iMac boot issue - sometimes no chime

[muess]

My late 2009 27" iMac had a kernel panic today and is now having serious boot issues. About 75% of the time it won't even get to the chime when powered on but some times it will. If I then let it boot to OSX it only gets to the blue desktop background and then the HDD spins down and it stays there with no OS. When booting into Windows it blue screens. When trying to boot into the installation disk it crashes with a kernel panic. I've tried all the usual fixes, PRAM, SMC, etc.

The only thing I can get it to do is boot into single user mode but when trying to run a fun system check (fsck) from there it crashes again. Unfortunately the back trace doesn't show anything useful just a crash for kernel_task.

Any ideas what the likely cause might be? Initially I was thinking failed HDD but with it now not completing the POST checks a lot of the time I'm worried it might actually be the logic board.

 

[blueroom]

Sounds like HDD failure. 5yrs is the typical lifespan of a HDD. Hope you have a backup.

 

[muess]

I use time machine so no probs there but could a failed HDD cause the machine to not even get to the chime a lot of the time? That means it's not completing POST checks which should run before the HDD is involved. When it doesn't get to the chime it just stays on a black screen with the fans going.