Epson CX 7800 E90 Error?

I am getting e-90 call for service message. I got that and in fooling with it, it told me 2 cartridges were out so I replaced those. Still got the message. Tried doing set-up utilities and then it tells me that 2 other cartridges were empty (not true!) so I replaced those. Still getting the message. I pressed down good on the top. there is some oily substance on the slider so I think it slides ok. Used compressed air. Don't know what else to do???