If you use the factory equipped oxygen sensor you must use the wire that supplies 0 to 1 volt signal to the computer. This gauge only displays a “Rich” or “Lean” indication, with the exception of 14.7 to 1. It is physically impossible to measure an exact air/fuel ratio with a standard exhaust gas oxygen sensor. Because every vehicle is different, you cannot calculate what each bar on the gauge represents.
NOTE: Do not solder any wires attached to the oxygen sensor. Soldering wires to oxygen sensor can cause sensor failure.
Connect the black wire from the gauge to a good ground, you can usually find an existing ground under the dash. We recommend using a “ring” style terminal to secure the ground wire. Be sure to check for a good ground, if you do not have a good ground the gauge will be inaccurate or not work at all. Connect the red wire from the gauge to a switched 12-volt source (12-volts with key on but 0-volts with key off). The easiest (and closest) place to find an in cabin power source would be from your radio or ignition switch. You will want to use a fused power source so that in case of any electrical problems you do not damage the gauge.
Now connect the gray wire from the gauge to the signal wire from the oxygen sensor. With a one-wire factory Toyota sensor you will splice the gray wire from the gauge into the only wire from the oxygen sensor. With a Toyota four-wire sensor you will splice the gray wire from the gauge into the blue wire from the oxygen sensor. With a Bosch four-wire or three-wire sensor the gray wire from the gauge will splice into the black wire from the oxygen sensor.