Most key fob "faults" are just a flat battery. This guide walks you through changing it yourself — what battery you need, how to open the fob without breaking it, and what to do when a fresh battery doesn't fix the problem.
Before you replace the battery, check the symptoms match a battery problem. Some "key faults" are actually fob synchronisation or immobiliser issues — battery replacement won't help those.
You used to unlock from across the car park; now you have to be next to the car. Classic battery-dying symptom.
Sometimes the button works, sometimes you have to press multiple times. Battery running down.
Most fobs have a small LED that flashes when you press a button. If it's dim or doesn't light at all, the battery is low.
Keyless-entry cars (Comfort Access, KeylessGo etc.) need fob battery power for proximity detection. If the car doesn't unlock on approach but still unlocks via button-press, the fob battery is on its way out.
The car says "Key Not Detected" or won't start even with the fob held against the start button. That's usually fob synchronisation or immobiliser fault — battery replacement won't fix it. Call us for diagnosis.
Most key fobs use a CR2032 lithium coin cell — but a few use CR2025, CR2016 or CR1620. Check the existing battery before buying a new one.
20mm diameter, 3.2mm thick. Fitted to BMW (CAS, FEM, BDC fobs), Mercedes (most FBS3 and FBS4 fobs), Range Rover/JLR (L322, L405, L494, L538 fobs), Audi, VW, Skoda, SEAT, Porsche (most modern), Volvo, Ford and most others. A few pounds from any supermarket or DIY shop.
20mm diameter, 2.5mm thick. Some older Audi/VW fobs, some older Toyota/Lexus. Looks identical to CR2032 but slightly thinner — will rattle in a CR2032 holder.
20mm diameter, 1.6mm thick. Older Audi A4/A6 flip-key fobs, some older Honda. Stacked pair sometimes used in older keys.
16mm diameter. Some Aston Martin fobs, some Volkswagen, some Volvo S60. Always check the existing battery before assuming CR2032.
Brand matters less than you'd think — Panasonic, Sony, Duracell, Energizer, Maxell are all fine. Avoid no-name cheap multipacks: they're often well past their shelf life and last weeks instead of years. Pay a few pounds for a quality single battery.
Most modern fobs are designed to be opened — but they're also designed not to fall open in your pocket. The trick is knowing where to look for the release.
Slide out the small mechanical key blade first (button on side or bottom). The slot the blade came from often contains a small lever — slide it across with a coin and the fob's top half pops off.
Slide out the mechanical key blade. Use the blade itself (or a coin) to lever open the rear case at the slot where the blade lives. The case has a small notch designed for this.
No mechanical blade visible from outside. Find the small chrome "tab" on the bottom edge — slide it across with a fingernail to release the rear case.
Press the button to flip out the key blade. With the blade folded out, look for a small slot near the hinge — insert a flat blade screwdriver and twist gently to release the case.
Never force it. If the case won't open with reasonable pressure at the obvious release point, look closer — there's always a release somewhere. Forcing breaks the plastic and the only fix is a new fob case.
Once you're into the fob, the actual battery swap takes 30 seconds. Here's the careful version.
Look at the existing battery before removing — note which side faces up (the "+" or "-" marking). Phone-photo it if helpful. Putting it back wrong is the single most common DIY mistake.
Use a non-metal tool (plastic spudger, fingernail, wooden toothpick) to lever the battery out. Avoid metal tools — they can short the battery or scratch the contacts.
If the contacts look corroded or dirty, wipe with a clean cotton bud and a tiny drop of isopropyl alcohol. Let dry before refitting.
Drop the new battery in, "+" side facing the same direction as the old one. Press gently until it clicks into place. Don't force.
Line up the case halves and press together until they click. There's no glue — the case is meant to click shut.
Press a button. The indicator LED should flash bright. Test on the car: lock, unlock, range. Should be back to normal.
A fresh battery doesn't fix every fob fault. Here's what's likely going on if your battery swap didn't solve the problem.
Some cars need the fob to be "re-synced" after a battery change — usually by holding it close to the start button and pressing a button on it. Check your handbook for the specific procedure.
Fobs can fail internally — broken solder joint, blown chip, water damage. New battery won't fix it. We can replace just the internal electronics (cheaper) or supply a new fob entirely.
If the car is reading the fob but refusing to start ("Key not recognised" message), the issue is usually the immobiliser module, not the fob. Common on Mercedes W204 (ELV), BMW E series (CAS faults), JLR (KVM faults).
CR2025 in a CR2032 holder will look the same but contact poorly. CR1620 in any larger holder won't fit at all. Double-check you bought the right size.
Sounds basic but happens. The "+" symbol should be visible (facing up out of the fob in most designs). Some battery holders have "+" embossed on the contact — match it.
Most fob battery jobs are genuinely DIY — a battery and 5 minutes, done. We're writing this guide because honesty serves us better than upsell. If your fresh battery doesn't fix the fob, then you have a different problem, and we're the people to call.
Every quote is fixed before we book. WhatsApp your registration and a brief description of the issue — most quotes back within minutes, 7 days a week.
Send your registration and we'll confirm exactly what's involved and what it costs — fixed price, no surprises. Most quotes back within minutes, 7 days a week.