IIS & Application Pools
Connect HTTP symptoms with App Pool state, worker processes, bindings and logs.
503 investigation chain
HTTP 503
→IIS
→Application Pool
→Worker Process
→Event Viewer
→Application logs
Application Pool checks
Started / Stopped
Worker process
.NET version
Identity
Recycling
CPU / Memory
Bindings
HTTP status quick map
| Code | Meaning | First investigation |
|---|---|---|
| 401 | Authentication | Authentication configuration |
| 403 | Forbidden | Permissions/access |
| 404 | Not found | URL/file/routing |
| 500 | Server/app error | IIS/application logs |
| 502 | Bad gateway | Backend/upstream |
| 503 | Service unavailable | IIS/App Pool/service |
| 504 | Gateway timeout | Backend/network/application |
Remember: the HTTP status points you toward an investigation area; it does not prove the root cause.