I once took apart a digifant I ECU to see what PROM chip it used. There were no
non-volital chips other than the PROM chip. There is nowhere for any fault codes
to go. When the ignition is turned off the power to the ECU is turned off too.
There is no power to retain any memory.

But here are the chips:

MC6803U4CP       40 pin          8 bit MCU, No rom, 256k ram, no a/d. 13 IO
lines

MC6821CP           40 pin          Peripheral Interface Adaptor

CD74HCT373E      20 pin          Tri-state Octal D-type latch

ADC0809CCN        28 pin         8 bit A/D (8 channel MPX)

AM2732A-3DI        24 pin          4K x 8 EPROM

I have the equipment to burn (and copy) PROM's like this but do not have the
assembler/dis-assembler to do any changes. Note: my ECU had all the chips
soldered in.

When you know how low buck (compared to today) the digifant brain is you can see
why problems are so hard to fix.