Mass Air with check engine light?

I did the mass air conversion the other week and was just messing around with it, so I unplugged the meter and drove it about 10 miles, and it ran fine. Kind of confused me a little bit. The check engine light is on but they are the 91 and 47 codes I think, They are the smog pump codes. I was just wondering, I know that some cars go in "limp mode" when the check engine light is on. Is this true with the mustangs? If the check engine light is on, does the computer use sensors to compute the air fuel mix or does it still use readings from the meter? Any help would be greatly appreciated. Thanks in advance.

Might depend on what codes triggered the light. Do you know why it was on?

Code 47 - measured airflow low at base idle. Dirty MAF sensor, obstructed air inlet to MAF sensor, bad MAF sensor. If you have a CAI, try rotating the MAF sensor housing (clocking the MAF)

Code 41(Right O2 sensor) or 91 (Left O2 sensor) - O2 sensor 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

I'm curious how you got your Check Engine light working. I was under the impression that SD cars didn't have CE lights.

Im under that impression as well. Theres a spot where I can see "check engine" but its never lit up since I bought the car and Ive pulled codes.

spray carb cleaner in the egr and check engine light will be gone Leo

Where's the OP?

i have known of people wiring the CEL into an original SD car, but it seemed like a pain. i never did.