The number of oxygen sensors depends on which engine you have but in general, on a car that age there will be just one. The code you described doesn't point to a sensor problem. It refers to the condition being detected, in this case, too much unburned oxygen in the exhaust system. You were right to suspect a vacuum leak but that will cause a high idle speed. The low engine speed is likely in response to something the Engine Computer is trying to do to correct the lean condition.
One of the common things to look for includes a leak in the fresh air tube between the mass air flow sensor and throttle body. That will let air sneak in that isn't measured so the computer won't add that to its fuel metering calculation. It thinks there's less air so it give it less fuel. The unburned oxygen that's really there is what keeps the O2 sensor lean too long.
Next, a misfiring cylinder due to worn spark plugs, wires, or ignition coil will send that unburned oxygen and fuel into the exhaust. O2 sensors only measure oxygen, not fuel, so they will see the lean condition again.
Finally, a less common cause is a leak in the exhaust system in front of the oxygen sensor. Between the puffs of exhaust gas flow, the momentum creates little puffs of vacuum that can draw in outside air where the oxygen is detected as a lean condition.
Saturday, July 6th, 2013 AT 2:53 PM