FYI, I have a 2000 Dakota 3.9 got the engine code p171 when the check engine light came on, the truck would idle rough and lose a lot of power,fuel mileage would go from 19 to 9. I couldn’t find any vacuum leaks, I did everything including changing the spark plugs. I decided to pull the upstream 02 sensor , it was completely covered in a thick black soot , replaced it and problem fixed. Never got a 02 sensor code Hope this helps someone down the road.