Code 13... causes???

It has happened twice already in about a 2 month period. While cruising @ 45 - 55MPH I keep getting Code 13 which relates to Oxygen (O2) Sensor-signal stays low (lean) during warm engine cruise...
Since it's pretty hot on this side of town, I usually "crank and go" while having the A/C ON all the time, but I'm not sure that would have something to do with it. Both times the SES light had come on like after 10 mins of driving giving the engine plenty of time to heat up to warming temp. The O2 Sensor was replaced about 2 years ago along with a full engine tune up... (High Flow CAT and Exhaust, Plugs & Wires, Fuel Pump/Filter & Regulator, etc, etc). Also, the EGR, Throttle and Plenum were cleaned back in November '04.
If anyone could shed some light I would appreciate it!

Unsure what you have there as far as year or motor, but on our 91 (3.1) the service manual lists possible causes as sensor, sensor connection(circuit), or ECM. Only way of testting for sure requires scan tool and good multimeter. Perhaps a simpler was known by other folks here, but I do not know of any. My bet would be ECM, or, something happened to burn out your O2 sensor. Since you replaced your CAT unsure what else would cause that. I know ours had the ECM replaced 5-6 years ago because it was giving false codes for this and EGR. They are prone to failure, a lot of times due to water leaking inside. Are you getting ANY other codes?

Sorry forgot to mention, it's a '91 3.1L v6... there are no other codes present!

The computer (ECM) may be bad, just like ours. Pretty common problem. Ours kept showing codes when we drove a certain distance, mostly EGR and O2. Since we had both replaced just weeks before I knew it was not them. The way the car is driven won't make any difference to set the codes. if you can get a scan tool and a tester I could advise what the book says to do to test, or, you might pick up a manual. Just to be sure, since they are cheap, you might consider replacing the sensor again, just is case it was defective. Check the ECM for dampness.

I would go with jsgold on this one if you don't have a code scanner I know where I live the autozone here have scan tools. I'd give them a call and see if they can scan your car for codes.