Diagnostic LEDs 3 and 4, 'ERR B' and 'ERR A' lit and Mac Pro won't start

Today I couldn’t get my sleeping Mac Pro (early 2008) to start up and drive my displays. It whirred and chimed as normal but my two DVI displays stayed blank.

I took the side panel off and checked out everything out, did a PRAM reset, but it didn’t help.

Then I noticed that two diagnostic LEDs were lit on on the mainboard. These were LED 3 and LED 4 on the logic board and are listed as ‘CPU B error’ and ‘CPU A error’ which according to the service manual are indicators that the CPUs have ‘halted on an IERR (instruction error).

I tried reseating the RAM and the RAM riser cards. I tried restoring the Mac Pro to the original 2GB RAM provided by Apple, and then tried it with just the extra 4GB that I’d added when I first bought the machine. None of this helped and the two LEDs stayed on.

I even tried resetting the Sys_RST System Reset switch on the logic board but that didn’t help either.

When I press the Mac Pro start button it seems to start normally, the machine chimes, and there is hard disk activity for about 30 seconds. Then the hard disk goes quiet and about 10 seconds later the two error LEDs turn on. At no stage is there any sign of life in the video monitor. I reduced my setup to a single monitor and reseated the graphics card but neither helped. I also had a look at my boot disk on another machine and it seems to be fine.

The machine has been working fine up to now, is in a cool and stable location, has not been ‘fiddled’ with in any way. I’m reasonably familiar and careful with opening up PCs, but not a techie, so I don’t know where to take it from here.

Anyone got any suggestions?

Thanks.