2005 Prius Alarm Won't Stay Off

Hello! Noob here. Been lurking the threads for a couple of months but some of them pertaining to this issue never had updates, or all of the things listed haven't helped.

I bought a 2005 Prius in March, and the alarm keeps going off randomly. She's got about 270k miles and has a regular service history. I have tried troubleshooting with many of the things listed & it seems the only solution I can find is pulling the horn fuse. The lights will still flash & it has actually burnt out a bulb.

I have replaced the fob battery twice, I've open/shut all of the doors, hood, and hatch. The batteries of the car itself is fine & according to CarFax, they have all been replaced within the last two years. I know it is not fob related because it will happen even with the batteries out and/or if the fob is far away.

I had initially noticed that it happened more after I filled the tires. I thought it was due to something with those & they needed replaced anyway, so I got new tires. No change. They checked the bearings, brakes, and shoes. I noticed it was happening more when the temp was around or below 60F & rainy. It's now 80 and dry most days, and it's still happening. I have no idea what is setting it off so much.

One thing is, it's at its worst at night. But, it will still have bad days where it won't stop going off in broad daylight.

The door actuators seem to be fine. I have sat in the car with the fob & waited for the locks to switch. It's gone off with me inside of it before, but it's only been once. I guess maybe I'm not sure exactly how to tell if the actuators are working...? But it has sounded fine, like they're having no issue locking.

Other things I've changed/replaced:
Headlights
Spark plugs
Muffler
2 oil changes
Checked transmission fluid, it's still bright hot pink and clean

I know these have a big chance of not being related, but I've been hopeful.

At this point I just want it off, without having to pull the horn fuse. Could I take this to a garage and possibly have the guy disable it? Is there something he can hook up to the computer and shut it off? I've looked at the buttons to the left of the steering wheel, and I don't seem to have the alarm-related switch people have discussed being there. There's only the dimmer switch & the slot next to it is capped with a plastic cover.

Probably unrelated, but the check engine light is on, with a failed cat code. The cat itself is newer. I'm thinking that the muffler being bad maybe backed up exhaust & has dirtied the O2 sensor (which also doesn't look old) and that needs cleaned? I've done Cataclean but I have read it doesn't do the best at cleaning the sensor. So, I bought a new sensor, and when I got under there, it was as new looking as the replacement. Can an O2 sensor go bad and cause the alarm? Would an extender possibly work?

I woke up to a note on my car threatening to call the police and have it towed if it keeps going off. So, I'm back to pulling the fuse every night until I find a solution. This car is such an anxiety bomb.

 

Thank you for that! A short is starting to be what I think, too. I will look into the wiring... I don't know the first thing about it. I just don't want to get swindled by a garage, so I will keep looking into it! Thanks again.

 

Every time I say this so far, I eat my words. But, I think I figured it out. The 12V battery had just been replaced & so I didn't mess with it because I knew it wasn't bad.

I unplugged the battery today to clear the codes after replacing my O2 sensor & oil. While unplugging it, I noticed the fuse and pulled it out to check it. Looked fine but it fell out when I put it back in. Turns out, these fools had the wrong voltage AND size of fuse in there. I put the horn fuse back in & I'm starting the waiting game again. It will be storming tonight, so it will be a good test!

Wish me luck! Also, the new sensor seemed to clear up any issues with the cat! Double win

 

Narrator: It was a indeed *not* it.