I have two ceiling fans on my enclosed back porch. Both are Harbor Breeze fans and are identical. Both fans are on the same light switch and the same power switch. We keep the fans set on high so all we have to do is flip on the power switch and they both come on. If we need the light they are both wired with a separate light switch too.

Yesterday when the switch was turned on for both fans one came on the usual high speed while the other one appears to be running on low speed. They both have chain pulls (no remotes) so we tried the chain pull on the slower one and no change. It remains on the slower speed. It will not shut off with the chain pull either. Just the slower speed. What could have happened from one day to the next to cause this? Any ideas as to what we might do? I only wish the speed had broken on the high speed if that is what happened. Ha! Thanks.

Check the reverse switch, if it is good, then it is probably a bad capacitor. The fact that it won't shut off at all BUT the pullchain wasn't touched is odd. That implies a bad pullchain, but a pullchain can't go bad if you don't pull it.

Ceiling fan capactors - troubleshooting and resources - Ceiling Fans N More