I just wrapped up my valve cover gaskets, got everything together and the car is throwing C1511. I took the battery back out and double checked all the steering ECU connections, everything looks tight there.
Figured it just might need the ecu re-calibrated since it was disconnected for a few days. When I try the calibration process with accessories on/engine off, I get the error "communication failed."
Tried it again with the engine on and I get the error "Torque Sensor Adjustment has failed. DTCs other than C15115/C1525/C1581 are present." I ran the health check and C15115 is the only CEL present...any advice?

Disconnect and reconnect your steering ECU. Something didn't get seated properly when you reconnected it.

It would be the forward connector on the engine side. Lexus notes it as C4.

Pulled everything apart, put it back together today, all the issues still persist.
I even followed the wiring down to the steering rack to make sure no connections were loose there. Anything else I'm missing before I have the dealership try and calibrate it?

Try to turn your steering wheel from one full lock all the way to the other full lock two two or three times.

OP that shouldn't have happened. Make sure no pins are pushed out, look close at the insulation to see if the wire(s) moved from their grommets. If you have a valid hardware and software, put a battery charger on it while doing the cal procedure as maintaining sufficient system voltage is critical.

The only other thing I would suggest is back probing the pins. I once had a problem with my Supra that drove me to the brink of insanity because the petal in the connector wasn't making contact.

Dealership is going to tell you to:

1. Replace steering ECU
2. Replace steering rack
3. Replace harness between ECU and steering rack

None of these are cheap. One (or more) of the pins below are not making the connection.

TIS says you don't have connectivity through the harness. Since you removed the steering ECU, it seems obvious the failed connection is in the connector with the gray latch (from your photo coordinating with what I see in TIS).

I back traced every wire, everything was fine. Ran techstream again, this time it's throwing B1421 and B1424, which are both related to the daylight sensor on the dash.
Looks like those codes being present are preventing me from doing the 0 point calibration. Did I mention how much I absolutely loathe working on "modern" cars?

A little bit of research indicated that driving it around (sans power steering) and getting some light to the sensor might help clear the codes. Spun it around the block twice, no joy.
I didn't touch anything close to the dash sensor....where do I go from here?

Mine always shows the solar sensors not working DTCs, but it has never prevented me from doing anything with Techstream.

Here's the results of the health check...only three codes are current so I figured those were the ones preventing me from running the zero point calibration. At this point I feel like I forgot to plug in a sensor, but the car runs and drives fine (albeit without power steering), a/c works, etc.
Not sure where to go from here. Maybe go to a junkyard and swap in another steering ECU as a test?

You can try that, but it's going to be hit or miss. If your pins are not making good contact, a different ECU won't matter. AMHIK.

Necro thread revive, just to say that Lobux was 100% right.
My electronics skills are questionable, so I ended up getting a $20 junkyard wire loom off an IS250. Took less than 20 minutes to swap it out. Techstream gave me the same connection error the first time, but was able to calibrate the steering the second time.
Oddly enough, the IS-F wiring loom has an extra ground that's missing from the IS250 version. This also seems to have solved the no-start condition I was experiencing, where I was having to hit the start button 2-4 times before it would actually fire up. I was afraid it was the starter going bad prematurely, but since changing this out it's fired up the first try every day.