I currently get some error codes from my 2004 XJ8:

C1430 Rear left damper [air strut] short circuit to battery
C2302 Leveling plausibility error
C1427 Rear right damper -- circuit open
B2496 Anti-theft horn circuit short to ground

The car hasn't got any symptoms, tho I haven't tested the anti-theft horn in a long time (and don't know how to -- what sets it off?)

UPDATE: I unplugged the electrical connection from the L air strut, and tested the resistance between each of the 2 sockets (on the wire connector, not on the air strut) and ground.
One was open, the other had a resistance of 8.5M ohms. Is that enough of a "short" to trigger C1430? If so, I can ignore it, right?
Seems to me a short through 8Mohms would take centuries to drain the battery.

But I need to drive far away, park my car for a week, and drive back. Could these short-circuits drain my battery by then?
Will those circuits still have power when the key is not in the ignition?

I think something is draining the battery, or the alternator is poor, because I drove 120 miles yesterday, and today apparently had several bogus error codes caused by a low battery.
The battery is about 6 months old, AGM, and meets or exceeds factory specs.

I'm also worried about how the open circuit on the R damper affects driving, though I haven't noticed any problem.
I recently replaced my Bilstein air struts with Miessler air struts; maybe they've gone bad.

 

Sorry to hear about this, @Xeno

Do you have a multimeter or an OBD II scan tool? If so, you can read the charging voltage. You should see something over 14 volts if it is charging correctly. That is where I would start since you have a newer AGM battery. If you have a meter that can read voltage, measure across the battery posts with the car completely off and then again with the engine running. With the car off it should be around 12.6 volts and with it running it should be over 14 volts. If you don't have these tools, you can also take the XJ to an auto-parts store, like O'Reilly or Autozone, and they should be able to perform a load test on your battery and charging system test on the spot. If your lucky, they might also be able to test for a current draw. Hope this helps.

-j

 

AGM batteries are unsuitable for these vehicles because they require charging voltages of 14.6 -14.8 V for optimum operation and a specific charging cycle that is monitored by a battery controller. There is no such battery controller in the XJ.

The effects of ground faults can be very different. However, it cannot be ruled out that they produce a battery discharge, if only because one or more control units therefore do not fall into sleep mode. A measurement of the quiescent current provides information about this. How this is done can be found on the WWW, the quiescent current on X350 should be less than 30 mA 40 min after shutdown.

Fritz

 

That's surprising -- most of the batteries that battery store websites say are compatible with my car are AGM.
Are you sure you aren't thinking of gel batteries? batterystuff.com says,
It has no such statement for AGM batteries. On another page it says,
This page says
(The website links to that page from a page about AGM batteries, and says AGM batteries are another type of sealed lead acid battery.)

More googling indicates that AGM batteries don't require a special charging cycle, but do use a higher charging voltage, like you said.
But then the question is whether this higher charging voltage matters, or whether it just charges the battery faster.

The factory battery is Jaguar C2D22312, but I can't find out what chemistry it uses.

 

The big one is drawing down your battery when parked. You may have a short somewhere. If concerned about parking for a week, first make sure you can open trunk with a key. Then disconnect the negative post from battery.Battery cannot be drained then.
I would reset everything and drive to get everything reset and then scan again. Low voltage can trigger all kinds of error readings.
My experience with AGM batteries tells me they work fine in a X356. Switched my el cheap AGM after 6 1/2 years. Still started car every time, but car sits for weeks sometimes. Was pulling down to less than 12.5 volts when not driven.
If you drive yours regularly it should be fine.
The reading of 8.5 Meg will pull down the battery. If you let it sit for a few years. I have let mine sit for over a month and it cranked right off. Same with he control system drawing 40 mA when parked. But a steady 8.5 Meg ohm leak will pull 0.0015 mA from the fully charged battery.

 

I carry a jump starter, a decent one not those little lithium toys. Of course, Murphy's Antilaw says I have never had to use it.
Consider disconnecting the battery (only a few things to reset), making sure your key works manually. Further info here:
Battery dead, trunk locked, key will NOT open trunk - Jaguar Forums - Jaguar Enthusiasts Forum

Oops, my jump starter is carried in the trunk. How clever is that?

 

Don't worry about it - plenty of people report using AGM batteries just fine.
Whilst it's true that the original spec is Silver Calcium, and the documentation says charge voltage can reach ~15.3v, I've only ever seen ~14.6v.
And there's another saving grace. The charging only maintains [~14.6v] for 5 mins exactly, after which it drops back to [~13.5v]. It might come on again whilst driving but only for a few minutes. This means the charging is actually compatible with nearly any battery.
I happen to have a Calcium battery, though I don't think it's Silver Calcium which is near unobtainable.

I see my measurement for quiescent (parked) current averages 36mA, or 0.86Ah/day, or about 50Ah in 60 days. (based on sitting on a charger so includes battery self-discharge)
The car systems esp. suspension do wake up occasionally. The battery is never FULL even on a long drive. A battery is never new even when bought new. If I was leaving the car in Timbuktu for a week, I imagine I'd concoct some emergency plan.
I believe power can be applied to either stud of fuse F35 in the front fuse box in the hood, not for starting but perhaps to open the trunk.

 

My quiescent current is 4.8 amps, not milliamps.

 

Have you attempted to use an ammeter connected through the negative battery terminal to monitor the parasitic draw? If you are having a draw of nearly 5 amps with the vehicle completely shut down, go circuit by circuit by individually removing one fuse at a time to see which circuit is causing the draw.

Once you determine the circuit, look at the components and modules on that circuit to find the cause.

 

That is a huge draw. If a constant 4.8 amps a thermal gun will find it. It has to be hot. The wire may be hot.
The resistance calculates around 2.6 ohms. If it sits overnight, what is the battery voltage reading?
Like was mentioned start pulling fuses. 10 amp and up. A 5 amp would not last last passing 4.8 amps.

 

My Fluke ammeter gives a different reading every time I use it, ranging from 2.8 to 5 amps.
But I put a 12V 0.6A fan inline instead of the ammeter, and it wasn't enough current to turn it on.
Then I put a 12V 0.25A fan inline, and it brought that up to full speed.
So the current is between .25 and .6 amps.

Then I pulled all of the fuses and relays, one at a time, and found one, passenger fusebox F38, "Radio head unit", for which the fan slowed down when I pulled it out.
Then I left it out, and went back and pulled all of the other fuses and relays a second time, finding 2 that had a very small impact on the current drain, also
in the passenger fusebox:
F42: FEM, REM, DDM, clock
F37: Driver window

But most of the quiescent current was still on after pulling all 3 of these fuses. Whatever short is draining most of the current isn't going through a fuse or a relay, unless it's one that isn't in any of the 3 fuseboxes. I don't know what to do now. A friend told me the alternator draws current that doesn't go through a fuse.
And there could be shorts from a wire to ground anywhere.

Since I still get the code
B2496 Anti-theft horn circuit short to ground

I figure that's a good place to start, but I can't make heads or tails of the XJ electrical guide.
I've attached one page from it, with a schematic of the security system, and am inserting here a picture of the upper-right corner, around the anti-theft horns.
I don't know whether "Anti-theft horn circuit" refers to the passive or the active horn.
I don't know how to interpret anything in the schematic.


On the "Passive security sounder", I guess GR means a green wire, and B means blue or black? But what do all the little marks like PS2, CR45-2, CR13-8 mean?
What do the square-and-cup pictures above those labels mean? I can't relate any of this schematic either to locations in the car, or to fuses or fuseboxes, or anything at all.

 

From what I can work out, you put a 20ohm or 50ohm resistor (a fan) in series with the battery? Naturally this would drop the voltage to the car system dramatically and cause "less current".
Your Fluke ammeter is likely to be more accurate. Just trust it even if the current varies.
Last time I looked, color codes are given separately nearer the top of the manual. Not that that's much help.
All the fancy numbers refer to connectors (I think). You could try disconnecting any accessible ones.
Not certain but it's possible an alternator with a blown diode could draw current. So disconnect it.

Other than that, connecting a fan in series with the battery as a diagnosis method, suggests you might want to visit an auto electrician?

 

I can't even find the anti-theft alarm horn. I think it's buried somewhere between the trunk and the right rear side panels.

 

Have you searched the forum?
B2496? Anti-Theft Horn? - Jaguar Forums - Jaguar Enthusiasts Forum

Personally, I think this car is more complex than one of Elon Musk's rockets.