Recently I went down from 42cm handlebars to 40cm ones, but I've been getting really tight/sore triceps whilst on the bike. I've read that usually the symptom of running bars that are too narrow is usually that you have breathing difficulties, but that aspect is fine for me. After about 20 minutes or so, my triceps start getting really sore, towards the elbow end of the muscle. Is this a sign that the bars are too narrow?
I went from 42cm to 40cm bars on a road bike and experienced the same symptoms. The switch to narrower bars can definitely cause tricep pain at the elbow. So, yes, tricep pain can be a symptom of narrow bars (it could be from other things too). Whether it is a temporary or a permanent symptom can be gleaned by how you feel after you've had some time to adjust to your new setup.
It sounds trivial, but make sure when you are riding in this new cockpit, that you keep your elbows bent and use your arms as suspension/dampening/etc. If you aren't able to ride comfortably with your elbows not locked out, this is a problem. If this is the case, I'd first add some rise to the stem to bring the bars up a bit. If you still can't get a comfortable position, then I'd recommend getting another set of 42's and riding what you know is comfortable and works for you.