Codes 159, 542 and 543, engine will not stay running?

It sounds like the PCM is going out but to be sure here is a guide to help check the wiring and the PCM wiring diagrams below:

https://www.2carpros.com/articles/how-to-check-wiring

The codes you provided appear to be OBD-I (On-Board Diagnostics 1) trouble codes for a 1994 Ford Ranger. These codes help diagnose issues within the vehicle's systems. Here's what each code corresponds to:

Code 159: Continuous Memory Test Failed

This code indicates that the vehicle's computer (PCM - Powertrain Control Module) has detected a failure in its ability to store diagnostic information in its continuous memory. This could be due to a fault in the PCM or its wiring.

Code 542: Fuel Pump Secondary Circuit Failure

This code points to an issue with the fuel pump circuit. It indicates that the PCM is not receiving the expected voltage signal from the fuel pump relay or the fuel pump's electrical circuit.

Code 543: Fuel Pump Secondary Circuit (Lack Of) Voltage

This code is related to the fuel pump circuit as well. It suggests that the PCM is not receiving the expected voltage from the fuel pump relay or the fuel pump circuit. This could indicate a problem with the power supply to the fuel pump.

Here is the location and how to replace the PCM in case you need it. Check out the images (below). Please let us know what happens.