2000 Dodge Dakota 4.7L with O171 Code: Poor Fuel Economy and Foul Exhaust Smell
My 2000 Dodge Dakota with a 4.7L engine has recently experienced a significant drop in fuel efficiency—down by 25%—despite having 95,000 miles on the odometer. The vehicle now averages only about 9 mpg, down from its usual 16 mpg in winter conditions. I've noticed a foul odor coming from the exhaust, but it does not smell like sulfur. The onboard diagnostic system has flagged code P0171 (Lean Bank 1), indicating that the engine control module detected a lean air-fuel mixture on the left side of the engine. My concern is that the vehicle runs smoothly with no noticeable power loss or vibration, which makes me suspect it's not due to a large intake leak. I want to confirm: does code P0171 mean the system detects a lean mixture and adjusts by enriching the fuel mix on the left bank? If not, what exactly does this code signify? Before starting any troubleshooting, I'm considering whether a faulty oxygen sensor on the left side could be sending incorrect signals to the engine control unit. Is that a plausible cause? Are there other potential sources such as vacuum leaks or issues with the intake manifold? Thanks for your insights, Mike
After receiving no response to my original post, I took my truck in for diagnosis. As expected and hoped, the issue was a failed oxygen sensor on cylinder #1 of the left bank. The faulty sensor caused the engine to run too rich on that side, which eventually led to carbon buildup and fouled spark plugs—explaining the sharp decline in fuel economy (from 16 mpg to just 9 mpg). Remarkably, the truck still ran smoothly despite this issue. I reviewed the forum's FAQs for P0171 codes and found common causes listed as a malfunctioning MAF sensor or vacuum leaks. However, no mention was made of oxygen sensors being potential contributors. Based on my experience: if it had been the MAF sensor (which is single in the 4.7L engine), I would have expected to see both P0171 and P0174 codes (lean right bank). Since only P0171 was present, I concluded the issue was isolated to the left side—likely due to a faulty O2 sensor or a cracked intake manifold.