# Key Fob Not Detected in Car 2019 Toyota Mirai: Fixes

> ClearTheCode — Is your 2019 Toyota Mirai displaying "Key Fob Not Detected" and won't start? Learn common causes and step-by-step fixes to get back on the road quickly.

Source: https://clearthecode.com/articles/key-fob-not-detected-5820-toyota-mirai-2019
Author: ClearTheCode

# Key Fob Not Detected in Car 2019 Toyota Mirai

Experiencing a "Key Fob Not Detected" message in your 2019 Toyota Mirai can be frustrating, especially when it prevents you from starting your fuel cell vehicle or even locking/unlocking the doors. This common issue often points to a problem with your key fob itself or the vehicle's ability to communicate with it. Understanding the underlying causes and knowing the right diagnostic steps can help you resolve the problem efficiently.

## What drivers notice on this 2019 Toyota Mirai

When your 2019 Toyota Mirai fails to detect the key fob, you'll typically encounter a very clear set of symptoms that prevent normal operation:

-   **Dashboard warning message:** The most prominent sign is a message on your multi-information display stating "Key Fob Not Detected" or similar wording.
-   **No start condition:** The vehicle will not power on, and the "Ready" indicator light will not illuminate. Pressing the start button will likely yield no response or an additional warning.
-   **Inoperative remote functions:** The buttons on your key fob (lock, unlock, trunk, panic) will not work, even when standing close to the vehicle.
-   **Smart entry system failure:** Touching the door handles will not lock or unlock the doors, indicating the passive entry system isn't recognizing the fob.
-   **Steering wheel lock:** The steering wheel may remain locked, preventing you from turning it.

## At a glance: difficulty, repair cost, and diagnostic workflow

- **Difficulty Rating:** 6/10 — Moderate
- **Common Symptoms:** Intermittent detection: The vehicle sometimes detects the fob, and other times it doesn't, requiring multiple attempts o; Reduced range: You might notice that you have to be much closer to your Mirai for the remote lock/unlock functions to wo; Delayed response: There's a noticeable lag between pressing a fob button and the vehicle responding.; Having to hold the fob near the start button: Before a complete failure, you might find yourself needing to physically t
- **Estimated Repair Cost:** $100–$900 (parts + typical shop labor)
- **OEM Tooling Required:** Standard OBD-II scanner and hand tools

**Diagnostic workflow:**
1. Confirm the symptom on your vehicle.
2. Scan for stored or pending codes with an OBD-II tool.
3. Inspect the most common causes in this guide (visual checks first).
4. Run verification tests before replacing parts.
5. Repair, clear codes, and verify on a test drive.

## Symptoms and warning signs

While the "Key Fob Not Detected" message is the primary symptom, other subtle signs might precede a complete failure or indicate an intermittent problem:

-   **Intermittent detection:** The vehicle sometimes detects the fob, and other times it doesn't, requiring multiple attempts or specific positioning.
-   **Reduced range:** You might notice that you have to be much closer to your Mirai for the remote lock/unlock functions to work.
-   **Delayed response:** There's a noticeable lag between pressing a fob button and the vehicle responding.
-   **Having to hold the fob near the start button:** Before a complete failure, you might find yourself needing to physically touch the fob to the start button for the vehicle to recognize it.

## How to verify and confirm the issue

Verifying the "Key Fob Not Detected" issue in your 2019 Toyota Mirai involves a few practical checks to narrow down the cause:

-   **Try your spare key fob:** If you have a second key fob, try using it. If the spare works perfectly, the problem is almost certainly with your primary fob.
-   **Replace the key fob battery:** This is the most common and easiest fix. Even if the fob seems to have some power, a weak battery can cause intermittent or complete detection failure. Refer to your Mirai's owner's manual for specific instructions on replacing the CR2032 battery.
-   **Perform the emergency start procedure:** Toyota Mirai vehicles, like many modern cars, have an emergency start method. Hold the key fob directly against the start button while pressing the brake pedal and pushing the start button. If the vehicle starts, it strongly suggests a weak fob battery or an issue with the smart key system's antenna, as the fob's transponder chip is still functional.
-   **Check for strong RF interference:** Are you in an area with heavy radio frequency interference (e.g., near a radio tower, power lines, or other strong electronic devices)? Try moving your vehicle to a different location.
-   **Visual inspection of the fob:** Check your key fob for any signs of physical damage, water intrusion, or corrosion on the battery terminals.
-   **Inspect the vehicle's 12V auxiliary battery:** While the Mirai is a fuel cell vehicle, it still relies on a conventional 12V auxiliary battery for starting the system and powering accessories. A weak or dead 12V battery can cause a myriad of electrical issues, including smart key system malfunctions. You can test its voltage with a multimeter.
-   **Scan for diagnostic trouble codes (DTCs):** Although not an OBD-II engine code in the traditional sense, a professional-grade scan tool can sometimes pull codes from the Body Control Module (BCM) or Smart Key ECU that relate to key fob communication or immobilizer issues. For a comprehensive guide to vehicle diagnostics, you can refer to All repair guides.

## Common causes (most likely first)

Understanding the most frequent culprits behind a "Key Fob Not Detected" message in your 2019 Toyota Mirai can guide your troubleshooting:

-   **Dead or weak key fob battery:** By far the most common reason. The fob uses a small coin cell battery (typically CR2032) that eventually loses its charge.
-   **Key fob damage:** Physical damage from drops, exposure to water, or extreme temperatures can internally damage the fob's electronics.
-   **Radio frequency (RF) interference:** Other electronic devices, strong Wi-Fi signals, or even certain environmental factors can temporarily block the signal between the fob and the vehicle.
-   **Vehicle's 12V auxiliary battery low or dead:** A struggling 12V battery can cause the smart key system to malfunction, as it requires stable power to operate correctly.
-   **Smart Key ECU or receiver malfunction:** The vehicle's internal computer module responsible for receiving the key fob's signal can fail, preventing detection.
-   **Wiring issues:** Damaged or corroded wiring connecting the smart key antennas or ECU can interrupt communication.
-   **Key fob unprogrammed or desynchronized:** Although less common, a fob can lose its programming, especially after a battery replacement or vehicle battery disconnection, requiring reprogramming.

### Related guides on this vehicle

- [Key Fob Battery Replacement 2019 Toyota Corolla](/articles/key-fob-battery-replacement-3898-toyota-corolla-2019)
- [2019 Toyota Tacoma Brake Pedal Feels Low: Diagnosis & Fixes](/articles/brake-pedal-feels-low-toyota-tacoma-2019)
- [Safely Jump-Start a Dead Car Battery 2016 Toyota Mirai](/articles/jump-start-dead-battery-safely-toyota-mirai-2016)

## Step-by-step diagnosis and fixes

Follow these steps to diagnose and potentially fix the "Key Fob Not Detected" issue in your 2019 Toyota Mirai:

-   **Step 1: Replace the Key Fob Battery.**
    -   Obtain a new CR2032 coin cell battery.
    -   Locate the small slot or tab on your Mirai's key fob and use a small flathead screwdriver or coin to gently pry the two halves apart.
    -   Carefully remove the old battery, noting its orientation (positive side up/down).
    -   Insert the new battery with the correct orientation.
    -   Snap the fob halves back together securely.
    -   Test the fob immediately.
-   **Step 2: Attempt the Emergency Start Procedure.**
    -   With the new battery in the fob (or if you suspect a weak battery), place the key fob directly against the start button.
    -   Ensure your foot is firmly on the brake pedal.
    -   Press the start button. If the vehicle starts, this confirms the fob's transponder chip is working, pointing to a weak battery or RF communication issue.
-   **Step 3: Test with a Spare Fob.**
    -   If you have a second registered key fob, try using it. If it works, the problem is isolated to your primary fob.
-   **Step 4: Check the Vehicle's 12V Auxiliary Battery.**
    -   Locate the 12V auxiliary battery (often in the trunk or under the hood, consult your owner's manual).
    -   Use a multimeter to check its voltage. A healthy battery should read around 12.6V or higher when the vehicle is off. If it's significantly lower (e.g., below 12V), consider having it tested or replaced.
-   **Step 5: Inspect the Key Fob for Damage.**
    -   Examine the fob for cracks, water damage, or signs of impact. If damaged, a replacement fob may be necessary.
-   **Step 6: Eliminate RF Interference.**
    -   Move your Mirai to a different location, away from potential sources of strong radio interference, and try again.
-   **Step 7: Check for Immobilizer System Issues.**
    -   If basic steps fail, there might be an issue with the vehicle's immobilizer system or Smart Key ECU. This typically requires specialized diagnostic tools available at a dealership or qualified independent shop.

## Repair options and cost factors

The repair options and associated costs for a "Key Fob Not Detected" issue on your 2019 Toyota Mirai vary widely depending on the root cause:

-   **Key fob battery replacement:** This is the cheapest and easiest fix, usually costing just a few dollars for the battery itself, and can be done DIY.
-   **New key fob purchase and programming:** If your fob is damaged or lost, you'll need a new one. This can be costly, as new Toyota fobs often need to be purchased from a dealership and programmed to your specific vehicle. Locksmiths specializing in automotive keys may also be able to provide and program fobs at a potentially lower cost.
-   **Vehicle 12V auxiliary battery replacement:** If the vehicle's 12V battery is the culprit, replacement costs are similar to those for a standard car battery.
-   **Smart Key ECU or wiring repair/replacement:** This is the most complex and expensive repair, as it involves internal vehicle components. Diagnosis and replacement of these parts typically require a dealership or a specialist with access to specific Toyota diagnostic equipment.

## When to see a professional

While many "Key Fob Not Detected" issues can be resolved with simple DIY steps, there are times when professional help is essential for your 2019 Toyota Mirai:

-   **After trying basic fixes:** If replacing the fob battery, using the emergency start, and checking for interference doesn't resolve the issue, it's time for a professional diagnosis.
-   **No spare fob:** If you only have one key fob and it's not working, you'll be stranded without professional assistance.
-   **Suspected vehicle electrical issue:** If you suspect the problem lies with the vehicle's 12V battery, wiring, or the Smart Key ECU, a technician has the tools and expertise to properly diagnose and repair these components.
-   **New key fob programming:** If you need a new key fob, it almost always requires specialized equipment to program it to your Mirai's immobilizer system.

## Frequently asked questions

### How do I start my 2019 Toyota Mirai if the key fob battery is dead?

If your 2019 Toyota Mirai's key fob battery is dead, you can typically perform an emergency start. Place the key fob directly against the start button while simultaneously pressing the brake pedal and pushing the start button. This allows the vehicle to read the fob's transponder chip directly, bypassing the long-range radio signal.

### Can I program a new key fob for my Mirai myself?

Generally, programming a new key fob for a 2019 Toyota Mirai requires specialized diagnostic equipment that is usually only available at a Toyota dealership or through a certified automotive locksmith. While some older vehicles allowed DIY programming, modern immobilizer systems are more complex for security reasons.

### What else could cause my Mirai to say "Key Fob Not Detected" besides the fob battery?

Beyond a dead key fob battery, other common causes for a "Key Fob Not Detected" message in your 2019 Toyota Mirai include a weak or dead 12V auxiliary battery in the vehicle, strong radio frequency interference, physical damage to the key fob itself, or a malfunction in the vehicle's Smart Key ECU or receiver. Diagnosing these issues may require professional assistance.

## Sources and further reading

For more detailed repair guides and information on various automotive topics, explore our All repair guides. If you're interested in other Toyota models or vehicles, you can [Browse vehicles](/vehicles) on our platform.

This information is not a substitute for OEM service procedures; always use a qualified technician for safety-critical work.

## Explore related guides on ClearTheCode

- [Browse OBD-II codes](/codes)
- All repair guides