We need to get a look at the 5v source that feeds your MAP sensor and output voltage of the MAP to get a better idea of what's going on. If your scanner (either one) has the capacity to directly read sensor voltages, check your key-on reading of the MAP sensor to begin with. If your altitude is near sea level, expect to see something around 4.5 volts, but here in mile-high Denver we have to make do with 3.7v. The code sets if key-on voltage is between 2.2v and 0.4, neither of which should be possible with the engine off. Voltage drops with pressure, as you noticed.
The 5v circuit is colored yellow-pink and it should show 4.8 to 5.2 volts. This stream should be steady, not changing if the engine is started or whatever. It's shared with a number of other sensors, so if it's low, we'll need to start unplugging them and rechecking voltage. The others on the same circuit are: EGR position sensor, trans pressure sensor, accelerator position sensor (the pedal assembly) and the camshaft position sensor. A separate 5v output serves the remaining sensors, which is an improvement over the way they used to do it ... one feed went to all and if it got shorted, the PCM went to sleep.
If OK (5.2 to 4.8v), check your MAP signal voltage with the key on and the sensor plugged in (wire color violet-brown). Realistically, anything lower than 3.5v if you live below 6000 feet is just plain wrong.
And last (for now), take your air cleaner duct off at the throttle body so you can get a better look at the throttle body blade. Make sure nothing has gotten stuck in the blade, like a chunk of air cleaner foam seal or something of that sort. ETC does a self-test at every key-on and if it doesn't like what it sees, you'll likely lose ETC function for the whole key cycle.
Then turn the key on. The throttle valve should do a quick dance, opening and closing as it does its self-test. You can hear something going on from the driver's seat, but it would be better if you had a helper to watch the event. BTW, wiring for the ETC is more visible with the ducting removed and this is a favorite critter chew area. Look at the front and left (driver) side of the intake for signs of hoarded hood pad, leaves, grass or freaking destroyed wiring. Man, they can really make a mess of things...