after you get this fixed, if you want to get rid of the P30 let me know. I'm looking for one.
first make sure that the ECU is reading this right.
ground the pressure switch line to the ECU. (C6)
this will make the ECU think that the switch is closed and the CEL should be off (after resetting the ECU) and everything will work fine.
Next remove the ground and place a multi meter on the wire with the key in run but the engine not running. There should be 12volts on the wire.
next take the meter and make sure that the black wire coming from the switch is actually ground. Then make sure that there is a solid connection between the wire at the ECU and the wire are the switch. If all those things check out, replace the switch.
the switch is located on the engine, next to the distributor plug.
I’d be checking for bad connections personally.