Immobiliser fault

[Ippypicasso]

Hi, hope someone can help.
This morning, after stopping to pick up a paper on the way to work, my 2.0 HDI 2002 Picasso displayed Immobiliser fault when I tried turning the ignition, and the car turned over but would not start. Tried turning key a few more times, same issue. Got out of the car, locked it, unlocked it, tried ignition again and no problems, she started fine. Decided to play it safe and dropped home to pick up the spare key, and used that for work instead of my main key. All fine, started it after I got to work, again no issues. Drove it lunchtime, and back, again no problems.
Any ideas what would have caused the above problem ?
Thanks in advance
Paul

[spider]

Could be dying battery. Plexed cars rely on decent battery voltage and won't put up with any low voltages.

Other causes are transponder aerial starting to die.

[c52002]

I've got a C5 and this has happened to me as well, except it happened when i was driving along all of the doors locked a service light displayed and if i pressed the door lock button it did nothing. I reached my destination opened the door, and the lock light went off, I tried turning the ignition on and gor the message immobiliser fault. I disconnected the battery, reconnected it and waited 5 minutes for everything to reboot, and it has been fine for a few months.
Weird !!!!

[Ippypicasso]

Thanks guys, battery seems fine and car has been fine all day. One thing I'm wondering, the problem this morning occurred as I said at the shops, but I remember now that I turned the engine on after getting back in the car, then thought I need to use the cash point so turned her off again and left the key in ignition while I contemplated whether I could be bothered to go back in, then thought I'll do it later so turned her back on again, maybe leaving the key in and doing that I may have confused things perhaps ?

[myglaren]

Thanks guys, battery seems fine and car has been fine all day. One thing I'm wondering, the problem this morning occurred as I said at the shops, but I remember now that I turned the engine on after getting back in the car, then thought I need to use the cash point so turned her off again and left the key in ignition while I contemplated whether I could be bothered to go back in, then thought I'll do it later so turned her back on again, maybe leaving the key in and doing that I may have confused things perhaps ?

While not impossible I'd say it was unlikely. I leave my key in the ignition all day at work - my son sometimes swipes it to go shopping and it saves him searching for me.

[Ippypicasso]

Ok that makes sense.
Strangely enough, car (touch wood) is fine now, a weird one.
Thanks

[Old-Guy]

And some people think it strange that an IT Pro (me) should be obsessive about keeping a 17 year-old Xantia (partly) because it has the minimum of electronics in it? I get enough grief (very occasionally) from the alarm and the ABS as it is - at least they don't stop the car being used.

[Ippypicasso]

I see your point, was just saying to a guy in the office today that things were so much easier 20 years back. My old Fords never had a computer that made decisions which could effect my journey from A to B