Glad to hear it's truly multithreaded.. that's how one should write stuff like this that writes asynchronously to a port. (My VB6 class isn't multithreaded b/c VB6 can't really do threads.)

Instead of waiting 300ms to send the next command, may I suggest just waiting until you get the ELM's prompt ">" back and then send the next command then? I think the ELM won't send a prompt if it can't accept the command; correct me if I'm wrong.

Any luck using the ELM in binary mode? Supposedly it's a little faster but I didn't have the patience to code for it.

Finally, there's gotta be some .NET class for serial I/O where you don't have to rely on an ancient ActiveX component. I have little .NET experience so I can't really help find it, but there's got to be something out there.