I wouldn't put too much faith in the error codes. Many years ago (pre-internet) I spent hours trying to troubleshoot a Memory Protect error on a Barracora ultimately it was the actual CPU that was faulty.
The Internal System3-System7 Diagnostic Firmware.
AKA, this is NOT "Bally World".
Lastly (really I promise!), the System3 to System7 internal diagnostic software is very limited. Remember the Bally power-on LED flash test, and how nifty that is at helping identify bad CPU board components? Well unfortunately, there is nothing like that in the Williams' firmware. Either a Williams system3 to system7 CPU/driver board boots, or it does not. There's not much middle ground here. The Williams' LED(s) at boot up provide some very basic information, but nothing like the information a Bally MPU LED provides. Yes there is a diagnostic test switch SW1 on the Williams CPU board. But in order for this test to work properly, the CPU board has to be successfully booted and running! But if the CPU board has successfully booted, the need for this test is really quite limited.
To make matters even worse, the diagnostic SW1 test, even on a working CPU board, can confuse even a veteran user. The diagnostics are a memory test only, and tests the CMOS RAM IC19 (5101) and the two static RAM chips (IC13/IC16). But the static RAMs IC13/IC16 rarely die. And the user will already know if the CMOS 5101 RAM is dead well before the diagnostics are run. If the game boots into "audits" mode, and the CPU batteries are good, it's 99% for sure the 5101 RAM is dead.
Also, even if the CPU board has seemingly "booted correctly", the flipper ROMs IC17 and IC20 can still have problems. These two ROMs hold the diagnostic code, and if one of these ROMs has a problem, false indications can result from the SW1 diagnostic switch (but usually the CPU board didn't boot anyway and the LEDs are indicating a locked-up board, and these two ROMs don't even get a chance to start working).
To make matters worse, the diagnostic LEDs just tend to confuse the newbie repair person. For example, the CPU board does not boot, but the user presses the diagnostic switch SW1 anyway. The LED reports back the suspected failed component. But that's the problem... since the CPU board never booted properly, the output from the diagnostic test CAN NOT be trusted! What ever the test indicates is surely incorrect, and the newbie is replacing good CPU board components, based on the failed/incorrect test results (I believe this is called, "chasing one's tail"). This is especially a problem if the newbie came from "Bally world", where Bally's LED actually has good boot-up component diagnostic results.
The bottom line is this: if the Williams System3-System6 CPU board's LEDs lock-on at power up, the CPU board is not working! Likewise for System7, if "0" comes on immediately at power-up, the CPU board is not working. Why that is happening, well, you're on your own to figure it out! Because the Williams diagnostic firmware is *not* going to help.