replace pulgs with some nippon or densos, make sure there gaped to.031, the carbon build up is from sometimed timed wrong or a fule issue i would replace the fule filter plugs also.
code 24 is in the aif flow meter "mass air flow" little nipple hanging from the top of it inside the air flow meter , code 31 is the mass air flow look inside of it and youll see a black bar up and down in the middle of the unit if you cleaned it, what did you use, could have ruined it, this unit is very delicate check the ohms on iti would double check the timeing a dohc motor are a real hassle if you didnt make refrence marks or who ever did the head and timing belt change one tooth either way or two teeth can make a big diffrence. you could have knock a pice of carbon down itno the egr valve when you clean the lines and stuff, and you would never know it. there are 2 cats on the supra one is right of the turbo and the other is not to far after it, the first one could have melted doen from bad plugs and wires or any number of things.
also dumb question for you did you plug the air flow meter back in ive done this and the car will start for 2 secondes then die????
but im guessing that you might have messed up the air meter like i said do a ohm test on it and this will tell you right off.
when you refered to the "signal" question what is it this is what a particular sensor is telling the computer.
check out these items i have said and let use know what happens if you need more help email me at
ryanweaver1977@yahoo.com
good luck
o-ya why did you start to tear into the supra if it was allready running fine, and what kind probs did the previous owner have?