Limp Mode without DTCs
 

I have a 1997 S 500 coupe. The gear selection switch for this car has been replaced. Now, it keeps going back into “limp mode”…won’t shift out of first gear. I am able to reset it by turning key to “On”, stepping down on accelerator for 5 seconds turning key to “Off.” Then, waiting two minutes. Any idea what might be causing this issue?

Are you sure there are not any codes? Are you able to pull up trans data?

Some of these cars had problems with leaking trans electrical connectors allowing oil to wick it's way into the trans computer causing all sorts of odd behavior.

The error codes for the transmission and engine are separate from the OBD diagnostic codes you can read with a inexpensive scanner. Those codes are primarily for the emission control equipment or related. The transmission codes can be read with a SDS or similar.

The transmission goes into one of two different 'Limp Home' modes in order to protect the transmission from additional damage.