Lean codes report the condition. There are totally different codes stored when a defective oxygen sensor is detected.
This is where you will need a scanner capable of displaying live sensor data while driving. A good starting point is to view short and long-term fuel trims. If those numbers are high positive, the Engine Computer is requesting additional fuel beyond what was programmed in at the factory. If the numbers are high negative, the computer is trying to reduce the amount of fuel entering the engine and may be not having enough control to do so.
You can also watch the switching of the front oxygen sensor. If it tends to stay in the "lean" state longer than the "rich" state, you might look for an exhaust leak ahead of the catalytic converter. In between the pulses of exhaust gas pressure, the momentum creates a small pulse of vacuum that can draw in outside air. The oxygen will be detected as a lean condition and the computer will counter that by adding fuel. Oxygen sensors just detect oxygen, not unburned fuel, so no matter how much fuel the computer adds, the O2 sensor will still be reporting a lean condition.
The same thing happens if there is a misfire in any cylinder. You might smell the unburned fuel at the tail pipe but the O2 sensor will see the unburned oxygen.
Friday, May 6th, 2011 AT 2:23 AM