I'm getting a code number 123455 on my 1990 Plymouth grand voyager what does it mean?

the check engine light should be on with a code P0700 present. This means that the transmission control module has codes in it. You will need a high end scan tool (not an OBDII code reader) to get those codes. Once you know what those codes are post them here and I'll be happy to assist you further.

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

P0167-/ 02 (oxygen) Sensor Heater Circuit Malfunction (Bank 2 Sensor 3)

Turn key to on ( not start) then off, then on, then off, then on and leave on, and have a pen and paper ready to record flashes of "check engine light". It will flash then pause between second number of 2 digit code. Then a short pause and flash again, pause, then flash. For example: if it flashes 2 times, then pause, then flashes 2 more times, that would be code 22. If it flashes 3 times, pause, then flashes 5 more times, that would be code 35. As I said, there will be a short pause between 2 digits, as well as after number. Write them down as they come up and get back to us with those codes for advice. When you get code 55 ( 5 flashes, pause 5 more ) that means end of message. Those codes should point everyone in right direction of trouble spot.

P1698 No CCD message from TCM , have no idea what it means though as a europeon model of the same vehicle just says cannot connect on my reader

just because the check engine light is back on, it doesn't necessarily mean it is for the same problem. it could be, but would need to pull codes again to find out what its for. literally hundreds of different codes could turn on check engine light. also, was original code for "insufficient egr flow" or "excessive egr flow"? it makes a difference.

Misfire at Cylinder # 3. One of three things:
1. Spark Plug malfunctioned.
2. Wire going to Cylinder # 3 is malfunctioning.
3. Electronic Coil is malfunctioning at Post # 3.

I would try a tune up.

Have the transmission computer scanned for codes. This trans is completely electroniclly shifted.

According to minimopar.knizefamily.net, fault code 31 is "Purge Canister/EGR Solenoid".

where is the ECM on a 96 voyager van? we need to replace it. thanks.