Dimension 4300 - unlisted ABCD light code and unlisted on-screen error message

I am a computer repair technician who is trying to repair a Dimension 4300 that fails to boot - the customer thinks it may have suffered lightning damage.

 

Having replaced the PSU, I can now get the PC to run its initial POST tests and to display POST messages on the screen. However the code of lights on the ABCD panel and the error message displayed on screen are not listed on the support page < ADMIN NOTE: Broken link has been removed from this post by Dell>.

 

The light code is GGOO (ie A=green, B=green, C=orange, D=orange) and the on-screen message is "I/O card parity interrupt at F000:B997: Suspend NMI / Reboot / any key to continue". If I suspend NMI, the lights change to GGGO which signifies "Other failure". There is no beep code from the speaker - not even a single-beep "speaker is working" confirmation.

 

This is with the modem and LAN cards removed. With the video card removed as well, the lights display the same code. With the modem present in the PCI slot, the PC fails even to run its POST tests: the monitor does not come out of standby, the ABCD lights do not come on and the green LED beside the orange "PSU is supplying power to motherboard" LED does not come on.

 

I suspect that the motherboard may be damaged. Can anyone confirm whether this error code/message indicates this. If so, how do I obtain a replacement motherboard (from Dell?) in the UK?