(Applies to Analog Narrow Band EFIEs Only)

The oxygen sensor gives it's information to the vehicle computer by putting a voltage on it's output wire. The voltage it produces is between .0 and 1 volts when measured against ground. However, when you're reading this voltage with a multimeter, it will show you a range of voltages of approximately .2 to .7 volts. The computer is able to read this constantly changing voltage, average it out, and adjust the fuel/air mixture accordingly.

Oxygen sensors have a warm up period before they operate properly. Some sensors just use the heat of the exhaust pipe to warm them. Others have an extra heater wire (and sometimes a separate heater ground too), used to heat the sensor more quickly.

Prior to the sensor becoming warm, it puts out an illegal voltage. The computer sees that the voltage is incorrect, and will ignore the data. The computer is said to be in "open loop". After warming up, the sensor will resume normal operation and will put out voltage as described above.

The EFIE has 2 test ports that you can plug the leads of your multimeter into. They are colored red and black, and you should plug in the leads with the same colors (for circuit board models, the red port is the same as the white wire, and the black port is the same as the green wire). When your leads are both plugged into the EFIE, you are reading only the voltage offset being produced by the EFIE. You are not reading what the sensor is putting out, nor what the computer is reading. You are only reading the voltage that the EFIE is adding to the oxygen sensor's output, before it gets to the computer's input. Note, that if the EFIE is turned off, then this reading will be 0 volts, as the sensor will be connected directly to the computer, and the EFIE will have no effect whatsoever.

So that is how you set the voltage that the EFIE is adding to the sensor's output. Both leads are to be plugged into the 2 EFIE test ports. To test the voltage that the oxygen sensor is putting out, attach your red meter lead to the EFIE's black test port, and your black meter lead to vehicle ground. If the car is running, then you should see a constantly fluctuating voltage between .2 and .7 volts on your meter. If you were to hook up an oscilloscope or other high speed testing device, you would see that the voltages actually range from 0 to 1 volts or so.

To test the voltage that the computer is receiving, plug your red meter lead into the EFIE's red test port, and attach your black meter lead to ground. Note that this will read exactly the same as the previous test if the EFIE is turned off. If the EFIE is on, you will read the voltage the oxygen sensor is producing plus the voltage being added by the EFIE. Lets say you set the EFIE to .25 volts. Let's also say that when reading the oxygen sensor in the previous paragraph, you saw a fluctuating voltage between .2 and .7 volts. When reading the computer's input voltage you would then see a constantly changing voltage in the range of .45 and .95. This is due to the .2 to .7 volts the sensor is producing, plus .25 volts that the EFIE is adding.

In a practical sense though, you should never need to take these last 2 readings, unless you suspect that the sensor is malfunctioning. The only measurement you need to take is between the 2 test ports, so you can see what voltage offset the EFIE is producing.