Jaguar XJ (X351) Battery Problems, Stop Start Fault
On the X351 battery trouble comes in three shapes: a worn or deeply flattened main battery, a charging fault right after a battery change, and stop/start that gives up. When stop/start stops on a 2012–2015 car, test the small auxiliary battery first — a new main battery won't fix it. After any battery change, put the battery monitor into relearn rather than paying for coding.
How bad is it · Safe to drive
Drive it, but don't leave it standing on a weak battery
A failed auxiliary battery only costs you stop/start. A main battery that keeps going flat will leave you locked out — and a deeply flattened battery loses capacity for good.
Which one applies to yours depends on your symptoms, your model year and what has already been done to it.
Likely causes, most common first
Read the check cost before the odds: a free check that rules a cause out beats a likely guess that costs money.
Failed auxiliary (small) battery
From MY2012 until single-battery cars arrived around MY2016, stop/start runs off a second small battery. It wears out on its own schedule, and the system shuts stop/start off when it does.
Stop/start has quit, and a new main battery and a battery monitor reset change nothing. The small battery shows 14 V on the charger and about 10 V a couple of hours later.
Charge the small battery, take it off the charger and measure again after two hours. If it has sunk to about 10 V, it is finished.
Replace it with the same type as fitted at the factory; Exide makes it, for example the EK151 at around £50.
A worn or deeply flattened main battery
A battery that has once been run completely flat loses capacity for good. A drop tester only shows it is good enough at today's charge, not how much it holds.
It goes flat after an hour of washing the car with the doors opening, or within two weeks of standing; a dealer reading of about 54% is already poor.
Measure across the battery before charging, charge it fully with a smart charger, then test again after an hour off charge and at idle.
A new battery when it won't hold charge. A small jump pack may turn the engine over and then run out on a completely flat battery.
Charging System Fault right after a battery change
The car's battery monitor has to be told a new battery is in; until then charging can be misjudged. If that isn't it, the alternator is.
Charging System Fault on the dash immediately after a new battery went in.
Have the battery monitor reset first. If the message stays, have the charging output checked.
A battery monitor reset in the best case; an alternator replacement in the other.
The car never had stop/start
Jaguar fitted stop/start from MY2012. An earlier car may carry an AGM battery without having the system at all.
A 2010 or 2011 car with no stop/start switch or menu option that has never stopped its engine.
Look for the stop/start button and the setting. No button means no system.
Nothing to fix. It can't be added at a sensible cost.
The order to check them in
The point of the order is money: each step either rules a cause out or makes the next one worth doing. Green is free, amber costs a little, red is the one to arrive at last.
Which car do you have
Up to MY2011: one battery, no stop/start. MY2012–2015: main battery plus a small auxiliary one, both AGM. From MY2016: a single battery.
freeStop/start gone: the small battery
Charge it, leave it two hours off charge, measure. A big drop condemns it, whatever the main battery reads.
free to testMain battery: charge, then test properly
Smart charger with positive on the battery post, negative on the chassis stud that the battery earth cable is bolted to, so the battery monitor sees the current. Then test after an hour off charge.
a smart chargerFlat and locked out
The mechanical key opens the door and the bonnet. Connect a pack to the jump terminal in the engine fuse box and a clean chassis point, press only the boot release, then connect to the battery itself to start.
freeChanging the battery
Like for like: AGM for AGM. Let the modules sleep first — doors shut, key out of range, half an hour — and backup power isn't needed. Coding isn't needed either: put the battery monitor into relearn so the smart alternator doesn't overcharge. Dealers charge about £250–280 fitted; an equivalent battery is around £105–190.
a batteryAfter the change
Reset the one-touch windows and the clock. If a locking fault appears, disconnect the battery again and leave it 30 minutes, rather than shorting the leads together.
freeCharging System Fault
Battery monitor reset first, then the alternator output.
free, or an alternatorWhat it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- Jaguar XJ (X351) Touch Screen Problems, Software UpdateIf the screen starts blanking or rebooting — check the battery first.
- Jaguar VIN decoderConfirm your exact model year, engine and factory options before ordering parts — free.
Ratings, fuse numbers and torque figures vary by model year and market. Read the legend on your own car before pulling anything, and use the figures in the manual for your car, not the ones in a video.
Check your VIN: exactly which car you have and which bulletins apply to it.