Look for ATF contamination in the connector where you plug in the TCU. ATF can migrate into the plug and insulate the pins. That's what caused my trans to get stuck in limp home mode (2nd gear). ATF can wick up the tape wrapped around the wiring harness. If that all checks out you could have a bad solenoid or valve body. You can get a JY valve body and solenoids for dirt cheap. I wouldn't replace the entire trans unless there was obvious evidence of damage like chunks of metal or clutch material in the filter or coolant contamination. Early 203s had valeo radiators that sometimes let ATF and coolant mix.

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CENSORED due to not family friendly words