OBD Code 23 & 56?

Just did the key diagnostics. What's a MAT sensor? manifold ambient temprature? Meaning my tb is dirty?

And 56? No code for that in my Chilton book.

HELP!!!

The MAT (manifold air temperature) sensor is the sensor that reads air temp. in the intake manifold. It's a brass looking thing that's located in the manifold behind the TB and it has a connector with two wires going into it. Make sure the connector is attached properly. If it is and you're still getting code 23, replace the sensor.

Well, I'm not sure what the code 56 is, and I can't find my Haynes book to double check for ya, but if you had a brain fart counting, then it may have been code 55, which means the end of code sequence.

My guess is miscounted flashes as well - 55 is 'end of codes' although there are a few higher numbers
http://allpar.com/fix/80s-codes.html
61 "Baro" sensor open or shorted
62 EMR mileage cannot be stored in EEPROM
62 PCM failure SRI mile not stored
63 Controller cannot write to EEPROM
64 Catalytic converter efficiency failure
65 Power steering switch failure