fast efi problem

Pishta is on the right track suggesting the O2 sensor and grounding. I find grounding ECU at engine is a good thing, grounding to battery -, not so good. It has to do with starting current, voltage drop in cable between battery and engine. Since many sensors including O2 and temperature are engine grounded.

The O2 sensors are much different, for wide band and narrow band. Narrow band is very high impedance, so undesired loading, or induced electrical noise can be a problem. Oil or mud on outside of sensor can make it not work. Many use wire insulation to provide oxygen reference.

Wideband sensors are more complex, and have circuit to feed sensor, and complexity in interface. If your sensor reads 00, significant problem may exist. Location of sensor is critical for reliable operation. Sensors are easily damaged, by silicones, lead in non seize, moisture...