Just an FYI, on the later WJs, a bad transmission speed sensor puts you into a limp home mode. (and on the 545 they recommend replacing both the input one and the output one - that’s how it tells how harsh the shift is and what gear its in)
As for your particular problem, most code readers can clear ‘current and history’ codes. If its current, it should come right back. If you really have low pressure, you will burn up the transmission if you drive it. Recommend you do a careful filter change and make sure the O-ring/seal at the filter is seated perfectly in the transmission. If you do the filter change, and sensors are good, I’m afraid the next step is serious transmission diagnostics (like pressure test).