How to Fix iOS Simulator Not Starting

In this tutorial, you'll learn about How to Fix iOS Simulator Not Starting. We cover key concepts, practical examples, and best practices.

The Problem

You click Run in Xcode and the simulator shows a black screen or never launches:

Simulator unable to boot: Domain: NSPOSIXErrorDomain, Code: 60
"Simulator connection became invalid"

Or the simulator opens but stays frozen on the Apple logo.

Quick Fix

Step 1: Force quit and restart the simulator

killall Simulator
open -a Simulator

This kills the simulator process and launches a fresh instance.

Step 2: Erase all content and settings

In the Simulator app: Device > Erase All Content and Settings.

Or via terminal:

xcrun simctl erase all

Expected:

Erased all contents and settings of all devices.

Step 3: Shut down all simulators

xcrun simctl shutdown all

Sometimes a simulator is stuck in a "booting" state. Shutting down all devices allows a clean restart.

Step 4: Check available runtimes

xcrun simctl list runtimes

Expected:

iOS 17.4 (17.4 - 21F79) - Available

If a runtime is marked as "Unavailable," download it in Xcode: Settings > Platforms > + or download a simulator from the Components tab.

Step 5: Clear derived data

rm -rf ~/Library/Developer/Xcode/DerivedData/*

Step 6: Restart CoreSimulator service

sudo killall -9 com.apple.CoreSimulator
sudo launchctl reboot userspace

This fully restarts the CoreSimulator framework that manages simulator lifecycles.

Step 7: Check disk space

df -h /

The simulator needs at least 10 GB of free disk space. If space is low, clear caches:

xcrun simctl delete unavailable
rm -rf ~/Library/Developer/CoreSimulator/Caches/*

Step 8: Create a new simulator device

xcrun simctl create "iPhone 16 Pro" "iPhone 16 Pro" "iOS18.0"

Creating a fresh device from the command line bypasses any corrupted device settings.

Prevention

• Always shut down simulators from the menu (Cmd+Q) rather than force-quitting.
• Maintain at least 20 GB of free disk space for simulator operations.
• Run xcrun simctl delete unavailable periodically to remove stale devices.

Common Mistakes with simulator not starting

1. Using return to exit a function early instead of wrapping a pure value in the monad
2. Mixing let bindings with <- bindings in do notation, producing type errors
3. Overlapping type class instances that cause GHC to reject the program with ambiguous dispatch errors

These mistakes appear frequently in real-world IOS code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

FAQ

DodaTech Tool Reference

Doda Browser's Device Manager provides a UI for managing simulator devices, runtimes, and paired watches, complementing Xcode's device management.