We own 2 2004 Aveo's since 2004 and I've maintained them myself . One automatic with 174K miles and a stick shift with 100K miles . Today I'm finally stumped with a check engine light problem . After doing everything else , I think it might be the control unit ( ECU ) but before I buy a used one I'd like to ask .

Will a used ECU from a running ( junked ) car just "plug & play' ( everything matches , AC Delco numbers , the right year & transmission , etc ) OR WILL IT HAVE TO BE REPROGRAMMED/FLASHED simply because it's coming from another vehicle ?

Thanks in advance for any help . old n' slow