what would cause stang to run lean???

Possibly the 24's, Try some 19's.

check fuel pressure first. then check O2 sensor. it may be bad, making car run lean.

If the EGR system is malfunctioning, it can allow one to run lean at part throttle, which can toss a lean code.

A vac leak can cause a code too. Those are easy, cheap checks (vs lambdas).

Good luck.

I had code 91 and it was a big vacuum leak. My wideband showed me running very lean. Could also be the eec and maf not working together correctly causing your issue.

So how would you fix that problem?

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.

The following is a Quote from Charles O. Probst, Ford fuel Injection & Electronic Engine control:
"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 400 Millivolts. 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 600 millivolts. 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." End Quote

Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. You'll have to pull the passenger side kick panel off to gain access to the computer connector. Remove the plastic wiring cover to get to the back side of the wiring. Use a safety pin or paper clip to probe the connections from the rear. 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.

Most of the common multimeters have a resistance scale. Be sure the O2 sensors are disconnected and measure the resistance from the O2 sensor body harness to the pins on the computer.

Note that all resistance tests must be done with power off. Measuring resistance with a circuit powered on will give false readings and possibly damage the meter. Do not attempt to measure the resistance of the O2 sensors, it may damage them.

The O2 sensor ground (orange wire with a ring terminal on it) is in the wiring harness for the fuel injection wiring. I grounded mine to one of the intake manifold bolts

CODE: 32 (KOEO) - EVP circuit below minimum voltage. Vref missing or broken wire or bad contact in circuit. Check for 5 volts on the orange/white wire. With the sensor removed from the EGR and still connected, press the plunger and watch the voltage change on the brown/lt green wire. Check for the same results on pin 27 on the computer. Check for continuity between the wiring at the sensor and the computer. The brown/lt green wire on the sensor & pin 27 should be less than 1 ohm. The black/white wire on the sensor & pin 46 should be less than 1 ohm. The orange/white wire on the sensor & pin 26 should be less than 1 ohm.

Do an advanced search on username "jrichker" and the keyword "code". You'll find the tests and fixes for almost any code you find. The codes you mention are common so won't take long to figger out...

Guaranteed it's not the injectors...

G'luck

EDIT: aw crap, he beat me to it...