315 MHz vs 433 MHz key fob: how do I tell which one I have?

How to find a key fob's frequency

From the FCC ID, in four steps:

1. Find the FCC ID on the fob. FCC rules require it on the device, so look over the case before you look anywhere else.
2. Split it in two. The grantee code is the first three characters, or the first five when it starts with a number. The rest is the product code.
3. Search the FCC's equipment authorization database. Enter the grantee code and the product code on the FCC ID search page.
4. Read the grant. It lists the frequency, or frequency range, the fob was certified on. In Canada, the IC number leads to the same answer in ISED's Radio Equipment List.

Or skip the search: Partexia shows the FCC ID and frequency with the OEM key for the VIN.

Six Hyundai fobs and their frequencies

Here are the frequencies on the FCC grants for the six Hyundai keys and remotes on Partexia's part pages:

Two things stand out. The older remotes are on 313.85 MHz, not exactly 315 MHz, so read the grant rather than a round number. And the test reports for the two smart keys also list a 125 kHz receiver: the low frequency GM's bulletin names for passive entry antennas, beside the 433.92 MHz the fob transmits on.

Part pages: Hyundai 95440-AA000 · Hyundai 95440-J9000 · Hyundai 95430-J0700 · Hyundai 95430-1R300 · Hyundai 95430-3K202 · Hyundai 95430-2E200

Why both 315 MHz and 433 MHz are used

Because the rules allow both. In the US, key fobs are certified under 47 CFR 15.231, the FCC rule for transmitters above 70 MHz that send short control signals, such as door openers and remote switches. A fob must stop transmitting within 5 seconds of the button being released.

315 MHz and 433.92 MHz both sit in the rule's 260-470 MHz row, where the field strength limit rises with frequency, from 3,750 to 12,500 microvolts per meter at 3 meters. Canada's RSS-210 sets the same line, and notes that 225-328.6 MHz, which takes in 315 MHz, is designated for Government of Canada use.

In Europe, CEPT's short range device recommendation, ERC/REC 70-03, lists a band at 433.05-434.79 MHz and none between 260 and 400 MHz.

Can one car use both frequencies?

Yes. Infiniti's service bulletin says its Intelligent Key systems use two radio frequencies, 315 MHz and 433 MHz. GM tells its technicians the common frequencies to test for are 315 MHz and 433 MHz, for remote entry and tire pressure sensors, and 125 kHz, for passive entry antennas.

Which one a given key uses is on its FCC grant. Partexia shows it with the OEM key for the VIN.

What to match when you replace a fob

The FCC ID first. A fob certified on a different frequency is a different fob, even when it looks the same. Then the part number for the VIN, the buttons and the kind of key.