Hello, my troybilt 46” horse, Model 13AT78BT023, would not turn left, YouTube showed need replace steering shaft so replaced, replaced and still same problem, finally found it was grass accumulated in steering gear plate ,cleaned and turned ok, then after mowing few minutes the steering would not turn wheels, the bottom steering shaft screw came loose, tighten it up and again same happened, put old steering shaft back on and same problem. What have I done wrong ? I don’t understand why the screw at end of steering shaft keeps coming unscrewed, hadn’t done this before.

First off, clean everything in the area spotless-Grass, old grease etc.
If just one screw is coming loose, causing your issue, you can clean (brake cleaner)
the hole and bolt, then use RED Loctite. Let it set up over night.
 
Specifically, which part is coming loose:
 
https://www.jackssmallengines.com/jacks-parts-lookup/manufacturer/troy-bilt/13-lawn-tractors/2019-models/13at78bt023-horse-46-auto-2019/steering 
 
 
 
 
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#13. I’ve used blue lock tite, and let it set for 24 hrs , then I drove it for a few minutes and it stayed on , not loose so far. I will be mowing later this week after all this rain stops to see if it holds.

Part #4, a "pan head screw" appears to be what's holding it together.
 
In the picture(up close) it has a RED colored loctite already on the threads. 
If the blue doesn't stay, use the red...
 
.

I will try the red stuff if blue doesn’t work, drove it for few minutes other day and didn’t seem loose, when weather clears u and I mow I’ll see if it stays on.

Finally got out and mowed front and backyard, so far the blue lock tite is holding.