Hi. I have a 1995 Dodge Neon with 70,000 on the (second) motor (2.0L SOHC) and 180,000 on the body. Since this new motor was put in (about three weeks), the car repeatedly pops the Check Engine light for a code 51, " O2 sensor detected engine running too lean". Car performance is fine, but mileage takes a huge drop. I have checked everything over, and the mechanic who installed the new motor checked it over too. Neither of us can find what the problem could be. The car runs fine for a trip (5-20 miles), but pops the code when you start it up for the next trip unless you reset the computer. The only other problem I have noticed is a rough idle from time to time, and a little sluggishness when accelerating, but this is usually only after I reset the computer. Any suggestions would be helpful. I do not want to waste the money on a new O2 sensor just to find out it is something else. Thanks in advance for any help.