7 minutes ago, Breezy_Pete said:

I had always thought that VCDS only worked when ignition was switched on or engine was running.

When the driver's is opened (Left for me, right for you) part of the car wakes up. Primarily BCM and fuel pump.

This is why (if wired correctly) you can use the Edmundsen navigation system, while the key is off and such.

Alarm horn replacement has jumped to the top of the work list. Was sounding with no obvious trigger this morning.

That's the second false alarm in a few months.

No fault codes in BCM, but I will replace anyway I think.

It's the factory original and so just over 14 years old now.

Bought an eBay one a while back with a recent date code (2024 I think).

Looks like wipers need to come off to get scuttle cover off and out of the way.

28 minutes ago, Breezy_Pete said:

Alarm horn replacement has jumped to the top of the work list. Was sounding with no obvious trigger this morning.

That's the second false alarm in a few months.

No fault codes in BCM, but I will replace anyway I think.

It's the factory original and so just over 14 years old now.

Bought an eBay one a while back with a recent date code (2024 I think).

Looks like wipers need to come off to get scuttle cover off and out of the way.

That is an early indication of a failing 12V battery, I kid you not. Test your battery with a proper battery tester that tests it under a sustained load, and I expect it will report a higher than normal internal resistance and cause a voltage drop.

Do you mean the main car battery?

That's very new.

Much more likely the rechargeables inside the alarm horn, from what I've read.

Skoda even suggested a five year life for the units at one time.

20 minutes ago, Breezy_Pete said:

Do you mean the main car battery?

That's very new.

Much more likely the rechargeables inside the alarm horn, from what I've read.

Skoda even suggested a five year life for the units at one time.

Oh, OK, I've had the alarm horn going off for no apparent reason on my last three Superb and the main car battery in each case was the cause. Just after the alarm started being triggered, the main battery failed completely. The last time the main battery was only 18 months into its 4 year warrenty. Replacing stopped the alarm being triggered.

Just 2 weeks ago, my 9 month old main battery failed on the day I should have been taking my wife to the airport.

So when you say it is very new, I wouldn't dismiss that completely

I have never had to replace the alarm rechargeable in any of my cars.

38 minutes ago, Graham Butcher said:

I have never had to replace the alarm rechargeable in any of my cars.

IDK what other cars horns batteries you've replaced but Skoda's from the era of Pete's Roomster are sealed for life units and the battery isn't meant to be replaced but they can be by butchering the casing though I wouldn't recommend it for the price of a replacement.

Edited 1 July1 Jul by Lee01

11 minutes ago, Lee01 said:

IDK what other cars horns batteries you've replaced but Skoda's from the era of Pete's Roomster are sealed for life units and the battery isn't meant to be replaced but they can be by butchering the casing though I wouldn't recommend it for the price of a replacement.

If you read my posts, I said that I have NEVER had to replace a horn battery or, indeed, for that matter, a horn. In all of my experiences of the car alarm being triggered, it was the result of the car's main 12V battery beginning to fail. It was also the cause of my neighbour's Ford Fiesta alarm being triggered several times in succession. Fitting a new 12V main car battery in each case cured it.

The car that was written off had an alarm that was 10 years old, and in the car's 10 years it had 3 new 12V main batteries, and twice the alarm was the sign that the battery was failing. The last time that happened and the battery failed to start the car, I was outside of the Essex Ambulance Service depot on North Weald Airfield, and they brought their breakdown truck over to me, tested the 12v main battery and pronounced it had a bad cell, then jump-started me, and I drove to GSF, and they replaced the battery under warranty. Bingo, no more alarms and no more starting issues.

I'll cut the old horn unit open after the swap, to see if there's anything to see.

1 minute ago, Breezy_Pete said:

I'll cut the old horn unit open after the swap, to see if there's anything to see.

This one's a MkII Octavia but probably very similar if not the same.

New alarm horn fitted.

One false step in trying (and failing) to remove a shear-screw that looked like it needed to come out.

[I cut the slot, wasn't sufficient].

Undid the M8 nut in the vertical face of the bracket, and found that there was then enough room to wriggle it out without anything else being undone.

I did remove the rainwater shroud from above/around the cabin air intake to make more room. Allowed for better cleanup of scuttle too.

No sign of anything wrong in old unit though.

NiMH battery at 6.8V, which isn't horrible for a 5-cell arrangement.

Capacitors on board all measured OK in value and tan-delta. Nothing else obvious.

Will just have to wait and see.

1 hour ago, Breezy_Pete said:

New alarm horn fitted.

One false step in trying (and failing) to remove a shear-screw that looked like it needed to come out.

[I cut the slot, wasn't sufficient].

Undid the M8 nut in the vertical face of the bracket, and found that there was then enough room to wriggle it out without anything else being undone.

I did remove the rainwater shroud from above/around the cabin air intake to make more room. Allowed for better cleanup of scuttle too.

No sign of anything wrong in old unit though.

NiMH battery at 6.8V, which isn't horrible for a 5-cell arrangement.

Capacitors on board all measured OK in value and tan-delta. Nothing else obvious.

Will just have to wait and see.

5.8V is perfectly acceptable. So it's looking like either the alarm itself is faulty or, as I said before, check the main battery for high internal impedance and SOH.

You said before that it was pretty new but never said if it was a brand new one, a new second-hand one or how old it was.

Edited 2 July2 Jul by Graham Butcher

Brand new battery fitted two Fridays ago, the day of the alternator failure breakdown.

It should be OK, but if you have a battery tester to hand, I'd still test it under heavy load conditions; you have no clue how long it has been sitting on the shelf for before you purchased it. If nothing else, at least it would remove it from one way or another from being a possible suspect to either the the prime suspect or remove it altogether.

I would not have suspected that a so-called brand new Yuasa 096 EFB battery would fail completely after 9 months (purchased 16/9/25) from GSF Car Parts on 15/6/26, but it did.

Interestingly, I just checked on their website, and it seems that they no longer sell this brand. Form your own conclusions as to why that might have happened.

I don't have a battery tester, I will just be patient and see what happens.

Could be something daft like a spider living near the ultrasonic sensor.

Yes, it could be. Is this a car that is in daily or frequent use, or are you still bringing it back to life?

But battery failures are nothing new; the last time I experienced something like this happening was back when I was the electrical engineer looking after well over 100 buses and coaches in my 20s. I was just watching a video about brand new cars having a total turbo failure, completely destroying engines on brand cars with less than 50 miles on the clock, so it does happen.

PS, A spider was what I blamed in the first instance when my alarms on the Superbs kept going off in the night, but that proved not to be the case in the end.

Edited 2 July2 Jul by Graham Butcher

The car's been in regular use, mixed journeys, for well over a year now.

Rarely sits idle for more than 24 hours.

I'll let you and everyone else know if there turns out to be anything wrong with this battery, or any further issues with the alarm.

Just looked for any evidence of bugs near/in the interior motion sensor; nothing found.

Car alarms are such a waste of time; so happy that my Fabia was built without one.

New development, thanks to the main driver having just used the car and remembered something. 😁

Sunblind that covers the panaromic roof has been playing up in the heat, sometimes the front half opens while driving or when a door is shut, with a fairly alarming (geddit?) bang when it hits the end stop.

She just popped out and found front half of the sunblind open, and was fairly sure it was shut last time she parked it up.

Maybe the heat is making it sometimes open while the car is unoccupied and locked? That would be a very reasonable cause of alarm actuation, I think.

Off to investigate the magnets that supposedly hold the blind shut...

Is the sun blind manual or powered? Is there a spring in it that would tend to assist it opening if the magnets aren't holding it shut?

Manual. Yes, it is spring assisted I think.

The front two magnets were loose and free to spin out of the correct orientation, for reasons unknown.

Rear two seem much more firmly retained and take positive effort to rotate.

Gaffer-taped the front two to each other and the top surface of the blind for now. Doubt that's going to stay there for long, but I did at least alcohol-clean the surfaces first.

That is what I was going to suggest, to find a way of wedging or preventing the blind from opening, or leave it fully open so movement can be ruled out and see how the alarm behaves.