2002 Sonoma P0171 (System too lean Bank 1)replaced ects, cat conv, 02 sensor, has been smoked for leaks none found, 02 Sensor is reading either 0 or 4 switching bt the two, does this just indicate lean or is it faulty. Has some black smoke running very rich, what else should I look for to test?
Two common causes of your multiple symptoms are an ignition misfire and an exhaust leak ahead of the front oxygen sensor. With a misfire, you will see the black smoke and smell the unburned fuel but the O2 sensor will detect the unburned oxygen. When the computer sees that unburned oxygen, it will command more fuel to go with it. No matter how much extra fuel there is, the O2 sensor will still be seeing unburned oxygen.
When there's an exhaust leak, between the pulses of exhaust flow, the momentum creates little pulses of vacuum that can draw in outside air. That oxygen in that air will be detected as a lean condition and the computer again will add more fuel. Regardless how much fuel is added, there will still be that air sneaking in.
Also look for a vacuum leak that lets in unmetered air that the computer doesn't know about. If that leak is close to one cylinder, the oxygen will be detected in the exhaust but the computer will add fuel to all four cylinders.