License Error


                            0xC004C008

Windows Activation Error 0xC004C008: Key Already In Use on Another PC

11 min read Updated October 5, 2026 Windows Activation & Licensing

Recommended fix

Windows 11 Pro Retail license

Original price was: 25,00 €.Current price is: 14,99 €.

Fix It Now

Fix it now

The activation service refused the key; nothing on the machine is broken. Microsoft documents two causes under this code: a KMS host key that has passed its activation limit, and a product key already used on another device or on more devices than the licence terms allow. Which of the two you have decides everything else.

Run these first, in an elevated Command Prompt, to see which key is actually installed

slmgr /dli
slmgr /dlv

If this machine is a KMS host and you installed a KMS host key on it, stop troubleshooting the machine. A KMS host key permits up to 10 activations across a maximum of six computers, and once it is flagged only a key reset through Volume Licensing support clears it.
If this is an ordinary client and the key is live on another machine you own, that machine has to give it up first. Remove the key there with slmgr /upk, then install and activate here.
If it is the same machine after a hardware change, open Start > Settings > System > Activation, run the troubleshooter and choose “I changed hardware on this device recently”.
If the key is on machines you do not control, stop here. This PC needs a licence of its own; no local command produces one.

The troubleshooter route works only where the digital licence was linked to a Microsoft account before the hardware changed. If it never was, that option cannot recover the activation.

If Windows now reports itself activated you can stop here. If not, the next section explains what the service is counting and why most of the advice you will find for this code cannot work.

Why it happens

Every key carries a permitted number of activations, and the service records a hardware identifier each time one succeeds. Reactivating hardware it has already seen is free, which is why reinstalling on the same machine costs nothing. Hardware it has not seen spends an activation, and when the permitted number is gone the next request comes back as 0xC004C008 – a decision taken at Microsoft’s end, about the key, not about your installation.

Microsoft publishes two quite different situations under this one code, and they do not share a remedy. The consumer reading is that the key has already been used on another device, or on more devices than the Microsoft Software License Terms allow. The server reading, on Microsoft’s dedicated page for the code, is a KMS host key that has exceeded its limit of 10 activations across a maximum of six computers – burnt through by rebuilding a KMS host repeatedly, or by putting a host key on machines that were never meant to be hosts.

What a key permits depends on its channel, and none of those limits is visible or adjustable from the machine. Retail moves between machines, one at a time. OEM stays with the machine it first activated. Volume keys have counts and codes of their own: 0xC004C020 is the Multiple Activation Key over-limit code, and if that is what you are seeing this is the wrong article.

A KMS host key has been used on too many hosts

You have this one if The machine is a KMS host, you installed a KMS host key rather than a GVLK, and that key has stood up hosts before – often test builds nobody decommissioned.

Stop making changes on the server. The refusal is held against the key.
Ask Microsoft Volume Licensing support for a key reset or replacement.
Have ready: agreement or licence ID, product name and edition, host activations needed, and why.

Only the host needs a KMS host key; clients take a GVLK. Installing a host key on a client is a documented way this limit gets used up.

The key is live on another machine you control

You have this one if You know exactly where the other installation is, and it is still switched on and activated.

On the old machine, remove the key with slmgr /upk and take it out of service or licence it separately.
On this machine, install the key: slmgr /ipk XXXXX-XXXXX-XXXXX-XXXXX-XXXXX, then slmgr /ato.

Removing the key on the old machine does not hand the activation back. The count is held at Microsoft’s end and is not decremented by anything you run locally.

Same machine, different hardware identity

You have this one if A motherboard or several major components were replaced, and the machine activated normally before the work.

Sign in with the Microsoft account the digital licence was linked to.
Open Start > Settings > System > Activation and run the troubleshooter.
Choose “I changed hardware on this device recently”, then pick this device from the list.

Microsoft states the troubleshooter works only with the connected Microsoft account. Linking has to have happened before the hardware change, not after it.

The key was sold to more than one buyer

You have this one if An unusually cheap key bought online that worked briefly or never worked, and a seller who has stopped replying.

Retype the key once with slmgr /ipk to rule out a transcription error, then slmgr /ato.
Ask the seller for a replacement, and treat silence as the answer.

If a retry returns 0xC004C003 instead, the key has been blocked outright rather than over-used, and a replacement from the same source will go the same way.

Full reference

What your channel actually permits

Channel	Moves to another machine?	What this code means for it
Retail	Yes, one machine at a time	The previous machine must stop using the key before this one can have it
OEM	No	The licence stays with the machine it first activated
Digital licence linked to a Microsoft account	Recoverable after hardware changes	The troubleshooter is the route, and only with the account that was linked beforehand
KMS host key	Six hosts, ten activations	A key reset through Volume Licensing is the only remedy
Multiple Activation Key	A finite pool of activations	Over-limit reports as 0xC004C020, not as this code

Microsoft’s own wording for the consumer case puts the limit in the Microsoft Software License Terms rather than in a number, so treat any page that quotes you a specific activation count for a retail key with suspicion. The rule is one machine at a time; the enforcement is a hardware identifier held at Microsoft’s end.

Codes that arrive at the same screen from somewhere else

Code	What it points at	Where that leaves you
`0xC004C020`	A Multiple Activation Key has exceeded its limit	A volume licensing question, not a transfer question
`0xC004C003`	The activation server has blocked the key	Permanent. Nothing on the machine changes it
`0xC004C004`	The key is invalid, or the server had a problem with the request	A different remedy entirely: check the key, then retry later
`0xC004C770`	Published by Microsoft in the same group as 0xC004C008	Read it the same way: already used elsewhere
`0x803FA071`	Published by Microsoft in the same group as 0xC004C008	The same refusal seen through the Settings activation client

A batch that all failed together

Where a whole deployment carries the same key from a reference image, every clone spends an activation against it and the pool empties in an afternoon. Remove the key from the image, generalise it with sysprep /generalize, and redeploy. Then activate through volume activation rather than distributing one key, which is what volume activation exists to avoid.

Things that do not work, and why people keep trying them

Reinstalling Windows. The refusal is about the key, and a reinstall presents the same key from the same hardware.
Registry edits under the licensing keys. They change what the local machine believes, not what the activation service has recorded.
Removing the key from the old machine and expecting the count to fall. It does not; the record is held at Microsoft’s end.
Third-party activation tools. They work by not asking the activation service, which leaves you unlicensed and, in a business, in a worse position than the one you started in.
Retyping the key more than once. Once is a genuine check for a transcription error. Twice tells you nothing new.

Before you conclude you are a licence short

Confirm which key is installed with slmgr /dli – the last five characters are enough to tell two keys apart, and they are often not the key you think.
Confirm the edition matches. A Pro key on a Home installation, or the reverse, produces a different family of codes and a different fix.
Check for a firmware-embedded licence on prebuilt hardware. A machine that shipped with Windows may hold an OEM key in firmware that Windows will pick up once the wrong key is removed with slmgr /upk.
If the machine is domain-joined and the key came from work, ask whoever manages the volume agreement before doing anything else. Volume keys removed from managed machines are hard to put back.

When a licence is the actual fix

Where the key is genuinely running on another PC, the honest answer is that you are one licence short. Windows licences one machine at a time and no troubleshooter, registry edit or reinstallation changes a count held at Microsoft’s end. Arco supplies Windows 11 Pro retail keys with an invoice and a traceable channel, which also matters the next time a hardware change needs a reactivation and someone asks where the licence came from. If you are not sure whether your situation is a transfer or a second machine, slmgr /dli on both PCs usually settles it in a minute, and we would rather tell you that you do not need to buy anything.

Every code this article covers

Code	What it points at	Source
`0xC004C008`	The activation service could not use the key: either it has already been used on another device, or a KMS host key has passed its limit of 10 activations across six computers	Microsoft Learn
`0x803FA06C`	An activation-service refusal in the same family, seen through the Settings activation client. The documented code in this group is 0x803FA071	not published by the vendor
`0xC004C812`	An activation-service refusal returned for this key. Read it as a decision taken at the server, not as a fault on the machine	not published by the vendor
`0xC004C004`	The service rejected the key as invalid, or hit a problem with the request, rather than reporting it as over-used	Microsoft Learn
`0xC004C780`	Another refusal from the same part of the activation service. The documented code in this group is 0xC004C770, published as already used on another device	not published by the vendor

Confirm the fix worked

slmgr /dli reports Licence Status: Licensed.
Start > Settings > System > Activation shows the machine as activated with no remediation link.
On the machine you transferred away from, confirm the key is gone and it is not quietly reactivating.
Reboot and re-check, so you know the state is permanent rather than a grace period.
If this was a KMS host, confirm clients activate against it before considering the matter closed.

Questions people ask about this

Can I move my Windows licence to a new PC?

A retail licence, yes, provided the old machine stops using it. An OEM licence stays with the machine it was first activated on. Microsoft frames the limit as what the Microsoft Software License Terms allow rather than as a number, so read the terms that came with your key rather than a count someone quotes you.

Does reinstalling Windows use up another activation?

Not on the same hardware. The service recognises the machine and reactivates it. It is new hardware, not new installations, that spends an activation.

I am setting up a KMS host and got this code. Is it the same problem?

No, and this is the case most articles miss. Microsoft’s own page for 0xC004C008 documents it as a KMS host key that has exceeded its limit of 10 activations across a maximum of six computers. Nothing on the server fixes it; you need the key reset through Volume Licensing support.

The seller says the key is genuine. Is it?

It may well have been genuine when it was issued. Genuine and unused are different claims, and this code speaks only to the second. If the seller will not replace it, the practical position is that you have bought nothing.

Why do I see a 0x803FA-something code instead?

That is the same class of refusal reported through the Settings activation client rather than through slmgr. Microsoft groups 0x803FA071 with 0xC004C008 under the same explanation. Codes very close to it are not separately published, so read them by what the service is doing rather than by the digits.

Need a genuine license? Choose your edition: