95 Bonny ECM Question

I have a 95 Bonny SE. Had problem a few months back and computer was not showing codes(Should have). Found problem through lucky elimination. Now I have a problem where the timing is being thrown off and engine is laboring and backfiring out the exhaust. Just recently the check engine light came on. Once engine turned off and turned back on the light is out(not storing codes). My question is: Where are the codes actually stored at? The ECM or the removable PROM? If it's not the PROM then I can save time and money and just get another ECM rather than the PROM. Any thoughts?