Mitsubishi Repair Problems? Ask a Mechanic for Answers ASAP
P1784 is indicating a problem with the low-reverse pressure switch in the transmission.
The most likely cause of this fault is going to be a failure of the solenoid/pressure switch assembly. They are extremely common to fail on these old Chrysler powered Eclipses.To test it properly you would need a full function scan tool (something with 420A transmission access, not just a code reader) and a transmission line pressure gauge set. If you can get access to those, I am attaching the diagnostic chart below.
Otherwise... most likely the solenoid/pressure switch assembly and you should be OK after that. The line pressure tests etc are still highly recommended in case there is something unusual going on (valve body issue, TCM etc); having it checked properly will avoid any potential unnecessary parts replacements.
Let me know if I can do anything else for you.