Updated: March 2022
One of the most common causes of errors (particularly Array Index and Object Reference errors) is broken mods and assets. Here's two quick ways to tackle those problems:
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2633433869
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=1846793796

Updated: January 2020

(If you are just looking for place to share log file, scroll down to "Sharing your log file" heading for a list of sites people commonly use)

If you are having crash bugs or other errors, it helps if you share your log file so that other people can see exactly what is going on (your hardware specifications, what mods you use and full details of any errors that occur).

Without a log file all people can do is make random guesses and that will waste your time and theirs. So share the log file (find out how below)!

Note:

Each time you launch the Cities: Skylines app, the old log file is deleted and a new one is created. So please take a copy of the log file when the error occurs and share that.

Windows:

<SteamFolder>\SteamApps\common\Cities_Skylines\Cities_Data\output_log.txt

Tip: To open the game install folder, right click your game in the Steam library, select Properties, go to the Local Files tab and select Browse local files.

Mac OS X:

Users/<username>/Library/Logs/Unity/Player.log

Tip: Find it quickly by searching for Player.log in Finder.

Linux:

~/.config/unity3d/Colossal Order/Cities: Skylines/Player.log

Tip: To open the game install folder, right click your game in the Steam library, select Properties, go to the Local Files tab and select Browse local files.

Log files are too big for Steam, so you'll have to upload the file somewhere else such as:

• https://uploadfiles.io/ - free, no sign-up, can handle large log files
  
• https://paste.ee/ - fast, no sign-up required (some log files will be too large for it though)
  
• https://zerobin.net/ - bit clunky, but can handle huge files, no sign-up required
  
• Google Drive[support.google.com] - requires sign-up
  
• DropBox[help.dropbox.com] - requires sign-up
  
• or some other proper file sharing site that can handle large text files.
  
• NOT "Pastebin" (small size limit) or "Dochub" (just awful).

After uploading, make sure it's publicly viewable, then share the "Share Link" or URL. A good place to get help is the official Cities Skylines Discord Server[discordapp.com]

Updated: February 2020

Game crashing while loading a save? It's usually lack of RAM! This is by far the most common crash error for this game.

You will often see several messages like this grouped together (usually with different probe numbers and different allocation sizes):


Severity: Crash - You do not have enough RAM to run the game.

The game needs minimum of 8GB RAM, despite what the requirements page says, and ideally 16GB or more if you use DLCs or Workshop content.

Most operating systems can also use disk as 'temporary' (but really slow) RAM using a "swap file" or "page file" - increasing the size of your swap/page file will usually get the game running again.

Read the RAM, Page file, Loading Screen Mod and DLCs, Assets and Mods sections of this guide to help solve that:
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=465790009
Also, remove any bloated or broken assets to free up more RAM:
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=1846793796
And kick your subscription addiction :)
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=449998850

Last checked: August 2019


Severity: Severe / Crash. See this troubleshooting guide[github.com]


Broken "Interchange" assets can cause this error. See this list[github.com]

Last checked: August 2019


Severity: Severe / Crash. See this troubleshooting guide[github.com]

I'm slowly working through this guide to update it - sections with a Last checked have been checked and updated with correct info. Everything else is possibly outdated or obsolete.

You can use Ctrl+F to search this guide (Mac users: Cmd+F). Keep your search strings short (the guide doesn't include full stack traces, workshop IDs, asset names, etc., for sake of brevity)

If you have an error that's not listed here, please start a new discussion in the main Steam Discussion Forum.

You can also get tech support via the official C:SL Discord chat server[discord.gg].

Last checked: August 2019


Severity: Severe / Crash - See this troubleshooting guide[github.com]

Last checked: August 2019


Severity: Severe / Crash. See this troubleshooting guide[github.com]

Last checked: August 2019


Severity: Harmless - this is normal part of mod loading, ignore it.

Last checked: August 2019

Severity: Severe / Crash - See "No audio" troubleshooting guide[github.com] ("FMOD errors" section is near the end of guide).

Last checked: August 2019


Severity: Severe - See this troubleshooting guide[github.com]

Last checked: August 2019


Severity: Low - See this troubleshooting guide[github.com].

Last checked: August 2019


Severity: Low - Indicates that C:SL couldn't get additional information about a subscribed asset. You are either running in offline mode, or the asset no longer exists in workshop. Should not cause any issues other than lack of detail about the asset in Content Manager. Generally safe to ignore.

Last checked: August 2019


Severity: Harmless - this is normal part of mod loading, ignore it.

Last checked: August 2019


Severity: Crash - Your steam client is likely out of date. Change content server to a differnet download region then Refresh your Steam files then exit Steam and reload it. Updated Steam client should download. If not, try this.


Severity: Crash - Do this. Also, if you have another copy of the C:S installed on your machine, delete it.

Last checked: August 2019


Severity: Severe / Crash. See this troubleshooting guide[github.com]

Updated: February 2020

This can _usually_ be ignored:

TFW: System.Reflection.ReflectionTypeLoadException: The classes in the module cannot be loaded.

Many mods need to check if another mod is loaded and try and access its features for compatibility. If the other mod isn't present, you'll see this "error" in the log - it just means the other mod wasn't present; it doesn't break anything.

Last checked: August 2019


Severity: Severe / Crash. See this troubleshooting guide[github.com]

Caused by Nursing Homes for Senior Citizens mod; unsubscribe it and also any building assets that require it.

Updated: February 2020

This error can usually be ignored:

System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.Xml.XmlException

It's not known to cause any problems.

Last checked: August 2019


Severity: Severe / Crash - Did you run the game via locale.bat or something like that? Don't! Make sure you launch C:S via the Steam client (or whatever gaming service you use, eg. Origin) or using Cities.exe.

Last checked: August 2019

Note: These errors refer to TM:PE, but they are not caused by TM:PE. It's almost always a different mod that causes the errors.


Severity: Severe / Crash - See this troubleshooting guide[github.com].


Severity: Severe / Crash - See this troubleshooting guide[github.com]


Severity: Severe / Crash - See this troubleshooting guide[github.com] (it's almost always a symptom of the "Array index is out of range" error of another mod).

Last checked: August 2019


Severity: Severe (performance loss, may cause crashes)

Note: If you get the d3d11 error your game will almost certianly crash when working with or viewing cargo train stations.


Caused by errors in your graphics card drivers and/or DirectX 11 driver:

• Upgrade to latest graphics card drivers direct from manufacturer website
  
• If error persists, add the "-force-d3d9" launch option and then restart C:S.

Severity: Severe (significant performance loss, may cause crashes)


This is likely due to a multithreading incompatibility between the Unity graphics engine and your graphics card drivers or hardware. There are two potential solutions:

• Ideal case: Upgrade to latest stable graphics card drivers direct from manufacturer website and remove the '-force-d3d9' launch parameter; if this doesn't fix the problem use safe mode...
  
• Safe mode: Add the '-force-d3d9' launch parameter (use DirectX 9) and also the '-force-gfx-st' launch parameter to force Unity to render in a single thread (lower performance, but will fix the problem)

For information on launch parameters and how to use them, see Launch Options guide.

Tip for end-users: You can identify bloaty assets (which cause lower fps and other problems) using the Mesh Info mod:

http://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=453956891
Unsubscribing bloated assets will reduce load times and improve performance in-game. Please always let the asset creator (and other end-users) know that there is an issue with the asset by commenting on their workshop page prior to unsubscribing.

Severity: High (broken asset, could cuase performance issues, broken saves or crashes)


It's not clear from log file which asset caused the error; I usually assume it's whatever asset is mentioned immediately prior to the erorrs but this might not be the case. (@CO/Paradox - any details on this?)


Severity: Unknown, possibly high


Not sure what this means. Suggest unsubscribing the asset.


Severity: Unknown, possibly high


Not sure what this means. Seems to be common for Belgian/Dutch tenement building assets. Suggest unsubscribing the asset.


Severity: Medium (increased load times, glitchy LODs in-game)


The LOD was missing from the asset so the game had to auto-generate one; this increases game load times and often results in glitchy LODs in-game.


Severity: Medium (performance loss)


If the LOD is not significnatly smaller than the base model, a warning will be generated. Ideally LODs should be less than 100 triangles. Anything above 500 tris is going to cause fps drops, especially for growables.


Severity: Unknown


No idea what this means :/


Severity: Unknown


No idea what this is :/


Severity: Low (asset won't spawn in game)


This is caused when a growable building (those that spawn in zones) has a road, path, pipes, powerlines or some other "net type" because they would adversly affect the surrounding area (for example, a road in the asset would cause changes to the zoning squares).

Users: Unsubscribe the asset but let the asset creator know that they have to fix their asset.

Asset creators: Don't put roads, pipes, electricity, rail, paths or anything like that in your growables!


Severity: Low (prop or asset will be missing in-game)


This is most commonly caused by a) asset authors not listing props in the "Required Items" list on their workshop page or b) end-users not subscribing to props used by an asset.

Note: Sometimes the error is generated due to unsubscribed building/park/etc. assets and then loading a game that was using those assets; this can cause the dreaded "System.NullReferenceException: Object reference not set to an instance of an object" error to appear. To fix, try re-subscribing the missing assets.

When subscribing to assets via a Steam asset collection screen, users are not prompted to subscribe to required items even if the are defined for the assets being subscribed. Currently no way round this issue other than to direct end-user to the workshop asset description page where they can manually review the required items and subscribe any that are missing.

Asset authors will usually be unaware if their asset does not specify all required items (as they have the items already subscribed so won't see the missing props).

Some props are only available to users who pre-ordered the game and got additional assets (and their props) such as botanical garden, dog shelter, etc. The most common missing props in this scenario are 'palm01', 'Botanical garden fountain' and 'dog-park-trash-can'; widely used yet most users aren't able to acquire the prop. CO/Paradox are considering ways to make the pre-order content available to all users, perhaps as DLC. It's recommend asset authors limit props to those available too all users via the workshop.


Severity: Unknown, likely low or harmless


No idea what this is :/


Severity: Unknown, likely low


Not sure what this is. Seems to be a common error for rail/metro stations, airports and docks.


Severity: Unknown



Severity: Unknown


Possibly related to or caused by the texture atlas shader not found error mentioned earlier. Possibly also results in System.IO.InvalidDataException error.


Severity: Unknown


Not sure what this is. Suggest unsubscribe asset.

http://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=449020940
Seems to be a generally stable and actively maintained mod which is compatible with most other mods.

Deprecated (old/broken) versions: Improved Assets Panel (by nlight).

Severity: Severe / Crash (main menu runs very slow at 1fps, game unplayable)

You will also see an error dialog "A Mod caused an error [ModException]" at the main menu.


Note: This same error is also caused by the deprecated version of Improved Mods Panel mod (see next section).

Unsubscribe the old version and subscribe to the newer fixed version shown at the top of this section.


Severity: Severe (causes error in main menu)


This bug has been fixed, make sure you have latest version installed: Exit the game, unsubscribe the mod and then re-subscribe (make sure you use the fixed version shown at top of this section).

http://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=448687553
Seems to be a generally stable and actively maintained mod which is compatible with most other mods.

Deprecated (old/broken) versions: Improved Mods Panel (by nlight).

Severity: Severe / Crash (main menu runs very slow at 1fps, game unplayable)

You will also see an error dialog "A Mod caused an error [ModException]" at the main menu.


Note: This same error is also caused by the deprecated version of Improved Assets Panel mod (see previous section).

Unsubscribe the old version and subscribe to the newer fixed version shown at the top of this section.

This mod has been discontinued and removed form the Steam workshop.

Workshop ID: 412101021 Author: Crispy

No updates have been released, no support is provided. The mod causes game breaking problems.

As the mod is no longer in the workshop, you cannot unsubscribe it from Steam app or website.

To remove, go in to C:S then Content Manager > Mods. Look for the "Lane Changer" mod and unsubscribe it.

If that fails search for the 412101021 folder via your operating system and delete it (if you installed the mod manually from a fan site, delete the folder you created while installing the mod).


The following mods provide lane changer functionality: Traffic++, Traffic Manager


Severity: Severe / Crash (significant problems in-game, may cause crashes)

The mod updates vehicle AIs and does not reset them when returing to main menu or launching asset editor. As such it will break vehicle AIs.

End-users: You will have to unsubscribe the broken vehicle assets and ask asset creator to fix them.

Asset creators: With the lane changer mod active (so you can open and edit your asset) use V10DeleteAI mod to fix your asset then save and republish to workshop, Finally, unsubscribe the Lane Changer mod to prevent further problems.


Severity: Severe / Crash (unable to save games, may cause crashes)


Unsubscribe the Lane Changer mod.

http://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=407335588
Seems to be stable and well supported, however it is incompatible with several major mods (Traffic++, Traffic Manager, etc).

Earlier versions of this mod had some issues that broke vehicle AIs in vehicle assets.

Alternate Mods: Traffic Manager, Traffic++

May cause in-game performance issues on low-end systems. May cause unknown side-effects.

Severity: Critical / Crash (unable to load saves if broken vehicle present and mod not active)

Early versions of this mod changed the vehicle AI in game, but did not reset it to default AI when returning to main menu; side effect was that any vehicle created/edited in the asset editor would be using the wrong vehicle AI.

End users: Report issues with vehicle assets to their creators.

Asset creators: See V10DeleteAI for details on how to fix the AI of your vehicle.


Severity: Critical / Crash (save games become unplayable)


Any vehicle using non-standard AI will cause the mod exception (see note above on vehicle AI bug for solutions).


Severity: Critical / Crash (save games become unplayable, new games will corrupt when saved)


Any vehicle using non-standard AI will cause the mod exception (see note above on vehicle AI bug for solutions).

Last checked: August 2019

This section is somewhat out of date and many of the errors have been resolved over the past few years, however it might prove useful in rare cases so I'm keeping it in for now. The earlier section of same name contains more up-to-date information.

There are multiple variants of the error:


Hints as to which type of asset caused the problem can sometimes be determined from the stack trace that appears after the error message:



Unsubscribe any mods that alter the game audio (ambinet sounds, sirens, music, etc) and try again.

Sometimes this error is caused by missing service or transport building prefab; look for "Unknown prefab:" errors earlier in the log file and re-subscribe the missing asset if possible.

There are some additional things to try in the Troubleshooting FAQ.