Dell Precision T5500 fails to power on and the error lights aren't in the maintenance manual.

OK my T5500 has failed on me and now has lights 3 and 4 flashing green with a constant amber power light then powers off after a few seconds after trying to power on.

I've taken it down to minimum config and also tried a different PSU without success.

I've also reset the CMOS by leaving the battery out.

I've checked the maintenance manual but it doesn't show the error lights I'm getting.

I've yet to try a different processor, this will be my next step but I thought I'd ask here first.

Any ideas?

Thanks