No, not every metal box blocks a car key signal. A metal box blocks a car key signal only when conductive metal surrounds the fob on every side and the lid closes metal to metal with no gap.
A sealed metal box works as a small Faraday cage. A steel biscuit tin, a cash box and a home safe block a car key signal once the lid seals, while a coffee tin with a plastic lid, a toolbox and an open key bowl leak. Five lid faults cause the leaks, and a four step lock test at the car proves a metal box in about two minutes. A foil lining and a sanded rim turn a leaking tin into a homemade Faraday box for car keys. A Faraday pouch, also sold as a Faraday bag, suits a pocket, and a household metal box for car keys suits a shelf at home.
Biscuit tins, aluminium foil, Faraday pouches, fridges and microwaves, metal types, relay attacks, home safes, travel, night storage, anti theft mode and spare car keys raise fifteen further questions.
The list below gives four key takeaways on a metal box and a car key signal.
- Not every metal box blocks a car key signal, since a gap at the lid lets the signal out.
- A steel tin with a tight lid on a bare metal rim blocks the signal as a small Faraday cage.
- A four step lock test at the car proves a metal box in about two minutes.
- A Faraday pouch replaces a metal box in a pocket, a bag or a hotel room.
Why does a metal box block key signals?
A metal box blocks key signals because the conductive walls form a Faraday cage, a closed metal shell that reflects and absorbs radio waves. Michael Faraday built the first cage in 1836.
A keyless car sends a wake signal at about 125 kHz, and the fob replies at 433 MHz or 868 MHz in the UK. A relay kit carries those two signals between the house and the driveway. A sealed metal box keeps the wake signal away from the fob and holds the fob reply inside, so the relay kit finds no car key signal to pass on.
Which metal boxes block a car key signal?
Steel boxes with a tight metal lid block a car key signal, and boxes with a loose lid, a plastic part or an open side leak. The list below gives seven household metal boxes in order of blocking reliability.
- A Faraday box, also sold as a signal blocking key box, carries a conductive fabric lining and blocks the signal from new.
- A steel home safe blocks the signal once the door closes fully against the frame.
- A steel cash box blocks the signal when the lid overlaps the base all round.
- A biscuit tin or a sweet tin blocks the signal when the push on lid grips bare metal.
- A coffee tin with a plastic lid leaks through the lid.
- A metal toolbox leaks at the hinges, the handle holes and the loose lid.
- An open metal tray or key bowl blocks nothing, since one open side frees the signal.
The first four metal boxes block a car key signal, and the last three leak. ADC Locksmith Liverpool advises a lock test at the car for every household metal box, since two tins of the same shape often give two different results.
Why do some metal boxes leak the signal?
Some metal boxes leak the signal because a gap or an insulating layer breaks the metal shell around the fob. The list below gives five faults that let a car key signal out of a metal box.
- A loose lid leaves a slot between the lid and the base.
- A painted or lacquered rim insulates the lid from the base, so the two halves never join as one shell.
- A plastic lid, window or seal opens a path with no metal.
- A hinge gap, a handle hole or a coin slot breaks the shell at a fixed point.
- A dented lid or base stops the two halves meeting all round.
A 433 MHz fob signal has a wavelength of about 69 cm. A pinhole far smaller than that wavelength leaks almost nothing, while a gap that runs all round a lid forms one long slot and frees the signal. Lid contact decides the result more than any other feature of a metal box.
Which metal and thickness suit a metal box?
Steel, tin plate, aluminium and copper all suit a metal box, and a thin wall blocks a car key signal as well as a thick wall.
A 433 MHz signal reaches only a few thousandths of a millimetre into steel or aluminium, so a biscuit tin wall of about 0.2 mm already stops the fob reply. Thicker steel adds strength against dents and adds no extra blocking. Tin plate is thin steel with a tin coating, and steel weakens the 125 kHz wake signal better than aluminium because steel is magnetic. No material blocks a car key signal without a sealed lid, so lid fit comes before metal type and thickness.
Which test proves a metal box works?
A lock test proves a metal box works by showing that the car stays locked while the fob sits sealed inside the box beside the driver door. The list below gives the four steps.
- Place the fob inside the metal box and close the lid fully.
- Carry the closed box to the car and stand beside the driver door.
- Pull the door handle with the fob still sealed inside the box.
- Read the result, since a locked door shows the metal box blocks the car key signal and an unlocked door shows a leak.
The test takes about two minutes. A mobile phone inside the box gives a rough check only, since a phone uses different frequencies from a fob. A repeat test every few months catches a lid that has worn loose.
How is a homemade Faraday box made?
A homemade Faraday box is made from a steel tin with a tight lid, a bare metal rim and an aluminium foil lining. The list below gives the five steps that turn a leaking metal box into a DIY Faraday box for car keys.
- Choose a steel tin with a lid that overlaps the base all round.
- Sand the paint or lacquer from the rim and from the inside edge of the lid until bare metal shows.
- Line the base and the lid with two or three layers of aluminium foil, often called tin foil, and fold the foil over the rim.
- Lay a cloth or a sheet of bubble wrap inside, so the fob rests clear of the bare metal.
- Run the lock test at the car, and add a foil layer if the door unlocks.
The same five steps make a small Faraday cage for a phone or a contactless card. Foil tears where the lid rubs, so a lock test every few months shows when the lining of a homemade metal box needs replacing.
Metal box or Faraday pouch, which is better?
A Faraday pouch is better away from home, and a tested metal box or a Faraday box is better on a shelf at home. The table below compares a household metal box, a Faraday box and a Faraday pouch on five points.
| Point | Household metal box | Faraday box | Faraday pouch |
|---|---|---|---|
| Blocking from new | Only after a lock test | Yes, by design | Yes, by design |
| Best place | Shelf at home | Shelf at home | Pocket, bag or hotel room |
| Keys held | Several keys | Several keys and cards | One or two keys |
| Wear | Lid loosens and dents | Lining and clasp wear slowly | Lining cracks at the folds |
| Typical price | Free from the kitchen cupboard | From about £10 | About £3 to £30 |
All three rely on the same closed metal layer, and all three pass or fail on the same lock test. A household metal box is the cheapest car key signal blocker, and a pouch is the most portable. The guide to the best car key signal blocker ranks five buying criteria, with two layers of metal lining first.
What else do owners ask about metal boxes?
Biscuit tins, aluminium foil, Faraday pouches, fridges and microwaves, metal types, relay attacks, home safes, travel, night storage, anti theft mode and spare car keys raise fifteen further questions about a metal box and a car key signal. The fifteen questions below answer each topic.
Frequently asked questions
Does any metal box block a car key signal completely?
No, a metal box blocks a car key signal completely only when the lid seals metal to metal.
A loose lid, a painted rim or a plastic part lets some signal out, and a lock test shows the leak.
Does a biscuit tin work as a Faraday box?
Yes, a biscuit tin works as a Faraday box when the lid grips bare metal all round.
A painted rim or a loose lid breaks the seal, so each biscuit tin needs a lock test at the car.
Is aluminium foil as effective as a metal box?
No, aluminium foil is less effective than a metal box over time, because foil tears at the folds.
Two or three tight layers block the signal on day one, and a sealed metal box lasts for years.
Do Faraday pouches work better than metal boxes?
Yes, Faraday pouches work better than metal boxes away from home, since a pouch fits a pocket.
A tested metal box matches a pouch on a shelf at home and holds several keys.
Does a fridge or microwave block the signal like a metal box?
No, a fridge or microwave does not block the signal like a sealed metal box.
Door seals leave gaps, cold and damp harm the fob battery, and a microwave switched on destroys the key.
Which type of metal works best to block car key signals?
Steel works best in a metal box, and copper and nickel fabric works best in a pouch to block car key signals.
The guide to what material blocks a car key signal ranks five metals from copper fabric to lead.
Is a household metal box safe for car key security?
Yes, a household metal box is safe for car key security once a lock test at the car passes.
An untested tin gives false comfort, since a worn lid leaks with no visible sign.
Why do thieves use relay attacks and how does blocking help?
Thieves use relay attacks because a relay kit unlocks and starts a keyless car in 20 to 30 seconds without the key.
Blocking helps by leaving the relay kit no signal to pass on.
Do Faraday pouches wear out?
Yes, Faraday pouches wear out, because daily folding cracks the metal lining.
Damp and torn seams add to the wear, and a lock test every few months shows when a pouch stops blocking.
Does a metal safe block a car key signal?
Yes, a steel safe blocks a car key signal when the door closes fully against the frame.
A wide door gap or a plastic keypad panel leaks, so a lock test confirms each metal safe.
What works in place of a Faraday box?
A steel tin with a tight lid, a home safe or three layers of aluminium foil works in place of a Faraday box.
Each substitute needs a lock test, since none carries a tested lining.
Does a metal box block a car key signal when travelling?
Yes, a tested metal box blocks a car key signal when travelling, in a hotel room or a caravan.
A tin takes up bag space, so a Faraday pouch suits airports, hotels and car parks better.
Where does a metal box sit best at night?
A metal box holds car keys at night best in a back room or upstairs, away from the front door.
The guide to car keys at night ranks four home spots and five risky spots.
Does a blocked fob trigger anti theft mode?
No, a blocked fob does not trigger anti theft mode, since the car reads no key nearby.
A failed key chip causes that fault, and the reset for anti theft mode takes 10 to 15 minutes at ignition on.
Does a spare car key need a metal box too?
Yes, a spare car key needs a metal box or pouch too, because a spare fob sends the same car key signal.
ADC Locksmith Liverpool cuts and programs spare car keys in Liverpool from £100 in 20 to 40 minutes.
Based in Liverpool and still stuck?
ADC Locksmith Liverpool repairs, programs and replaces car keys at the roadside across Liverpool and Merseyside, usually on the same day. Craig Evans holds SERMI approval and a 5.0 rating from over 300 Google reviews. A call to 07731 544220 brings a fixed price before Craig travels.
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