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Will Any Metal Box Block a Car Key Signal?
No. A metal box blocks a car key signal only when the metal forms one continuous conductive shell around the key and the lid closes metal to metal, which rules out plastic lidded boxes, loose lids, hinged gaps and metal effect finishes over plastic. Being metal is not the test. The longest dimension of any opening governs how much signal escapes, so a press fit biscuit tin and a hinged cash tin leak where a sealed pouch does not. A measured biscuit tin returned well under half the attenuation of a purpose made pouch, and the reading shifted by several decibels when the key moved a few millimetres inside it.
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Which containers hold up and which leak
Sort the container by its join rather than by its material. The body of nearly any tin is continuous metal, and the lid is where the argument is won or lost.
| Container | Verdict | Why |
|---|---|---|
| Steel tin with a tight press fit lid | Capable | One piece body and a lid that meets metal to metal all the way round |
| Biscuit tin with a loose or rolled lid | Leaks | The rim gap is an opening, and its longest dimension sets the leak |
| Cash tin or toolbox with a hinge | Leaks | The hinge line and the lock slot stay open even with the lid shut |
| Box with a plastic lid or a plastic seal | No | The conductive shell is broken wherever the plastic sits |
| Metal effect plastic container | No | A painted or anodised finish over plastic carries no current |
| Safe with a continuous steel door and frame | Capable | Thick continuous metal, although the door seam still has to be tested |
Capable is not the same as proven, which is why every row in that table ends at the same place, a test against your own car.
The lid is the whole argument
Power escaping through seams and openings outweighs power passing through the walls of an enclosure, and a single breach removes the benefit of everything else. Measured enclosures have returned around 14 decibels where the material on its own predicted well over 100, and the gap between those two numbers is the gap around the lid.
Thickness does not rescue it. Reflection loss, the portion of the signal bounced straight off the surface, is independent of how thick the metal is, and only absorption depends on thickness. A heavier tin with the same open rim leaks exactly as much as a light one.
Enclosures designed to shield solve this deliberately, with conductive gaskets or contact springs that keep a continuous electrical join right around every edge and cover. A domestic tin has no gasket, no spring and no conductive paint on the rim, and that is the honest reason it underperforms a purpose made pouch.
How to test the box you already own
This takes two minutes and it is the only way to settle a specific container. Do it with the key you actually leave out overnight.
- Put the key inside, close the lid fully and press it down all the way round the rim.
- Carry the box to the car and press lock, then unlock.
- On a keyless car, pull the door handle and then try the start button with the key still inside the closed box.
- Move the key to a different spot inside the box and repeat, because a few millimetres of movement changes the result.
- Any response from the car at all means the box leaks, and it is not a storage place for that key.
- Repeat every few months, which is the interval police guidance gives for signal blocking storage, and after the box takes a knock.
What the measurements say about tins
Bench testing of phone sized containers puts a metal tin at roughly 29 to 44 decibels of attenuation across 1 to 6 GHz, against roughly 89 to 111 decibels for purpose made signal blocking pouches. Plain mylar and antistatic bags came in under 9 decibels, and carefully wrapped aluminium foil swung between about 50 and 90 decibels from one attempt to the next.
Two findings from that testing matter more than the headline. Position inside the tin was critical, with only a few millimetres of movement producing several decibels of difference, so a key that rolls around gives a different answer every night. And the tester judged the tin insufficient to provide meaningful assurance of isolation at any of the frequencies measured.
Carry the caveat with the figures. Those measurements were taken from 1 to 6 GHz, not at the frequencies a car key uses, so they rank construction quality and continuity rather than giving performance against a fob.
What a box does not protect you from
One verdict on how far this goes. Shielding reduces the chance of a relay attack succeeding, and it does not guarantee that a car cannot be opened or started. Anyone promising the second thing is overselling a tin.
A box does nothing about a diagnostic port used to unlock and start some vehicles, signals injected into the car network, a device that emulates a key, or a jammed lock signal that leaves the car unlocked after you walk away. It also does nothing about hands. Car keys were taken in 8 per cent of domestic burglaries with entry where anything was stolen in the year to March 2025, nearly 59 per cent of burglars came in at the front of the property, and 67 per cent of those burglaries happened in darkness.
A tin on the hall table passes the radio test and fails the other one, which is why the location of the box matters as much as the box.
A tin also does nothing for a fob that has stopped transmitting on its own account, which is key fob repairs rather than storage.
Where the box goes once it passes
Keep it indoors, away from the front door, the letterbox, outer walls and windows, and out of sight. Police guidance names a metal tin or box, or keys kept upstairs, as the storage to use, and it includes the spare key in the same instruction.
A protected first key alongside an unprotected spare in the hall drawer protects nothing, because the spare answers a relay attack exactly as well as the one in the tin. Give both keys the same treatment and test both.
Who Wrote This
Darius is the owner of Herts Car Keys and the locksmith on every job. He has six years in the trade and has completed over 7,000 car key and lockout jobs.
He is approved under the SERMI scheme, DBS checked and fully insured, and every key he supplies carries a 12 month warranty.
Car Keys Made at Your Vehicle
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Frequently Asked Questions
Call 07308 888825Will any metal box block car key signal completely?
No. Only a continuous conductive enclosure with no significant opening does, and even then the honest claim is functional rather than absolute, that the car no longer responds to the key. Full cancellation does not occur in any enclosure, so the test to trust is whether your own car reacts, not a specification.
Does aluminium foil block car key signals?
Foil blocks them when it is wrapped in two or three layers with no gaps, and the result does not repeat reliably. Measured attempts have ranged from about 50 to about 90 decibels of attenuation on the same foil. Opening the wrap destroys it, so there is no way to test the one you are relying on. Use it as a stopgap only.
Can a microwave oven block car key signals?
No, and do not use one. There is no reliable measurement either way at car key frequencies, and the objection stands on its own without one, because a metal bodied key sits inside an appliance that another person in the house can switch on. A tin with a tight lid or a tested pouch does the same job with no hazard attached.
Will a metal safe work to block my key signal?
A safe with a continuous steel body and door is the strongest domestic option, because the metal is thick and the door frame is engineered to close tightly. The door seam is still a seam, so run the lock, unlock and start test before you trust it with the only key you own.
Why do I need to block my car key signal?
Because a relay attack carries the conversation between car and key further than it was built to travel, with no code broken and no damage done. Police guidance puts a thief within a few metres of your key, through the wall of your home. Signal relay was the single largest entry method for thefts of whole cars in England and Wales in the year to March 2025, at around 47 per cent. A key that has already been taken is a different problem, and that one is lost car key solutions.
Can a small tin box block my car key signal?
Sometimes, and size is not the deciding factor. A small tin with a tight press fit lid outperforms a large one with a rolled or hinged rim. Bench testing also found that moving a key a few millimetres inside a tin changed the result by several decibels, and a small tin gives a key less room to shift.
Do stainless steel containers block car key signals?
They can, when the body and the lid are both steel and the lid seats metal to metal. A plastic seal, a rubber gasket that holds the lid off the rim, or a loose catch breaks the shell. Steel conducts less well than copper or aluminium, so the lid does more for the outcome than the grade of metal.
How can I test if my metal box blocks the signal?
Shut the key inside, close the lid fully, walk to the car and press lock and unlock. On a keyless car, pull the door handle and try the start button as well. Any reaction from the car means the box leaks. Move the key inside the box and repeat, then retest every few months.
Is a tin as good as a Faraday pouch?
No. In bench testing a metal tin measured roughly a third of the attenuation of a purpose made pouch and varied with the position of the contents. A tin is a reasonable answer where the lid is tight and it has been tested. A pouch is the stronger option, and it also goes in a pocket. Pairing the signal side of a fob to the car is remote key programming, which is a separate job from storing it safely.