lambda control doesnt necessarily mean its the O2 sensor, it just means the sensor thinks your AF ratio is outside the normal operating range. this can signify fuel system or vacuum leaks. the condition of your spark plugs may be able to narrow that down.
although if the previous owner followed normal replacement intervals, you are a few thousand miles (or maybe much more) past due for a new one anyway, so id replace it.
i would unplug the ECU and put the battery on trickle charge while you change it, so when you start the car up again the computer will relearn from scratch rather than trying to adjust to a radically different input signal.
Alusil, Dinan DME | ITG Air Filter | Eibach/Bilstein HD | 26/20 Swaybars | Iridium plugs | Depo/ProLumen HID | Optima batt. | no AC | Stoptech brake lines, Frozen rotors, brass bushings, Superblue