designed to. The reason is that you are fooling the car's computer and making it run out of spec, or differently than it was designed for. The EFIE was designed to make the car run according to spec despite an increase in fuel combustion efficiency.