After ECU swaps the check engine light (CEL) is sometimes active. Honda were kind enough to build a system into their cars to let the users check their own error codes, here's how.

Take off the passenger footwell panel as if you were getting to the ECU. Now look above the ECU and you should be able to see the following plug:

Dig about in this rubber plug and get the 2 wire connector out. Once you've done that you need to connect those 2 wires together. I used a paper clip, but anything that makes a connection will be fine

Sorry about the quality of the 2nd photo, but you get the idea. Once this is done sit in the drivers seat and set the ignition to the on position. The CEL should come on and then disappear. Then the error codes will start coming out

The system uses flashes of the CEL to tell you the error codes, a long flash indicates the 1st digit, a short flash indicates the 2nd. For example. long long short short = error code 22. Good luck with finding your fault, the table for looking up the errors in on the next page