Most likely you'll need to replace the switch.

I've attatched the troubleshooting guide from the Helms manual for a code 22 for a more detailed explanation.

Just to clarify the location of where exactly the pressure switch is, on the first page of this file there's a picture that points to what part of the overall solenoid valve housing the pressure switch is. The solenoid valve is mounted to the cylinder head next to the distributor.