 O2 sensor problem, or ...?
Hello,
I have a 1999 Escort that has had the engine replaced after almost 190K miles. About 1000 miles after the motor swap the Check Engine light came on and my ScanGauge II code reader identified a P1131 trouble code. The engine seems to be a little slow in responding to throttle and doesn't idle smoothly, but at highway cruise speeds everything seems OK, and no major drop in fuel economy.
The information I've come across so far indicates that P1131 is related to fuel mixture or the upstream O2 sensor. This makes sense, as the ScanGauge also indicates an OPEN LOOP fuel system condition the vast majority of the time - which I understand is not as it should be. Occasionally I'll see it revert to CLOSED LOOP, but it doesn't stay there long.
The O2 sensor harness seems to be clean and free of any obvious problem. Is the next step a new sensor, or are there other things to check that might cause these symptoms? | 
