Bad ground 

Start with getting your charging system (alternator + battery) load tested. Then check your fuses and relays

The easy way is to use a good scanner to see if the scanner can see other control modules and it just isn't seeing the PCM. But if you don't have an expensive scanner which can do a full system scan of all the modules you can troubleshoot this with a $10 multimeter and a test light.

You're getting Power at the Data Link Connector because your code reader can power up. However, it can't communicate with the PCM.

So the questions are: Is the PCM powering up?  Is the PCM functioning correctly?

One easy way to determine the answer to the first question is to check if the PCM is powering up and providing the 5 volt reference to the  thermistor sensors in the engine control circuits.

And the MAF sensor is "easy pickin's" because it's right there when you open the hood and it's in close proximity to the battery.

Incorporated in the MAF sensor is the IAT (Intake Air Temperature) Sensor which is a thermistor.

You don't even need a wiring diagram for this.

So you grab your multimeter. If it's a cheap one set it to 12 volts DC (20 volt range) and attach the Black lead to the Battery Negative Post.

Then touch the Red lead to the Battery Positive Post to make sure the multimeter is working. You'll see around 12 volts if the battery is OK.

Next, disconnect the MAF sensor wiring connector. Then turn the Key to ON.

With the multimeter negative lead (black) still connected to the Negative Battery Post, probe each pin of the MAF sensor connector with the multimeter positive lead (red). 

One of those pins should show close to 5 volts on the multimeter (provided by the PCM to the IAT sensor) to let you know the PCM is powering up.

If you want to do this let us know what you find out and we can walk you through the next steps ((where the test light comes into play))