Got some probs with some codes

In my 96 GT I have codes 171 and 174. They went away with a new maf replacement once before but now they are back. Fuel pressure is good, maf is good, pcv and hoses all good, sprayed carb cleaner (the flammable kind) around the intake manifold gaskets and nothing. I don't hear any sounds or anything else. The last time I had these codes the car still ran alright until I replaced the maf. This time the car cuts out on acceleration and all throughout the driving range. Basically, it is not drivable. Now I have noticed that before these codes came on that when I turned my defrost on that the rpms would jump up to around 1k back down to around 550. If I turned the defrost off and changed it to another setting there were no problems. I replaced the intake a few years ago and I know that it is common for them to leak. The intake I purchased came with these gaskets already on it. Does anyone else have any ideas before I go and yank the intake off to replace the gaskets with something else? I was unable to detect any leakage from it with carb cleaner but I am officially stumped.

Also, if I try to drive it, the cel light flashes at me.

I have replaced the IAC about 10k ago. I have not replaced the o2's. I have roughly 180k on motor. I considered the possibility of bad o2 sensors and have not ruled them out. I thought it would have to be both upstreams if that was the case, but, I have found that in cases like this the o2 is usually just the messenger. I was just driving the other day and when this started and at first when the cel came on, the car still ran fine but later on it started with the cutting out and flashing cel. I am attending school (ha ha even though I'm 33) and will replace those this weekend. I think I would rather try that first before I take the intake off due to time constraints.

I did check that out. I didn't find any leaks or anything that would indicate a problem as all connections were solid. I know that there is a vacuum line that runs into the firewall into the cabin and was able to trace it to a connection behind the glove box. No dice, it was good.

Randy may be right as the IAC is an Autozone brand. However, the o2's, all four of them, I'm sure are probably original. The only issue I have with the o2's going out is that seems odd that more than one would/went out at the same time. If I just got code 171 or 174 by themselves, I would immediately suspect an o2 if I knew all the things I mentioned earlier were good, which they are in my case. I tend to believe the problem is sensor related, one that governs both engine banks since I can't find a vac leak anywhere. I don't know what the issue with the defrost is except to guess a bad head unit. The defrost issue has been going on for almost eight months without ever kicking a code, so I don't think they are related, but I'm not entirely sure. I have the next four days off so I'll be tearing into the car some.

Not the IAC either. The car runs fine and idles fine until I drive it and then it cuts out and when I park it, it still idles fine. I think I will test the vacuum. I tried to move the vehicle to a different place in the driveway after the motor was well warmed and it struggled to move forward on its own power. Took quite a bit of pedal to get it to move.

Thanks to all who replied: Problem Solved
A vacuum test revealed a sticking egr valve, hence why it cut out. So, Autozone strikes again! Lesson learned from those rat b!@tards. I replaced it with one from Bonner Springs Ford and the code went away and the car runs just like it used to. So for anyone who has both po171 and 174, don't forget to check the stupid egr!!!

I did it myself. I can tell you how if you have a vacuum gauge. When vacuum was pumped to 15hg at the egr valve, the rubber bladder would hold pressure and not slowly bleed down, which it should have after a minute or so. This was done with the motor off. With it running, I had someone hold the tach steady at 2k and pumped it up again to 15hg, which should have made the motor start to idle poorly, which it didn't. It was stuck. a bad egr was the problem and a new one fixed it. I stated earlier in the post that I had tested many things in the process of elimination. The only thing I had left to test if it turned out not to be the egr would have been the o2's. This doesn't mean this is what is wrong with yours but definitely something to consider since the first thing everyone always suggests is an intake leak, and then a rotted hose. Most often this is correct but if it is not any of those things then you have to start troubleshooting .

I did it like this:
-checked the pcv valve and vacuum/emissions hoses
-using a flammable carb cleaner (check the can. they do have non-flammable) I checked for air leaks in and around those hoses, all around the intake manifold, the throttle body mounting areas and the galley area under the intake (I also used propane to try to locate)
-tested the fuel system pressure and looked for leaks, including listening to see if injectors were working (hold a screwdriver and place the tip on the injector and put your ear to the handle end and listen for a clicking....this just means they are working, they may still be clogged)
-checked the maf
-checked the IAC
-checked the egr, where I located the problem at.

Good Luck and let me know if you need any information.