Google throttle body electrical schematic for ls430 and you will see a club lexus link to overall electrical LS430 schematic pdf. On pdf search throttle and it will bring up throttle position sensor and motor leading to ecm. If you look you have the wires and the unit and it is most likely a bad sensor inside the module. There are used and rebuilt ones available on ebay. The car is going into limp mode.

 

Let me update this. With a summary.

first of all, I appreciate those who made suggestions. I’m learning a new platform. This is our second Toyota product…. The last one being a 98 Corolla we bought new and traded a year later….. ssssoooo….
yea. New to me.

Cut to the chase, the ECM was the absolute culprit.

1- Throttle body responded to power when it was applied directly.
2- Harness tested good to ECM plug.
3- I triggered misfires while trying to find which cylinder was dead…. Never threw a code.
4- Starting the car with the pedal down revved the engine for as long as I was willing to do it… until it returned to idle.

The persistent code was the TAC voltage low. The throttle actuator motor gets its voltage from the ECM. Both power and ground seem to be managed through the ECM.

I spoke to a tech at one ECM repair shop who said he has heard of this on later model Lexus, but not the LS430…. And his shop classified it as unrepairable. So based on what I observed, I order a “new” ECM from another shop since his shop didn’t sell “new ECMs.

today, I dropped it in and followed the provided instructions. The car started and ran perfectly (and this is important) on all 8 cylinders. Wife and I just got back after a 10-12 mile ride. It runs like a champ.

the VSC service alert came back after a couple of miles, but no check engine light, no misfires and not unresponsive throttle.

this fault is fixed.
now I only need to fix:
worn suspension
misadjusted trunk latch
center console lid (well… done now)
brakes
prob an alignment
replace the speakers.