btw, 02 sensor wont keep it from starting, it runs with presets for the first few seconds minimum, 02 reading doesn't affect that to my knowledge. No ecm codes is perplexing, but on the up side, any readings you get before it's actually run will be mostly useless. It will pick up a few things, but if it's indeed "trying" to start, I suspect fuel delivery to be at least one of the issues.
I'm assuming here that it's been confirmed that the ecm is in working order of course, CEL comes on, then goes off after a few seconds, etc. Again I assume to check the read outs, the OP is turning the key, on, off, on, off, on then waiting for the CEL to go out, then however long it takes for the CEL light to start blinking codes, usually 10ish seconds after the CEL initially goes out?
I got the dirty, low down, lovin my YJ Blues
Black Jeeps have longer hoods!