How to fix ultrasonic cat repeller?

Unless a local 'handyman' can fix it, it is not economically practical to try and fix it.

Try the test
Motion sensor on walk past or wave in front of it if the red light can be seen you have been detected to test the sonic function turn up sonic dial to hear the alarm harassment noise
If not heard get it looked at
If heard how old is dog as may be going deaf to ultrasonic sounds

There are some cases that one cat flea product wouldn't work properly against cat flea. If this is your scenario, you should rotate to different types of cat flea product each week this way you'll reduce the cat flea resistance significantly.

dont' think those things work for any lenght of time - best thing is poisons or sprays or just good hygiene (remove all food sources)

You can install an ultrasound rat repellent like carcat.
See link and faqs
Rid your CAR of RATs CARCAT Ultrasound rat repellent for car rat in car... Ultrasound rat repellent for CARS CARCAT

yes i have a thought,,, on poison though!!!! think on this for a minuet,,
poison's dont discriminate,,,ok,,you may get the rat,,then the owl eats the rat,,,the owl then dies...then the crow eats the owl,,,that also dies,,
then your neighbour's cat eat's the crow,,,that then dies,,,,,and what if any of them fed there young ones on this poisons rat??? they all die!!!
and i would not wish it on my worst enemy to watch an animal die from poisoning,,it takes hours or days to die from poisoning,,,in horrible pain!!
please think on this first before you use poison again,,,,,
im not trying to make you feel gilty or sad,,i hate rats too, i found the ultrasonic system works at close ranges,, but an engine block could stop the sound so you may be better using a fuw not just the one!
the more you plug in the better the cover you get,,,they do work!
but so will an air gun? and its much more fun shooting rats!
its faster than poison,,,and much safer than poison as you only get what you hit,,the rat!,,im sorry if i upset you that was not my intension

Where can I get a replacement output nozzle?