Question: I have a 2001 Dodge Durango 4x4 with the 4.7-liter and an automatic transmission. I received code 171, replaced both oxygen sensors, and checked the vacuum system and catalyst. The code remains and the truck is running rich and producing rich exhaust.

Answer: Trouble code P0171 indicates a lean condition, and replacing both oxygen sensors (assuming you don't have four) eliminates them from the scenario. A poor connection in an O2 circuit is still possible. When an oxygen sensor sends an incorrect "lean" signal to the Powertrain Control Module, the PCM will send a "rich" command to the injectors to compensate. But there are several other items that can produce a true lean condition. Top of the list is fuel pressure, which needs to be tested with an accurate fuel pressure gauge. It may be a bad pump, or a restriction due to a clogged filter or in-tank strainer. Contaminated fuel is also a possibility. After that you need a scan tool to evaluate incoming sensor data, such as improper TPS, ECT, or MAP signals. And, of course, it can be some other hard-to-find stuff like a hidden vacuum leak, mechanical engine problems, a faulty PCM, or poor connections in the wiring to and from the PCM.

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