Uhmm.. broken o2 sensor

Probably not. The older engines are more forgiving with things like that.

you may be stuck in open loop and not even have realized. What's your gas mileage been like?

Open loop is the mode the EEC goes into during startup and when cold. The car runs on preset fuel tables and takes input from a very few number of sensors. A lot of things have to be "right" in order for the EEc to go into closed loop and vary spark and fuel tables to run as effiently as it can. Something as small as a bad ECT sensor or coolant temp sensor can cause the EEc to remain in open loop. What I don't know, is if having only one of two 02 sensors operating correctly will cuase that condition. If the motor is running strong and not killing you in gas then leave it. If you ever go to hook that sensor back up, it will likely need replacing. running that long wihtout a preheater in teh exhaust stream has surely ruined it by now.

If anything (assuming the EEC isn't able to do it's best with just one 02), you may see in increase in mileage.