PSP 3.71 M33: CSO Games Won't Start — Update CFW to 6.61 Infinity

Custom Firmware (CFW) 3.71 M33 is very old (2007). Modern CSO/ISO tools, plugins, and game patches assume newer CFW (5.00 M33, 6.60 ME/LME, 6.61 Infinity/Pro).

Likely Causes & Fixes

1. Update CFW: Install 6.61 Infinity (permanent) or 6.61 LME/Pro (temporary) on any PSP 1000/2000/3000/Go. This fixes 99% of compatibility. Backup NAND first.
2. ISO/CSO Corruption: Re-dump from UMD or re-download (verify SHA-1/MD5 against Redump.org). Convert ISO→CSO with UMDGen or PSP ISO Compressor (not random online converters).
3. Wrong Folder: Place ISO/CSO in /ISO/ or /GAME/ (for homebrew) on Memory Stick root. Not in /PSP/GAME/.
4. Missing/Conflicting Plugins (seplugins): Disable all in VSH.txt/GAME.txt/POPS.txt (Recovery Menu → Plugins). Enable only inferno.iso or m33_620.prx if needed for specific games.
5. Memory Stick Issues: Fake/cheap MS Pro Duo causes corruption. Use genuine Sony/SanDisk. Format FAT32 (32KB clusters) via PC. Check free space > 2x game size.
6. Game-Specific Fixes: Some games need Prometheus / ISO Tool patches (for 5.xx/6.xx kernels) or EBOOT.BIN patching (for PS1/PSN games). Check PSP ISO compatibility lists (e.g., wololo.net, GBAtemp).
7. Kernel Version: In Recovery Menu, ensure "Kernel for GAME" is set to 5.XX or 6.XX (matching your CFW), not 3.71.

Start with CFW update. 3.71 M33 is the root cause for most modern backups.