These instructions apply to the MG HS Mk1 2019-present.
For other models, please choose your vehicle here.
If the smart key for your MG HS (2019–present) only locks the car when you are almost touching the door handle, needs a second or third press, or has stopped waking the car at all, the coin cell inside it has almost certainly gone flat. Every HS — petrol, the plug-in EHS/PHEV, and the facelift — uses the same style of keyless smart fob: a one-piece proximity remote with a hidden metal emergency blade and a single coin cell you can change on the driveway in a couple of minutes. Here is the exact battery, how to open the case without cracking it, and why the car sometimes needs a quick re-sync afterwards.
How to Tell the Fob Battery Is Dying
- Shrinking range. You end up standing right next to the HS, or pressing the fob against the door handle, to get it to lock or unlock. This is almost always the first sign.
- Keyless entry gets flaky. The car no longer wakes as you approach with the key in your pocket, and passive lock/unlock on the door handle becomes hit-and-miss.
- A key-battery warning in the cluster. The HS shows a low-key or “smart key battery low” message in the instrument display when the fob voltage drops.
- Cold-morning dropouts. A tired lithium cell loses voltage in the cold, so intermittent response on frosty mornings often means the battery is on its way out.
A coin cell in an HS fob typically lasts two to three years. Leaving the key sitting near the car so its proximity sensor keeps waking shortens that noticeably. If yours is a couple of years old and doing any of the above, fit a fresh cell before you suspect anything more serious.
The Battery You Need
The MG HS smart fob uses a single CR2032 — a 3-volt lithium coin cell, 20 mm across — the most common fob battery there is, sold in any supermarket, hardware shop or petrol station. Buy a good-quality branded cell. Owners on the MG forums repeatedly trace “the new battery didn’t work” back to weak or counterfeit cells bought cheaply online, so a name-brand CR2032 from a shop you trust is worth the extra pennies. If you are ever unsure, match the code printed on the flat face of the cell you take out.
How to Change the MG HS Fob Battery
- Release the emergency blade. Press the small catch on the side of the fob and slide the metal emergency key straight out. You will use its tip as a lever, and it also uncovers the seam.
- Split the case. Slip the tip of the emergency blade (or a thin, flat plastic tool) into the seam where the blade came out and gently twist to pop the cover off. Work slowly around the edge — the case is plastic and only needs light pressure, not force. Avoid a sharp metal screwdriver, which chews the seam.
- Note how the old cell sits, then lift it out. Take a photo first if you like. Ease the flat cell out from under its retaining clip.
- Fit the new CR2032 positive (+) side up. The marked “+” face should point up, toward the cover, so it seats under the clip. Push it home until it sits flat and secure.
- Click the case back together. Line up the two halves and press until the cover snaps shut all the way round, then slide the emergency blade back in until it latches.
- Test all the buttons and the keyless approach before you rely on it.
⚠️ Keep your fingers off the flat faces of the cell. Skin grease on a coin-cell contact is enough to cause short range or dropouts; handle the new battery by its edge and wipe the contacts with a dry cloth if you touched them. Fit it the wrong way up and the fob simply won’t work — so copy the orientation of the cell you removed, positive up.
Check It Worked — and Re-sync If Needed
Stand back a few metres and lock, then unlock, the HS. Full range should return at once, and the keyless approach should wake the car again. The low-key warning clears the next time the car reads a healthy fob.
If the fob feels sluggish or the car won’t recognise it straight after the swap, it may need a quick re-sync. MG HS owners report a reliable method: drop the fob into the bottom of the centre-console cupholder and press the Start button — the car reads the key at close range and re-pairs it. Setting the vehicle to the READY / ignition-on state with the fob inside also re-establishes the link.
If a Fresh Cell Doesn’t Fix It
- Re-check polarity and seating. Positive up, sitting flat under its clip against clean contacts — the single most common mistake.
- Suspect the battery itself. Test it with a meter if you can; a “new” cheap or counterfeit cell reading under 3 V is a frequent culprit. Try a known-good branded CR2032.
- Wipe the contacts. A little corrosion or finger grease on the terminals causes exactly the intermittent behaviour of a flat cell.
- Try the cupholder re-sync described above before assuming the fob is faulty.
- Rule out the car’s 12 V battery. A weak vehicle battery can make the HS slow to wake and mimic key faults; if the car itself struggles to power up, that is worth checking too.
New Key or Spare — Why It’s a Dealer Job
Changing the cell keeps the fob’s existing coding, so nothing needs reprogramming — the key works as before. Adding a new key or replacing a lost one is a different matter: MG smart fobs have to be coded to the car’s immobiliser with diagnostic equipment, blanks are ordered against the VIN, and owners note the pair of keys is often reprogrammed together. That makes a spare or replacement key a job for an MG dealer or a qualified auto-locksmith — so keep the battery in your spare fresh too, and you are never down to one working key.
Frequently Asked Questions
What battery does an MG HS key fob take?
A single CR2032 3-volt lithium coin cell. The same cell fits the petrol HS, the EHS/PHEV and the facelift.
Which way up does the battery go?
Positive (+) side up, facing the cover, so it seats under the retaining clip. Fitted upside down the fob won’t respond, so match the orientation of the cell you removed.
Do I need to reprogram the key after changing the battery?
No. A battery swap doesn’t affect the coding. At most the fob needs a quick re-sync — drop it in the centre-console cupholder and press Start, or switch the ignition on with the key inside.
How long should an MG HS key battery last?
Usually two to three years. Storing the fob right next to the car keeps its proximity sensor active and drains it faster.
My key still won’t work after a new battery — what now?
Re-check the polarity and seating first, then the battery quality — a weak or counterfeit CR2032 is a common cause. Clean the contacts and try the cupholder re-sync. If a known-good cell still doesn’t help, the fob may need attention from a dealer.
Can I drive the HS if the fob is completely dead?
Yes. Use the emergency blade hidden in the fob to unlock the driver’s door, then hold the fob against the engine Start/Stop button and press it — the HS reads the key at close range and starts even with a flat cell, so a dead battery won’t strand you.
If the remote stopped working suddenly — right after the car’s 12 V battery was disconnected, say — rather than fading gradually, the pairing or a stored fault is a likelier cause than the cell; you can look up any logged code on autodtcs.com.
Independent guide — not affiliated with MG. Always follow your official service manual and safety precautions. Read our full disclaimer.