This is a rebuilt 91 ford explorer. The CEL comes on and from what the tech book says code 41 means that the heated oxygen sensor isn't switching but a simple wire test confirmed it was. The sensor was replaced with the exact same number too and the problem still persists. It seems to come on when the brake is applied after about fifteen minutes of driving. Shot the engine off and the CEL goes out until another fifteen or twenty minutes. The heated oxygen sensor switches over at 600 degrees.

Also the idle speed seems to speed up and down. Like the engine is accelerating for a second and then back down and on and on it goes. No black smoke no nothing. The main computer has been replaced too.

Any experts out there wanna give this a shot? I would VERY much appreciate it. Hell I might even pay ya!!! lol

 

It seems very likely that the O2 sensor itself is good. How about the wiring? All good wiring from the O2 sensor to the PCM? PCM ground any good? Is the O2 sensor on a '91 three or four wire? Sometimes the O2 sensor grounds through the exhaust pipe, which means that a bad engine ground will yield a bad O2 sensor ground which yields bad O2 sensor readings.
The other thing it could mean is that the mixture is really off that bad. What's fuel pressure read? MAF problems? Vacuum leaks?

 

I have a 91 explorer w/ the same problem. It seems to run perfectly, though - no idle issues. I haven't tried replacing the sensor yet, but code 41 means that the right o2 sensor is recording a lean condition. Do 91's even have a left sensor or is there just one for the both the L & R cylinders? I was thinking that if there are L & R sensors, then the sensor is probably bad. Private Citizen, did you get it fixed?

 

'91 only has one O2 sensor. I think it's '93 when Ford started putting 2 O2 sensors in.

 

Apparently, this was happening to me because a wire connector had come apart. It was the black connector underneath the wire harness connector that sits in a bracket on the lower alternator bolts. I'm not sure what else runs through this connector, but the signal for the dash temp gauge does (this should help you id the connector). I plugged the two ends together and the check engine lights are gone. Others may be having a problem w/ a sensor that runs through this connector or something. Good luck