OK, Truth is I actually have a 97 Mercury Tracer which just an escort under a different name. I posted here because no one has answered a Tracer question in months. My check engine light has been going off and on for months. I just got an OBD2 reader. I got code 1131 which means Lack of upstream heated o2 sensor (HO2S11) Switch, Sensor indicates lean (Bank #1). What does this crap mean? Should I replace the sensor? Any help is much appreciated.

I unplugged the top o2 sensor which is in the exhaust manifold in front and reset the codes. After driving a while I still got the same code and one for the unplugged sensor. How many o2 sensors are in this car? The one I unplugged is obviosly not the one giving the code. I can see only one more behind the motor. Should I replace or look for something that could cause the code. I have already checked all wires and connections. The weird thing is, it will come on and stay on a few days but then go away for a couple weeks. Any help is much appreciated.:shakehead

Your Tracer has 2 Oxygen sensors. One upstream in the exhaust manifold and one downstream after the catalytic converter. 9 times out of 10 the up stream sensor should be replaced when you are getting a code. If you have over 85k it is time to do it. It will increase your gas mileage 10-20%. If you unplugged the upstream sensor it will cause a code for the downsteam sensor. The two are related. If you're getting a code off and on you might check the wires to the O2 sensors. Get that upstream sensor before you do anything else.

Thanks, I will get one and see if it helps. I have not noticed any power loss or poor gas mileage but I'm sure it has the factory sensor. The car has 162,000 miles. I better hurry my inspection sticker is dead as of last month and with this problem I'm sure it wont pass NC inspection.