2011 iMac 27": NVRAM problem? Only works correct if started with CMD+OPT+P+R

[Cenobyte]

My late 2011 iMac 27" has developed a strange thing: it works very slow, booting takes 5 minutes and the screens acts strange, building windows up very slow. There is no sound, the volume does not work.

After trying some things I tried to reset the NVRAM with COMMAND+OPTION+P+R and that partially worked: the computer booted up (no reboot as it should) and it worked perfectly. But when I shut it down and restarted it normally, it got it's problems back. Every time I boot with the NVRAM key combination, it runs great but this does not "keep". It seems like it's resetting the NVRAM, but it does not store this new setting.

 

[Roman78]

Backup Battery maybe. Should be CR2032.

 

[Cenobyte]

Thanks! I'm also thinking it's the battery.

Last night I also did an SMC reset (pulling the plug for +15 seconds) and this fixed the problem. However I expect it will return sooner or later if it turns out to be a dying battery. When that happens my next step will be replacing the CR2032.