My 1998 Dodge Neon reads DTC P1390. What's that mean?

SOURCE: 1998 dodge neon

haines manual. more than likely a crank sensor or cam sensor.

SOURCE: How to read trouble codes on 1998 Toyota Camry w/o scan tool

You can go to any auto parts store and they will usually read them for you for free. Around the late 90s many of the auto computer systems began requiring scan tools rather than the older method of simply flashing lights on the dash if you jumpered the correct pins.

SOURCE: what is code 1494 mean for 2001 dodge neon

P1494 Leak Detection Pump Switch or Mechanical Fault
Part of the early evaporative emission system. (Fuel tank fumes)

It’s there to allow the computer to run an integrity test of the fuel tank, lines and carbon canister’s ability to contain all of the fumes coming from the tank and all line associated with the system.
The code could be set for a number of reasons such as a faulty gas cap, cracked vacuum lines, solenoids & vent valves stuck to the pump itself.
It can be a very difficult fault to cure being it doesn’t take a very big leak anywhere in the system to set the code and that’s if it’s a leak and not an electrical or mechanical fault.

SOURCE: 03 dodge durango trouble code "p done" appeared where the milage is displayed, what does it mean?

"P done" means there are no codes stored. If there were codes you would see P and 4 numbers.

SOURCE: 1998 Dodge Neon, has code: PO403, EGR circuit

egr valve is located near exhaust manifold...exhaust gas recirculation or egr valve ports some exhaust fumes back to the inlet manifold in order to meet emmissions requirements...so look in this area