Consistent Crashing at Light Build


I get this error almost every time I try to render on my PC.

I’m not sure why this happens, but when I try the same thing using the same file on another PC it works without issue. Does anybody know the cause of this?


Any substantial difference between the 2 PC’s? CPU? RAM? Graphic Card?

My pc uses a GTX 980ti, where the other pc has a K5000. Other than that the PC’s are virtually the same. The other is a bit newer, but still uses windows 10, same Ram and CPU.

Ok. Did you already try to delete the content of the Build, Intermediate, Saved folders and have UE4 rebuild them from scratch? Just to make sure, make a full copy of your project first.

Hi Vr_marco,

I’m just about to try this, I’ve found the other folders but I can’t seem to find a “build” folder? Could you point me in the correct direction for that?

AH HA! Never mind, I was able to fix the problem by opening the swarm agent and “Cleaning” the cache and then “validating”.

That seemed to fix the issue, its possibly a bug in 4.18. Anyway, thanks for the help vr_marco

It could be that since UE4 is crashing before building the lights there is indeed no Build folder. Try to remove everything except Config, Content, Source if any and the .uproject file. Make sure you make a backup copy first.


No problem. Fortunately the process is very quick and easy. I think the cache folder is getting full and unable to continue to build.

When you try to build lighting, in your system tray you’ll get a little yellow and black “S” icon, that’s the swarm agent. Double click that and it should open the swarm agent. Then all you need to do is click the toolbar at the top called “Cache” - there you will see “clean” and “validate”.

I chose to hit both of these a few times just to make sure it properly worked. Then you should get your rendering abilities back. Also, in settings there is a “MaximumCacheSize” option. I can only assume that is related to the cache folder size. By default it is set to “10” i dont know if thats MBs or GBs but i just increased that to 1000. That seemed to prevent me from having to clean the cache as frequently.

Hope that helps


I have exactly the same problem, could you explain in details how did you open swarm and your next steps, thank’s a lot!

thank you!
Before you answered I deleted all meshes, builded light on the empty scene and then imorted the same geometry again and it works fine now. Somehing strange happens often with UE…
But your method will be helpfull anyway, because reimporting mesh is quite problematic solution

Warm regards:)

Hey Again, I did the same thing at first and it did work. I can only guess that the light build will work when you have less geometry in the scene, because there will be less light-maps to render, and therefor less space taken in the cache folder (there still may be a small amount of space remaining in the cache folder). Just a guess, but i think that was happening with my files.

Anyway, good luck with everything :slight_smile: