If you have replaced one Sony WF 1000XM4 earbud and it refuses to pair with your phone, the problem may not be Bluetooth at all. The two earbuds can have different firmware versions, particularly when one unit is purchased separately, and that mismatch can lead to messages such as “connection failed” or “pairing rejected by the earbuds”. In this guide, I will walk through the process I used to check the firmware, update the replacement earbud, and successfully connect the pair again.
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The problem: one replacement earbud would not pair
I originally had a working pair of Sony WF 1000XM4 earbuds, but my wife lost the right earbud. Rather than replace the entire set, I purchased a separate replacement unit. This seemed like the simple solution, but the new earbud would not synchronise correctly with the existing one.
The earbuds appeared to be in good condition. The charging case worked, the replacement unit charged, and my phone could detect the earbuds. However, the Sony Headphones Connect app would not complete the registration process. Each attempt ended with an error indicating that the connection had failed or that pairing had been rejected by the earbuds.
When a replacement earbud is from a different production batch, it may contain firmware that does not match the firmware installed on the original earbud. The earbuds need to communicate with each other as a matched set, so having different software versions can prevent normal pairing.
Before attempting any firmware work, it is worth checking the basic possibilities. A low battery, dirty charging contacts, an outdated phone app, or an incomplete Bluetooth reset can create similar symptoms. Firmware should not be the first thing you change without checking the simpler causes.
Quick checks before updating
- Charge the case and both earbuds to a comfortable level.
- Clean the charging contacts carefully with a dry cotton bud.
- Update the Sony Headphones Connect app from the relevant app store.
- Remove the earbuds from your phone Bluetooth settings.
- Place both earbuds in the case, close the lid, wait briefly, and try again.
- Test the earbuds with another phone if one is available.
If the replacement unit still refuses to pair after these checks, compare the firmware versions. That gives you a much clearer indication of whether the software is causing the problem.
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What you need before starting
The method shown in this guide uses a Windows computer, a USB cable, the charging case, and a firmware utility obtained from AudioSpare. The transcript used version 2.1.0 of the utility because a newer version appeared to have compatibility concerns in that particular situation.
Firmware utilities obtained outside the official Sony support channels should be treated carefully. Sony normally expects firmware updates to be completed through the Headphones Connect app, and a third party utility may carry risks relating to compatibility, security, warranty support, or damage caused by an interrupted update. Only use files you trust, scan downloaded files with current security software, and make sure you understand the instructions before continuing.
The safest approach is to try Sony’s official app and support process first. The computer based method is most relevant when a replacement earbud has a different firmware version and the official app cannot complete the pairing or update process.
Do not start this process if the computer is likely to shut down, restart, or enter sleep mode. It is also a good idea to disconnect other unnecessary USB devices so the charging case remains easy to identify.
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Step one: download the firmware files
The firmware utility used in the original process was found through AudioSpare. The guide was listed under the blog section and described as a firmware upgrade or downgrade guide for the Sony WF 1000XM4.
After opening the guide, scroll to the download area and select the utility version that matches the instructions. In the example, version 2.1.0 was downloaded. The files included a version checking batch file and the tools required to begin the firmware update.
Keep the downloaded files together in one folder. Avoid renaming individual files unless the instructions specifically tell you to do so, because a batch file may rely on the original filenames and folder structure.
It is sensible to create a separate folder on the computer, such as “Sony earbuds firmware”, and extract the downloaded archive into that folder. Before running anything, inspect the contents and read any included instructions. If Windows displays a security warning, do not ignore it automatically. Confirm the source and scan the files before deciding whether to proceed.
Important: A firmware update is not the same as installing a normal app. The process changes the internal software of the earbuds, so an interruption can leave one or both units unusable.
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Step two: connect the charging case to the computer
Use the USB cable to connect the Sony charging case to the Windows computer. The case is the part that communicates with the firmware utility, so the earbuds need to be placed correctly inside it.
Insert both earbuds into their charging positions and confirm that they are making contact. If the earbuds are not seated properly, the computer may detect the case but fail to read one of the units. Check that the charging indicators respond as expected before running the utility.
Once the cable is connected, open the folder containing the firmware files. The first file to run is called version check.bat. A batch file opens in a command window and checks the firmware information stored in the earbuds.
Windows may display a warning before allowing the file to run. Review the warning and only continue if you have verified that the files came from a source you trust. The command window may open and close quickly, so follow the supplied instructions carefully if the result is not immediately visible.
The first check showed that the left earbud had firmware version 2, while the replacement right earbud had firmware version 1.5. Windows displayed the two values separately, making the mismatch clear.
How to interpret the version check
The important point is not simply that the numbers look different. The left and right earbuds are running different firmware packages, which may prevent them from behaving as a single pair. The charging case may also report its own information, so do not assume every number shown belongs to the same component.
Version numbers can be presented differently by Windows and the utility. One screen may display a shorter value while another includes additional digits. Record the information before changing anything. Taking a screenshot is useful because it gives you a reference if the update does not work as expected.
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Step three: choose a compatible firmware package
The next stage is to use the firmware tool to bring the two earbuds onto a matching version. In the original example, version 2.1 was selected rather than version 2.3 because the newer package was believed to have compatibility issues with that setup.
This is an important distinction. The newest available version is not always the best option when you are trying to recover a mismatched pair. A package that is unsuitable for the hardware, utility, or replacement unit may create more problems than it solves.
Use the instructions provided with the tool and do not experiment with random firmware files. Confirm that the package is intended for the Sony WF 1000XM4 and check whether the guide recommends a specific order. Some tools may update the left unit, the right unit, or both units as a combined operation.
At this point, make sure the case is still connected and both earbuds are seated correctly. Close other programs on the computer so you are less likely to accidentally interfere with the update window.
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Step four: start the firmware update
Run the update command supplied with the utility. The command window should indicate that the firmware installation has started. It may show a progress percentage, status messages, or a data transfer stage.
Firmware updates can appear to pause for longer than expected. This does not necessarily mean that the process has failed. The utility may be transferring files, checking the data, restarting the internal device, or waiting for the case to respond.
During the update, do not disconnect the USB cable. Do not remove the earbuds from the case, close the command window, shut down the computer, or allow the computer to sleep. These actions can interrupt the transfer and potentially damage the firmware.
The utility in the example showed the firmware progressing towards completion. The original pairing problem was believed to be caused by the two earbuds using different versions, so the goal was to install matching software on both units.
Do not rush the progress bar. Reaching one hundred percent may only mean that the files have transferred. The utility may still need to verify the data and instruct the target device to complete the installation.
It is also worth keeping the computer connected to mains power. A laptop with a low battery can shut down at the worst possible moment. If you are using a desktop computer, avoid starting the process during a time when a power interruption is likely.
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Step five: wait for verification and completion
Once the transfer reaches one hundred percent, the tool may begin verifying the data. Verification checks that the files arrived correctly before the firmware is executed by the target device.
In the example, the command window displayed a message stating that the target was executing the firmware update and that the user should wait until the update completed. It then asked the user to press a key to continue. That message does not necessarily mean that it is safe to disconnect the case immediately.
After pressing a key, the first version check timed out. A timeout at this stage can be confusing, but the earbuds may still be completing the update internally. The safest response is to wait rather than repeatedly press keys or unplug the cable.
What to do if the utility times out
- Wait for a reasonable period after the update message appears.
- Keep the case connected until the utility has clearly stopped communicating.
- Do not remove either earbud while the update may still be running.
- Read the utility instructions for the correct reconnection procedure.
- Only disconnect and reconnect the cable when the instructions allow it.
In the original process, the next action was to disconnect the USB cable and reconnect it. The version checking batch file was then run again. This second check showed that both earbuds had reached version 2.1.
The reconnection step appears to have allowed the case and earbuds to restart their communication with the computer. It also gave a fresh reading rather than relying on the progress message from the first update window.
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Step six: confirm that both earbuds match
Run version check.bat again after reconnecting the case. The result you want to see is a matching version for the left and right units.
In this case, both earbuds reported version 2.1. This confirmed that the replacement right earbud had been brought into line with the original left unit.
A matching version does not guarantee that every problem has been solved. The earbuds may still need to be reset, removed from the phone, or registered again in the Sony app. However, the version check removes one major cause of the pairing failure.
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Step seven: pair the earbuds with your phone
With the firmware versions matching, open the Sony Headphones Connect app on your phone. If the previous failed attempt left a saved Bluetooth connection, remove the old entry first. This gives the phone a clean opportunity to register the earbuds again.
Open the charging case and take the earbuds out when prompted. Follow the app setup screens and accept the permissions that you are comfortable granting. The app may request notification access so it can show battery levels, connection messages, or firmware notices.
In the original test, the app moved through the setup process successfully after the firmware update. The earbuds registered correctly, connected to the phone, and displayed the expected battery information and usage statistics.
If pairing still fails, work through the following sequence rather than repeating the firmware installation immediately.
- Turn Bluetooth off and on again on the phone.
- Restart the phone and reopen the Sony app.
- Forget the earbuds from the phone Bluetooth menu.
- Place both earbuds in the case and close the lid briefly.
- Reset the earbuds using the official Sony reset instructions.
- Try the pairing process with another compatible phone.
- Check that both earbuds are charging and responding separately.
If only one side connects, check the charging contacts and confirm that the affected earbud appears in the version check. A hardware fault, damaged contact, or incompatible replacement component can look similar to a firmware problem.
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Common mistakes to avoid
Disconnecting too early
The most serious mistake is removing the cable as soon as the progress bar reaches one hundred percent. The tool may still be verifying the files or instructing the earbuds to execute the update. Wait for the completion message and follow the stated reconnection instructions.
Using the wrong package
Do not install firmware designed for another Sony model. The WF 1000XM4 has its own hardware and software requirements. A file that looks close in name may still be unsuitable.
Assuming Bluetooth pairing proves the update worked
A temporary Bluetooth connection is not the same as a complete repair. Check both firmware versions and test the app, audio playback, left and right channels, touch controls, and charging behaviour.
Ignoring the official support route
Third party firmware utilities can be useful in unusual replacement situations, but they should not replace official support when the earbuds are under warranty or when the hardware may be faulty. Sony support may be able to confirm whether the replacement unit is compatible and whether an official repair option is available.
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A practical recovery checklist
Before the update
- Confirm that both earbuds are the correct model.
- Charge the case and earbuds.
- Install the latest Sony Headphones Connect app.
- Try removing and adding the Bluetooth connection.
- Record the original firmware versions.
- Read the complete utility instructions.
During the update
- Keep the USB cable connected.
- Keep both earbuds inside the charging case.
- Prevent the computer from sleeping.
- Do not close the command window.
- Wait through verification and execution messages.
After the update
- Reconnect the case only when instructed.
- Run the version check again.
- Confirm that both firmware values match.
- Forget the old phone connection.
- Pair again through the Sony app.
- Test sound, controls, charging, and battery reporting.
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Frequently Asked Questions
Why will my replacement Sony WF-1000XM4 earbud not pair?
A replacement earbud may have a different firmware version from the original earbud. This mismatch can prevent the earbuds from synchronising and may result in connection failures or pairing being rejected.
What should I check before updating the firmware?
Charge the case and both earbuds, clean the charging contacts, update the Sony Headphones Connect app, remove the earbuds from your phone’s Bluetooth settings, reset the connection and try pairing them with another phone if possible.
How can I update the firmware on the WF-1000XM4 earbuds?
The safest method is to use Sony’s official Headphones Connect app and support process. If the app cannot complete the update, a Windows computer, USB cable, charging case, both earbuds and a compatible firmware utility may be required. Follow the utility’s instructions carefully and keep all downloaded files together.
Is it safe to use a third-party firmware utility?
Third-party utilities may involve compatibility, security, warranty and device damage risks. Use Sony’s official method first. If you choose to use an external utility, download it only from a source you trust, scan the files with current security software and make sure you understand the process before proceeding.
What precautions should I take during the firmware update?
Ensure the earbuds and charging case have stable power, and prevent the computer from shutting down, restarting or entering sleep mode. An interrupted firmware installation may leave one or both earbuds unusable.
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