Sims 4 "Unable to start" after adding mods? How to find the bad mod
Quick answer: it's usually one mod, often a script mod that's out of date after a game patch. Update your script mods, clear localthumbcache.package, then find the bad file with the 50/50 method: move half your suspect mods out, start the game, and keep halving the half with the problem. ModNest's Find Bad Mod does the halving for you.
What usually causes it
- Out-of-date script mods: after a game patch, script mods made for the old version can stop the game from starting. This is the most common one.
- Duplicate script mods: two versions of the same mod in your folder.
- A broken download: a file that didn't download all the way or got damaged.
Try these first
Update your script mods. Check each creator's page for a version made for the current patch.
Clear the cache. Delete localthumbcache.package in Documents\Electronic Arts\The Sims 4. The game makes a new one.
Look for duplicates. Search your Mods folder for the same mod name twice and keep the newest.
Test with no mods. Move your whole Mods folder to your Desktop and start the game. If it starts, it's a mod. If not, repair the game in the EA app.
The 50/50 method
This finds one bad file even in a huge Mods folder, because you split the suspects in half each time.
Close the game. Make a folder on your Desktop called Mod Test.
Move half of your mods (or half of the ones you added recently) into Mod Test.
Start the game. If it starts, the bad mod is in Mod Test. If it doesn't, it's still in Mods.
Take the half with the problem and split it in half again. Repeat until one file is left.
Move the good mods back, and keep the bad one out until its creator updates it.
| Mods to check | Launches needed (about) |
|---|---|
| 100 | 7 |
| 1,000 | 10 |
| 5,000 | 13 |
Tip: pick only the mods you added since the game last worked. Fewer suspects means fewer launches.
Let ModNest do the halving
This is exactly what Find Bad Mod in ModNest does, without you moving files around by hand:
Close The Sims 4 and open ModNest.
Click Find Bad Mod and pick the last day your game started fine.
Click Start search. Each round, launch The Sims 4, wait for the main menu (or the error), close it, and tell ModNest what happened.
ModNest moves the bad file out and puts everything else back.
Questions
Why does The Sims 4 say "Unable to start" after I add mods?
Usually one mod is the cause: most often a script mod that doesn't match the current game version after a patch, a broken download, or two copies of the same script mod.
How do I find which mod is breaking The Sims 4?
Use the 50/50 method. Move half your suspect mods out of the Mods folder and start the game. Keep halving whichever half has the problem until one file is left. About 1,000 mods take around 10 launches.
Should I delete localthumbcache.package?
Yes, it's safe. It's a cache in Documents\Electronic Arts\The Sims 4, and the game makes a new one the next time it starts. Clearing it after updates or big mod changes helps with old pictures and odd loading problems.
The game still won't start with no mods. What now?
Then it isn't a mod. Repair the game in the EA app (or Steam's Verify integrity of game files), and make sure your game is fully updated.