Code 41 or 91 - O2 indicates system lean. Look for a vacuum leak or failing O2 sensor.
The computer sees a lean mixture signal coming from the O2 sensors and tries to compensate by adding more fuel.
Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. The computer pins are 29 (LH O2 with a dark green/pink wire) and 43 (RH O2 with a dark blue/pink wire). Use the ground next to the computer to ground the voltmeter.
The O2 sensor ground is in the wiring harness for the fuel injection wiring. I grounded mine to one of the intake manifold bolts
"When the mixture is lean, the exhaust gas has oxygen, about the same amount as the ambient air. So the sensor will generate less than .4 volt. Remember lean = less voltage.
" When the mixture is rich, there's less oxygen in the exhaust than in the ambient air , so voltage is generated between the two sides of the tip. The voltage is greater than .6 volt. Remember rich = more voltage.
Here's a tip: the newer the sensor, the more the voltage changes, swinging from as low as 0.1 volt to as much as 0.9 volt. As an oxygen sensor ages, the voltage changes get smaller and slower - the voltage change lags behind the change in exhaust gas oxygen.
Because the oxygen sensor generates its own voltage, never apply voltage and never measure resistance of the sensor circuit. To measure voltage signals, use an analog voltmeter with a high input impedance, at least 10 megohms. Remember, a digital voltmeter will average a changing voltage."
Charles O. Probst, Ford fuel Injection & Electronic Engine control