Jaguar XF (X250) Auxiliary Battery, Stop/Start Not Working
On the facelift XF the usual answer is the small auxiliary battery under the boot floor: if it reads far below 12 V it's finished, and a new one brings stop/start straight back. Before buying it, measure the main battery too and check the boot fuse box — and rule out the car simply not being ready to stop.
How bad is it · Safe to drive
Safe to drive; you only lose stop/start
The car drives normally with stop/start out of action. Watch the main battery, though — if that one is weak too, the next step is a car that won't start.
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.
A dead auxiliary battery
Stop/start cars carry a second, small AGM battery under the boot floor. Once it can't hold a charge, the car won't stop the engine and usually says so on the display.
Auto Stop/Start not available or ECO/Stop/Start not available, sometimes with cruise control dropping out too. The small battery reads far below the main one — for example 9 V, or 7.8 V against 12.4 V — and won't take a charge.
Measure it at its terminals with everything off: a healthy one sits in the high 12s to 13 V. Around 8–9 V means it's finished.
Fit a new one. The original is made by Exide, and the EK151 AGM is the same battery without the Jaguar label. It's a straight swap — leave the main battery connected.
A missing fuse or a wet boot fuse box
The dual battery system is fed through the luggage compartment fuse box. A fuse left out after a battery change keeps the message lit permanently, and water in the box does no good either.
ECO/Stop/Start not available all the time, whatever the weather, with the Eco symbol struck through — often ever since the main battery was changed. Hazard lights that won't switch off point to water in the boot.
Open the luggage compartment fuse box and look for an empty slot or a fuse lying loose. On 2013–2015 cars F14, 20 A, feeds the dual battery module. Check the seal where the tailgate loom passes into the body.
Refit the 20 A fuse, dry the box and replace a missing loom seal; the message goes out.
A weak or undercharged main battery
Stop/start needs both batteries healthy. Smart charging eases off at about 85%, so an ageing main battery on short trips drifts below the level stop/start wants without being faulty as such.
Stop/start works for a day or two after a long run, then drops out on short journeys; a low battery, system will shut down warning with the radio on and the engine off; a start button that doesn't pulse red. A battery can pass a shop test and still fall from 12.68 to 12.32 V within minutes on headlamps alone.
Rested, the main battery should read 12.6 V or more; 12.4 V is low. Have it load tested for cold cranking amps rather than trusting a meter. Then charge it overnight: if stop/start returns and fades again over the next days, state of charge is the cause.
Worth trying: a full charge, then regular runs of half an hour or more. If it won't hold, fit an AGM battery made for stop/start — for example a Varta Silver Dynamic G14 — not an ordinary flooded one.
A new battery that was never registered
The battery monitoring system goes on charging a new battery as if it were the old one until it is told otherwise. Until then stop/start can stay off for good.
No stop/start at any time since a battery was fitted, even after long motorway runs.
Ask whether the new battery was registered with SDD or an aftermarket tool that can do it, such as some iCarsoft or Autel versions.
Worth doing after any main battery change. For the small battery opinion is split: there may be no reset for it at all, and a new auxiliary often works at once without one.
A corroded or loose terminal on the small battery
The small battery sits under the boot floor, where damp collects. A broken clamp or a dirty earth looks to the car like a poor battery.
Stop/start dead while the battery itself measures fine; white or green deposits on the small battery's terminals.
Lift the boot floor and check both terminals and both batteries' earths for corrosion and tightness. The vent tube should be connected to both batteries.
Worth doing first, as it costs nothing: clean the terminals with a baking soda solution and tighten or replace a broken clamp.
Not a fault — the car isn't ready to stop
Stop/start holds off by design whenever stopping would cost more than it saves. No fault is stored, and it returns by itself once the conditions are met.
The engine won't stop early in a journey, at or below freezing, with the climate on auto in hot or cold weather, while a diesel's DPF needs to regenerate, or after using the paddles. An A/C system low on gas that never reaches its set temperature does the same.
Try again with the engine fully warm and the climate turned down. After paddling down, pull the right paddle to go back to Drive.
Worth trying before spending anything: a longer run that lets a regeneration finish, and less demand on the climate at the lights.
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.
Rule out the normal reasons it holds off
Engine fully warm, above freezing, Drive selected and the climate not working hard. If it still never stops, look for a cause.
freeMeasure both batteries
Engine off, against a body earth. The main one wants 12.6 V or more, the small one the high 12s to 13 V; 8–9 V on the small one ends the search.
freeCheck the boot fuse box and the terminals
Look for a missing 20 A fuse or water in the luggage compartment fuse box, then for corrosion and loose clamps on both batteries.
freeCharge the main battery properly
Use a charger suited to AGM: positive on the battery, negative on a good body earth rather than the negative post, so the battery monitor sees the charge.
a chargerReplace the auxiliary battery
An Exide EK151 AGM. Change it on its own and leave the main battery connected: disconnecting the main one before the modules sleep can leave the car not recognising its key.
about £54Replace a weak main battery and have it registered
A stop/start AGM matching the original rating, then registration with SDD or a capable aftermarket tool.
an AGM battery and a diagnostic sessionThe fuses named above — where they sit
The highlighted position is the one the procedure talks about. Pulling a fuse is a stop-gap, never a repair: check what the circuit feeds before you pull it.
Circuit descriptions from the Jaguar handbook, 2013 edition, p. 1, 2. Drawing by jagaux.com from the factory electrical guide. Fuse layouts change between model years — read the legend on the lid of your own box before pulling anything.
What it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- Jaguar XF (X250) Battery Not ChargingIf the dash says the battery isn't charging or the main battery keeps going low.
- Jaguar XF (X250) Smart Key Not Recognised or Not DetectedIf the key isn't found after the batteries were disconnected.
- Jaguar XF (X250) DPF Full, Restricted PerformanceIf a diesel also warns about the DPF — stop/start waits for the regeneration.
- XF (X250) fuse boxes and full fuse listThe luggage compartment fuse box with the dual battery module fuse.
- Jaguar XF (X250) Battery Drain, Low Battery WarningIf the main battery goes flat when the car stands.
- Jaguar XF (X250) Problems After Battery ChangeIf warnings or locking faults appeared after a battery change.
- 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.