Here are your parts
Parts for Whirlpool DP840SWKX0 Dishwasher - AppliancePartsPros.com
See the attachment for the tech sheet.
I need someone to tell me what the source of the problem would be so I know which part to order please?
Sorry but it is not that simple as there are a number of things that can cause your problem.
What if any troubleshooting have you done?
Need more symptoms re: what works and what does not, does the unit make any sounds, etc?