"My key fob has died" is one of the most common car emergencies — and one where the diagnosis matters a lot, because the fix ranges from £2 for a new battery to £450 for a replacement smart key. The good news: you can usually rule out the expensive scenarios in 5 minutes. This guide walks through the diagnostic sequence in order of likelihood.
The 5-minute diagnostic sequence
Step 1: Test within 1 metre of the car
If you usually use the fob from 5+ metres away (driveway, across a car park), try standing within touching distance of the car. If it suddenly works, the battery is weak. Replace it.
Step 2: Replace the battery (almost always the fix)
Most fobs use a CR2032 or CR2025 coin cell. £1-£3 at any supermarket or hardware shop. Procedure:
- Find the seam or notch on the fob — usually requires a small flat screwdriver or coin to pry open
- Note the orientation of the existing battery (which side is up)
- Pop out the old, slide in the new in the same orientation
- Snap the case back together
- Test
For smart proximity keys, battery replacement sometimes triggers a reset — you may need to resync (see Step 4).
Step 3: Try the spare fob
If you have a spare and it works, your main fob has failed (electronic fault inside, water damage, etc.). Spare doesn't work either? The car's receiver might be the issue, or both fobs lost sync simultaneously (rare but happens after a flat car battery).
Step 4: Resync procedure
Fobs sometimes "forget" their pairing — usually after a dead car battery, a battery disconnect, or a fob battery swap. Resync procedures vary by make:
- Ford: Insert key, turn to ON 4 times within 6 seconds, press lock + unlock on fob
- Vauxhall: Key in ignition, turn to ON, press fob lock button within 5 seconds, turn off, repeat for each fob
- VW / Audi: Often requires VAG-COM or OBD tool — locksmith job
- Toyota: Key in ignition, turn to ON, press lock/unlock sequence (usually 5 presses then unlock)
- Honda: Insert key, turn to ON, press unlock on fob
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Step 5: Use the physical key blade
Almost every modern fob has a hidden physical key blade — a slide-out metal key. Use it to manually unlock the driver's door. Once inside, try starting the car: if the immobiliser detects the fob in the cabin, the engine will start even if the remote buttons aren't working. (This is a useful workaround for a dead fob battery in a proximity-key car.)
Push-button start cars — special case
If your car has push-button start and the button doesn't respond, three common causes:
- Dead fob battery. Most cars allow you to physically touch the fob to the start button to bypass — check the owner's manual.
- Brake or clutch not pressed. Modern cars require brake (auto) or clutch (manual) for start. Sometimes the brake switch fails — try pressing very firmly.
- Immobiliser fault. Dashboard immobiliser light stays lit. Try locking/unlocking with the fob 2-3 times to resync. If persistent, locksmith diagnostics (£40-£100).
"Ghost" fob behaviour
Sometimes a fob works intermittently — locks but doesn't unlock, or unlocks but doesn't start the engine. Most common causes:
- Weak battery not fully dead — replace and retest
- Worn buttons inside the fob — circuit board contacts dirty or button rubber deformed
- Water damage — common after fobs get wet (in a coat pocket, dropped in a Cornwall puddle)
- Signal interference — car parks near hospitals, transmission towers, or military bases have strong RF that can occasionally jam fobs (rare)
What it costs in Cornwall
| Job | Cost |
|---|---|
| Fob battery (CR2032 / CR2025) | £1-£3 |
| Fob diagnostics (mobile auto locksmith) | £40-£100 |
| Fob resync (if locksmith needed) | £40-£80 |
| Replacement fob shell (worn buttons) | £15-£40 part + £30-£60 to reprogram |
| Replacement fob (cut + programmed) | £100-£250 typical / £250-£500 premium |
| Immobiliser ECU reset (if fault) | £100-£250 |
When to call a Cornwall mobile auto locksmith
- Battery replacement didn't fix it
- Spare fob also doesn't work
- Resync procedure doesn't apply or doesn't work
- Dashboard shows immobiliser warning
- Fob is physically damaged (cracked, water-logged)
Need a car key fob fixed in Cornwall? Submit your postcode — we'll match you with a mobile auto locksmith who comes to your car, does diagnostics on the spot, and quotes before any chargeable work.
Frequently asked questions
Why has my car key fob stopped working?
Most common: flat battery (CR2032 or CR2025 coin cell, £1-£3 at any supermarket). Other causes: lost sync after a flat car battery, water damage, worn button contacts, or rarely an immobiliser fault. Test within 1m of the car — if it works close-up but not at distance, the battery is weak.
How do I change the battery in my car key fob?
Find the seam or notch (usually requires a small flat screwdriver or coin to pry open), note the existing battery orientation, swap for a new CR2032 or CR2025 in the same orientation, snap shut, test. Some smart proximity keys need to resync after a battery change.
What if my push-button start doesn't work?
Three common causes: dead fob battery (try touching the fob physically to the start button — most cars have this backup), brake/clutch not pressed firmly enough, or immobiliser fault. If the dashboard immobiliser light stays lit, try locking/unlocking with the fob 2-3 times. If persistent, locksmith diagnostics £40-£100.
How much does a replacement car key fob cost in Cornwall?
Replacement fob shell (worn buttons): £15-£40 part + £30-£60 reprogramming. Full replacement fob cut and programmed: £100-£250 for mainstream cars, £250-£500 for premium proximity keys. Always 50-70% cheaper from a Cornwall mobile auto locksmith than from a main dealer.
Can I reprogram a car key fob myself?
Sometimes — Ford, Vauxhall, Toyota, and Honda have manual resync procedures documented in owner's manuals. VW, Audi, BMW, Mercedes usually require diagnostic equipment (locksmith or dealer). Search '