OK, that makes sense to me. My "mechanic" is telling me that the fuel pump pressure is too high and flooding the engine. Wants to replace the pump for about $600. From what I have read, there is a pressure regulator on the fuel rail that should take care of high pressures. My gut feeling is that the o2 sensor(s) are the cause and not the fuel pump.

Does the p0171 code mean that the controller is ADDING more fuel to compensate for what it believes is a lean condition possibly caused by the o2 sensor input?

Do I check the o2 sensor BEFORE or AFTER the catalytic converter? Or both? Do they operate the same?

Did some more testing and here are the results.

CHecked upstream O2 sensor (single wire)
- ignition ON, not running sensor voltage is 405mV.
- Start engine, 380mV at idle, engine hot. Runs good. Ran rpm up to 3000 for about 30 seconds then readings start to drop to 40, 60, 100 etc then rpms drop and engine runs rough with lots of black smoke out the exhaust. Reduce rpm to idle and readings return to 380mV. Engine smooths out.

Checked downstream O2 sensor (4 wire)
- ignition ON, not running sensor voltage is 430mV measured at the purple/wht wire in the engine compartment harness.
- Start engine, 830mV at idle, engine hot. Runs good. Ran rpm up to 3000 for about 30 seconds then engine rpms drop and starts running rough with black smoke out the exhaust. No real change in sensor voltage (850mV)

Please advise on where to proceed. Many thanks.