I am stumped with a 2007 Honda Civic 1.8 sedan with automatic transmission. Other the car failing to shift into 4th gear all other transmissions functions are fine. It has a trans error code (p0843 high) indicating a failing pressure switch. Replaced the switch and it did bot correct the problem. I tested the voltage to the terminal that connects to the switch and did have a good ground with 5 -volts
If you replaced the switch and you still get the code then you probably have a plugged up line. This would keep the switch from seeing the expected pressure.
This might be fixable with a flush and fill or if you can pull the switch and inspect inside the hole you may be able to see the plug and extract it.
When was your trans fluid changed last?