Replace the Key Fob Battery

If the remote on your SsangYong Rexton (Y400, 2017–2022) only locks the car when you are almost touching the door, responds to some button presses but not others, or has stopped working altogether, the coin cell inside the fob is the first thing to change — and usually the only thing. The Y400 — the fourth-generation Rexton, also badged Rexton G4 and sold as the Mahindra Rexton in some markets — is normally supplied with a keyless smart fob carrying a hidden mechanical blade, the same key family SsangYong fitted to the current Musso pickup and Korando. Some trims and markets instead came with a folding “flip” remote. Both take the same battery but open differently, so this guide covers each and explains how to tell a flat cell from a genuine fault.

How to Tell the Fob Battery Is Dying

• Shrinking range. You have to stand right beside the Rexton, or hold the fob against the door handle, before it will lock or unlock. This is almost always the first sign of a tired cell.
• Keyless entry gets fussy. On the smart fob, the door handle stops sensing the key in your pocket, and the push-button start no longer wakes the car unless you hold the fob right against the start button.
• A key-battery warning. A weak smart key can trigger a low-key-battery message in the instrument cluster — treat it as a prompt to swap the cell rather than a fault to diagnose.
• Cold-morning dropouts. A cell that is nearly flat sags further in cold weather, so the remote that worked yesterday afternoon may do nothing on a frosty start.

A coin cell in a Rexton fob typically lasts two to four years. If yours is around that age and showing any of the above, fit a fresh battery before you suspect the receiver, the immobiliser, or the car’s own 12 V battery.

The Battery You Need

Both the smart keyless fob and the folding remote use a single CR2032 — a 3-volt lithium coin cell, 20 mm across, the most common fob battery made and stocked by any supermarket, hardware shop, or petrol station. Buy a good-quality branded cell: cheap coin cells sag under the brief high current the transmitter draws and can give poor range even straight out of the packet. If the battery you lift out is marked differently from CR2032, always match the exact code printed on that cell rather than assuming.

Method 1 — Smart (Keyless) Fob

1. Release the emergency blade. Slide or press the small catch on the fob and pull the mechanical key blade straight out. That blade both unlocks the door if the fob is dead and doubles as a lever for the next step.
2. Split the case. Slot a flat plastic tool — or the emergency blade itself — into the seam the blade leaves behind and gently twist to part the two halves of the housing. Work around the joint rather than forcing one spot, so you don’t crack the plastic.
3. Note the old cell, then lift it out. Before it comes free, look at which face — the flat “+” side or the side with the code — is pointing up. Press the fob buttons a few times with the cell out to drain any residual charge.
4. Fit the new CR2032 the same way round as the one you removed, then click the two halves back together and slide the blade home until it latches.

Method 2 — Folding “Flip” Remote

1. Fold the metal blade closed so the fob sits flat and is easier to hold.
2. Look for a small retaining screw on the back. If there is one, undo it with a fine cross-head (Phillips) screwdriver and lift the cover off; if there is no screw, the case simply clips, so prise the seam apart gently with a flat plastic tool or a small flat-blade screwdriver.
3. Note the orientation of the old cell, lift it out, and drain the fob by pressing the buttons a couple of times.
4. Drop in the fresh CR2032 the same way up, refit the cover, and — if it has one — drive the screw back in snugly. Don’t overtighten it; the case is plastic.

⚠️ Match the polarity. A coin cell fitted upside down will not power the transmitter and can slowly discharge against the contacts, leaving you with a “new” battery that is already flat. Always copy the exact orientation of the cell you took out — that is why it is worth glancing at which face points up before the old one comes free.

Did It Work?

Stand back a few metres and lock, then unlock, the Rexton. Full range should return straight away. On the keyless fob, walk up to the door and try the handle sensor, then the push-button start, to confirm proximity sensing is back. Any low-key-battery message usually clears once the car has read a healthy fob, occasionally after a drive cycle rather than the instant you fit the cell.

If a Fresh Cell Doesn’t Fix It

• Re-check the polarity and seating. An upside-down or loosely seated cell is by far the most common reason a battery swap “doesn’t work.” The cell must sit firmly under its clip against clean metal.
• Wipe the contacts. A smear of finger grease or a little corrosion on the terminals is enough to cause dropouts; clean them with a dry cloth or a cotton bud.
• Try the other fob. If your spare works at full range with its own battery, the problem is the first fob’s electronics, not the car.
• Rule out the car’s 12 V battery. A weak main battery can make a Rexton slow to wake and mimic key faults; if the car itself is sluggish to respond, have the vehicle battery tested.

New Key or Spare — Why It’s a Dealer Job

Changing the cell keeps the fob’s existing coding, so nothing needs reprogramming and the remote works the moment the fresh battery is in. Adding a new key is a different job: a blank smart fob or flip remote has to be ordered against the car’s VIN, the blade cut to the lock, and the transponder paired to the Rexton’s immobiliser with diagnostic equipment. That means a SsangYong dealer or an auto-locksmith with the right kit — so keep the battery in your spare fresh too, and you are never down to a single working key.

Frequently Asked Questions

What battery does a SsangYong Rexton Y400 key fob take?

A single CR2032 3-volt lithium coin cell. The keyless smart fob and the folding flip remote both use the same size, so you only need one type to hand.

Do I need to reprogram the key after changing the battery?

No. Swapping the coin cell doesn’t touch the fob’s coding, so it keeps working straight away. Only a brand-new key that has never been paired to the car needs programming.

Which way round does the battery go?

Fit the new CR2032 the same way up as the one you removed — glance at which face points up before you lift the old cell out. A cell fitted the wrong way will not power the fob and can flatten itself against the contacts.

How do I open the smart key without breaking it?

Pull the emergency blade out first, then ease a flat plastic tool into the gap it leaves and twist gently to split the halves. Work around the seam rather than levering hard in one place, and the case parts without cracking.

How long should a Rexton key battery last?

Usually two to four years. Heavy use of the remote, or leaving a keyless fob sitting close to the car so it wakes constantly, shortens that.

Can I still drive the Rexton if the fob battery is completely dead?

Yes. Use the hidden mechanical blade to unlock the driver’s door, then hold the smart fob against the push-button start (or the marked spot on the column) so the car can read it directly, and start as normal. A flat coin cell won’t strand you.

If the remote died suddenly right after the car battery was disconnected or went flat — rather than fading gradually over weeks — the cause is more likely the car’s electronics than the coin cell, and a stored fault code can point you to it. You can look up what any logged code means on autodtcs.com.