I do a similar job to what you do - troubleshooting and repairing electronics, and like you it has to be logged electronically. I too have forgotten to log into my work order logging station - on a number of occasions, and when there is a screw-up (and yes they invariably will happen) I have always fessed up. You get more respect for admitting to a mistake, then correcting it and learning from the experience than you ever will by letting a coworker take the fall for your screw-up.

The longer you go without fessing up the worse things will be for you when your boss does find out. I am surprised they didn't look at the date/time stamps for when the work was done - that should have shown that your coworker didn't make the mistake as she would have been clocked out and out of the building. I'm surprised your coworker didn't have the smarts to ask when the bad parts were made.

Just tell your boss that you just noticed the timestamps on the messed up parts/work and realized it was likely your mistake and not your coworkers and ask what you can do to make it right - because your bosses will eventually find out and whether or not you admitted to your mistake will shape thier opinion of whether or not they can trust you.