stalebrim Sorry for confusing you with the E9420 ...
Whoops. Brain dead, me.
For @Rolts, this is the Dell Support page for the E6420 and this is the E6420 Service Manual. The diagnostic lights are on pages 141-143.
In general, the two models (E6420 and E9420) seem very similar in terms of construction and servicing, although components are somewhat different. I checked the diagnostic light sections of the two manuals (E6420 and E9420) and they are identical between the two models.
You might want to check and see if you get any error codes from the lights. That might help you identify a hardware issue, if one exists.