What you need to know first is the fact that the car is running RICH or LEAN. Measure out the voltage (digital multimeter) from the O2 sensor while the engine is running. When its running lean, the voltage is between 0 and 200 mV. When its running rich, the voltage is 500 to 750 mV.

If the engine is not running and the O2 sensor is cold, the voltage is ca 550 mV (ignition on).

With a normal working injection system, the ECU regulates the mixture between lean and rich, with a frequency from about 1 sek. This gives the optimal working conditions for the cat.

If the O2 sensor is cold, it gives no signal. This is the reason that the O2 sensor is heated elektrically by a resistance. The green wire is the signal to the ECU, the connector with two black wires in it is the heating. To measure the signal, connect the + from the multimeter to the green wire, the - terminal to ground.

After the engine is running, the O2 sensor needs about 30 to 60 sekonds to heat. That means that the mixture from a cold engine is not regulated by the O2 sensor !

First things to suspect by this kind of failure :

- Fuel injectors clogged

- AMM out of spec

- Fuel pressure regulator

- Air intake leak

There is a cheap way to force the engine running richer, I did it two years ago, and still no check engine light !