996 Key battery - The Return

Hi Everyone, I've been following the forums as a 'silent reader' for a number of years, and it has helped me on several occassions with my 996 (3.4 C4). I have a new query relating to, what seems to be the age old problem, of the key fob battery dying. So this might help other owners, and hopefully find me a better solution. If I replace the battery in the key, it works fine. The battery probably lasts around 6 weeks before it packs in again. Replacing the battery kicks it into life and starts the loop again. However if I don't drive the car for just 2 days the battery packs in early, it has all the signs of a very low power / old battery in the key, ie, the flashing lights become very dim, or don't flash at all. So, because I really need to lock the car (even if I don't need the alarm sounder on) I have a couple of questiosn - 1) Is it likely that the key itself is shorting slightly, possibly a burnt out resistor or something that is causing the battery to drain? Is this what most people experience. 2) If it is, and the key fob is really beyond repair or economic replacement, if it possible to lock the car but not have the alarm sounder working? 3) Is it a good idea to start replacing bits on the circuit board (youtube has an daunting, way to easy, video on how to do this ) Why? Because a secondary fault with the interior sensor sets the alarm off it the car has either - been parked in the shade, and then the sun heats the car up, or vice versa. I've seen a few comments suggesting fitting a switch under the hood that could 'disconnect' the alarm sounder under there, or a friend suggested that it is someting that could, he thinks, be turned off within the ECU when the car is plugged in. Does anyone have any suggestions or experience of these two ideas. All help would be massively appreciated as usual. Thanks GIW

Check your owners manual, there is a simple procedure for turning off the interior sensor, something like double locking the car or something, like when you leave a pet in the car and run in for a bit... I don't ever use this so can't remember exactly the sequence...

Hi, thanks for coming back so quickly. It must be bloody early there. You're right about the deactivation, but unfortunately it only works with the key fob not the key. (lock the car with the key fob, car beeps, unlock the car with the fob, BUT don't open the door, the car will re-lock after 30 seconds without the alarm being activated). If it worked with just the key, or via and 'internal button' I would be fixed, unfortunately when the battery goes, I don't have the option.

Quotes source post: 19651805

Well, it 5:30 AM and I haven't went to bed.. yet.......lol ...you must be in the UK with the "bloody" accent ?? I'm retired so I don't have a schedule...right now I am on the night shift so as to be in-time with my helper..or her with me..

lol, once upon a time you could leave your castle unlocked. . . . . Have a great day

I've heard of people replacing the buttons on the fobs, that could maybe help here but is a total shot in the dark. I've also heard of people putting the whole fob in the oven at a certain temperature for a few minutes to try and re-flow any bad connections, I tried this and melted the plastic battery holder, so if you try that be more careful than I was. Its purely anecdotal, but I bought a RF measurement device to troubleshoot some of my key fobs and saw a transient failure mode where the fob continued to broadcast after the button was no longer pressed and the lights were no longer flashing, didn't happen every time but did happen multiple times in testing. If this is actually a possible failure mode I could see that draining the battery real quick, but have no idea what the cause could be. For the alarm horn you can just unplug it, its easily accessible in the frunk, disabling all alarm noise may or may not make sense in your threat model.

Thanks dsbl, I have seen a few videos which 'resoldered' the connections within the keyfob. Sounds easy enough, but it seems like a lot could go wrong. It definately looks like its draining the battery if its not used - no idea why using it would keep it topped up for longer, unless there is a sort recharge side effect when its in the ignition barrel. I will try your second suggestion, probably with a switch this weekend and keep everyone posted. Have a good day

I don't think there is any interaction between the key and the car save for when the button is pushed. You driving it or not driving it is a bit of a red herring. Your suggestion that there's something invisibly incorrect with the operation of the key is likely correct. I would remove the board, check for high resistivity at connections and/or try and reflow the board. If you remove it from the key there should be no issue with trying this.