Alarm keeps coming on! :(

Welcome to PC MrBlue:

Sorry to hear about your problem. You did give us plenty of information, but one piece more would be helpful. Does the problem occur even if you leave the car unlocked or only if its locked. If the later, then its likely just a bad door sensor. If the former, then that will be harder to track down

 

Factory alarm or after-market?

Tom

 

Anything in the overhead sunglasses box? Assuming it's an oem alarm system...

 

I had a similar problem, alarm was very sensitive, just touch the car and it would go off. Dealer finally fixed it by replacing the glass breakage sensor.

 

I did not think about a hood switch. There is no light there so it did not occur to me that it could be there. I wanted you to check the lock situation in part to eliminate the key fob (panic button) as a source. That would have still made the alarm go off even if unlocked.

Where is the hood switch?

 

I am having this problem again with my 2004 Prius with 130K miles. I replaced the 12V battery with an Optima battery a year and a half ago. I don't remember if my alarm issue was around the time of needing to replace the battery or not. When I had the alarm issue before I didn't know that the battery could be the problem and didn't do a voltage test. I didn't replace the battery until it was absolutely necessary. If the car sat for a couple days, it wouldn't start. If it was used the evening before, you couldn't be sure it would start the next morning if it was a cold winter night.

This time it is in the summer and no starting problems. I did the voltage test as advised on this forum and it seemed to say the battery is good. I had 12.4 volts if the car was recently running, 12.3 if tested right after the alarm used some of the power. Putting the load on it by pushing the power button, it held steady at 12V; headlights turned on the same time would make it hold steady at 11.9V.

A couple days ago the alarm went off at 10:00 at night. Last night it went of at 4:00AM and again at 6:00AM. The other times it would only go through one cycle and stop before I got to it. This time it kept going and I had to start it. I wish I knew about checking the voltage at that time. The battery may have been drained by the previous two cycles (maybe more if I slept through other episodes). After starting the car at 6:00 to get it to stop, it would want to keep starting again as soon as I shut the car off. I noticed that the tiny red light on the smart key was blinking, even when the car was running. I think it usually only blinks when a button is pushed. Does this light indicate a need of a new key battery? After the car ran for a bit, the light stopped blinking. When I shut the car off while the light was not blinking, it seemed that I would have more time before the alarm would go off again. And it seemed that if I shut it off while the light was still blinking, the alarm would sound immediately. This could be coincidence, or maybe that the extra time of running to get the light to stop blinking may have charged the 12V battery some more.

I am going to try changing my smart key battery to see if that helps, thinking the red light may indicate a low battery. I can't see that the signal from a key with a low battery could reach my car from inside, but I'm hoping that the previous communication with the car could have made a difference in the state the car was in.

I am still in a troubleshooting mode. Perhaps I can post if I come to a conclusion. But I find it hard to believe that my Optima battery, which was supposed to be superior to the original battery, is bad after a year and a half when the original lasted seven years; and I know the original was drained dead at least a few times if not several times. This battery hasn't been drained dead yet.

BTW, for a while I didn't realize why my original battery was getting drained. I eventually realized that it was the headlights. The headlights will go off automatically if the car is turned off while they are on. But if you turn the car on to accessory the lights will come back on, but won't go back off again when you turn it off from accessory. I found myself after parking the car (with the headlights on) deciding to put my windows up in case it would rain. I was only putting it in accessory mode to wind them up and therefore the lights would stay on.

 

Is it a dodgy button on the smart key? If it's worn out, dirty (inside) or jammed it could be continuously transmitting and causing the alarm issue.

Couple other things for you to check; is the hatch completely shut as sometimes it doesn't always fully shut. Are the seat belts all fully up when you get out the car? If not, sometimes they'll slide up on their own setting the alarm off. Finally, you sure it's not a cat or similar animal jumping up on the car? There have also been reports of mice making a home in the cabin filter housing inside the car (not the air filter housing in the engine bay). Might be worth a look in there to double check.

 

You're right, the little red light on the fob only blinks while a button is depressed. It looks like your panic button is sticking. If you take the battery out of the fob, the alarm won't sound.

Sorry, I don't know how to un-stick the panic button.

Can you use the other fob for now?

I think you can still use the fob in the dash slot to power up the car (without the battery in the fob) and use the metal key to lock and unlock the door while you figure out how to un-stick the button.

 

My key is not in good shape. I used to allow it to hang on my belt or in my pocket along with other keys. The face of it wore out so two of the buttons aren't there, including the panic button, and dirt can get inside. I had wrapped electrical tape around it to keep the dirt out. This has been a few years like that. So, while it would seem this is likely the problem, I believe that the key was out of range when it has been going off. And I don't think I mentioned in my other post, the alarm also went off if I tried leaving the key in the car.

Thanks for suggesting the other possibilities. I will keep them in mind.

 

Since the red light was blinking even sitting in the car with it running, it makes sense that it could be a sticking button. But I can't make it go off by pushing the button at the range that the key was when it went off by itself. I think it has been a few years since I changed the battery in the fob. It won't hurt to try that and I will do that later today.

Thanks for the suggestion of using the key without the battery. I'll do that if the problem persists. If it still goes off then, it would need to be something in the car setting it off, or a 12V battery, which I did the voltage tests for. I can't use the other key because I lost it a few years ago.