Read the FAQ.

When you unplug the AMM, look inside the female male plug - sometimes one of the pins will get pushed back and make a bad connection.

Was the car recently serviced?

This is one of the few connectors that gets unplugged during service, as one usually unplugs this to check or change the air filter.

If the pin is pushed back, try to gently pull it out of the plug by the wire - if it comes out, you will see a small tang that is supposed to keep it in place - gently bend the tang up a bit so that it catches.

One thing you can do is to peel back the rubber boot, and if the wires are all in place, run a sharpie across all the wires so that you can check to see if all are in place when the plug is seated.