1993 Honda Civic. Can I replace the e.c.m. with a 94 e.c.m.? The idle waves and engine light is on . can't smog.

 

Yes 92 to 95 are basically the same car. You need to match the Pxx number in the middle of the ECU part number. They are different for manual vs. automatic and VTEC vs non VTEC, and the high mpg VX version is entirely different.

But before that, realize that it is very seldom the ECU itself causing symptoms that you have. When the CEL is on, read the code and check stuff related to that system. Generally it's going to be something under the hood.

 

Yes you can assuming the engine and transmission were the same in the donor car, as stated though it's almost never the ECU that's the problem. If you haven't already done so see what codes are being thrown and investigate further before spending money.