Honda OEM 4-Button Smart Key Fob (FCC ID KR5T21) – Key Details
Fitments for the Honda OEM 4-Button Smart Key Fob (FCC ID KR5T21) include the 2021 Acura TLX, where it serves as the factory proximity key for passive entry and push-button start. Across this platform, the fob communicates at 433.92 MHz and pairs with the vehicle’s immobilizer through a Philips ID47 transponder chip embedded inside the housing. Each Honda OEM 4-Button Smart Key Fob (FCC ID KR5T21) initiates a coded handshake with the body control module and engine immobilizer every time the driver approaches the door handle or presses the start button. Owners replacing the device should expect a dealer-level or advanced locksmith programming session, as the proximity system requires bi-directional communication with the vehicle’s security architecture. When ordering a Honda OEM 4-Button Smart Key Fob (FCC ID KR5T21), confirm that the FCC ID printed on the back of the unit reads KR5T21 to ensure electrical and protocol compatibility with the 2021 TLX.
Technical Specifications of the Honda OEM 4-Button Smart Key Fob (FCC ID KR5T21)
The fob carries four buttons — typically lock, unlock, trunk release, and panic — arranged on a slim housing consistent with Honda’s current-generation smart key form factor. Internally, the remote operates on a 433.92 MHz RF carrier for keyless entry commands, while a separate low-frequency antenna coil embedded near the emergency key slot enables the ID47 transponder challenge-response cycle needed to authorize engine cranking.
The ID47 chip inside this unit is manufactured by Philips (now NXP Semiconductors). This transponder family uses an AES-128 encryption scheme, making it significantly more resistant to relay attacks and cloning attempts compared to older fixed-code or rolling-code-only systems. During ignition authorization, the vehicle’s antenna ring sends a low-frequency interrogation pulse; the ID47 chip responds with an encrypted token that the immobilizer ECU validates before releasing the starter circuit.
Because this is a genuine Honda OEM part, the housing dimensions, rubber button pads, and internal circuit board are identical to what ships from the factory. Aftermarket shells may look similar but can differ in antenna placement or button membrane thickness, potentially causing intermittent signal issues. Using the OEM unit eliminates those variables.
Programming and Registration Procedures
Registering a new smart key to a 2021 Acura TLX requires a diagnostic scan tool capable of communicating with the Honda/Acura immobilizer module via the OBD-II port. The procedure generally involves the following high-level steps:
- Connect a compatible diagnostic platform to the vehicle’s OBD-II connector.
- Navigate to the immobilizer or smart key registration menu for the TLX.
- Follow the on-screen prompts to place the new fob inside the vehicle — usually on the center console near the push-button start switch — so that the low-frequency antenna can communicate with the ID47 transponder at close range.
- The tool writes a new cryptographic seed to both the fob and the vehicle’s immobilizer ECU, binding them together.
- Cycle the ignition to confirm that the engine starts and that passive entry functions respond correctly at all four door handles and the trunk.
All previously registered smart keys should be present during the procedure. Most Honda and Acura immobilizer systems erase the key memory and re-register every fob in a single session, so any unit not present at the time of programming will stop working. Automotive locksmiths experienced with Honda proximity systems can typically complete the process in under thirty minutes.
Emergency Mechanical Access
Like most modern smart keys, this fob includes a concealed mechanical key blade housed inside the body of the remote. Pressing or sliding a release tab on the rear of the housing ejects a thin metal blade that fits the driver’s door cylinder. This backup access method is critical when the fob’s battery dies or the 433.92 MHz RF signal is jammed by local interference. Once inside the vehicle, the driver can still start the engine by holding the dead fob directly against the start button, allowing the immobilizer’s low-frequency antenna to power the ID47 chip inductively and complete the authentication handshake without a functioning battery.
Battery Replacement
The KR5T21 fob uses a standard CR2032 coin cell. When the battery voltage drops below the operating threshold, the vehicle’s instrument cluster will display a key-fob-battery-low warning. Replacing the cell is straightforward: remove the mechanical key blade, insert a thin pry tool into the seam along the fob’s edge, and carefully separate the two halves of the housing. Swap the old CR2032 for a fresh one, observing correct polarity, then snap the housing back together. No reprogramming is needed after a battery change — the ID47 transponder and the stored RF rolling codes remain intact in non-volatile memory.
OEM vs. Aftermarket Considerations
Aftermarket smart key housings and complete fob assemblies for the KR5T21 exist in the supply chain at lower price points. While some aftermarket boards use legitimate Philips ID47 transponder chips, others substitute clones of varying quality. A cloned chip may program successfully on the first attempt but exhibit reliability issues over time — intermittent no-starts, reduced passive-entry range, or failure to wake from sleep mode. For a vehicle like the 2021 Acura TLX, which relies entirely on the smart key for everyday operation, the cost difference between aftermarket and OEM is generally small relative to the inconvenience of a malfunctioning proximity system.
Locksmiths and dealers can verify authenticity by cross-referencing the FCC ID KR5T21 printed on the unit with the FCC’s equipment authorization database, which lists the original grantee and technical parameters including the 433.92 MHz operating frequency and permissible power output.
When to Contact a Locksmith
Vehicle owners who have lost all smart keys for a 2021 Acura TLX face a more involved recovery process. Without at least one working fob present, the immobilizer ECU may require a PIN code or seed file extraction before new keys can be written. Qualified automotive locksmiths equipped with advanced Honda diagnostic software can handle this scenario on-site, often faster and at lower cost than towing the vehicle to a dealership. Even for simple add-key situations, a mobile locksmith with the correct scan tool can program the fob in the owner’s driveway, eliminating the need for a service appointment.
Specifications and fitment
| Key type | Smart Key |
|---|---|
| Transponder chip | ID47 |
| Chip manufacturer | Philips |
| Button count | 4 |
| Frequency (MHz) | 433.92 |
| FCC ID(s) | KR5T21 |
| Region(s) | USA |
| Product type | Smart Key |
| Compatible makes | Acura |
Listed vehicle fitment
| Make | Model | Years |
|---|---|---|
| Acura | TLX | 2021– |
| Acura | TLX | 2021– |
Fitment must be confirmed against the existing key’s FCC ID family and the vehicle’s exact configuration before cutting or programming. VIN or photo verification may be required.
Questions and answers
Which vehicles does Honda OEM 4-Button Smart Key Fob (FCC ID KR5T21) fit?
Honda OEM 4-Button Smart Key Fob (FCC ID KR5T21) is listed to fit Acura applications such as Acura TLX, Acura TLX. Confirm the exact year range and the existing key’s FCC ID family before ordering, since fitment can vary by trim and region.
Does Honda OEM 4-Button Smart Key Fob (FCC ID KR5T21) need programming?
Yes. Because it carries a transponder/remote function (chip ID47), it must be programmed (paired) to the vehicle’s immobilizer and, for remote functions, to the keyless-entry system. Many vehicles also require at least one working key to add a new one.
Does the blade need to be cut?
A push-to-start smart key has no traditional blade to cut in normal use, though many include an emergency insert key that can be cut separately.
Is Honda OEM 4-Button Smart Key Fob (FCC ID KR5T21) OEM or aftermarket?
This listing is an aftermarket-grade replacement intended to match the original key’s electronic and mechanical specification. It is not required to carry a vehicle-brand logo and should be matched by FCC ID family and chip type rather than by appearance.
Check Fitment and Key Help
Low Rate Locksmith, a mobile automotive locksmith, can confirm whether Honda OEM 4-Button Smart Key Fob (FCC ID KR5T21) matches an existing FCC ID family and coordinate cutting and programming when the vehicle configuration requires professional service. For dispatch and support, call (833) 439-8636 or email info@lowratelocksmith.com.
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