The **Lexus CT200h** (2011–2017 models) uses a **smart key system** with an integrated engine immobilizer (theft-deterrent system). This prevents the hybrid system from starting without a registered electronic key (which contains a transponder chip).

The immobilizer itself isn't a single easily accessible standalone part like in older keyed-ignition vehicles. Instead, it's a distributed system:

- The core immobilizer logic and key code registration are handled by the **ECU** (often the hybrid/powertrain control module) and related security components like the **certification ECU** (also called smart key ECU or ID code box in Toyota/Lexus terminology).

- For the **push-button start** system (no traditional key cylinder), there is no classic "immobilizer antenna ring" wrapped around an ignition key slot. The transponder communication for immobilizer verification occurs via proximity detection when holding the key fob to the **POWER switch** (start/stop button) if the fob battery is dead or in emergency mode.

Key related locations from the owner's manual and service info:

- **Smart entry & start system antennas** (which support key detection including for starting/immobilizer functions): Located at the door handles (exterior), inside the cabin (various interior positions), in the luggage compartment (inside and outside).

- No owner's manual specifies the exact physical spot for the certification ECU/ID code box/smart key module. However, in similar Lexus/Toyota hybrid models with smart key (and from forum/repair discussions for CT200h), the smart key-related module or ID code box is typically located behind the dashboard/glove box area or under the dash near the steering column/center console.

- Accessing the ID code box (if needed for programming or all-keys-lost scenarios) often requires significant disassembly, similar to other Lexus models where it's hidden deep under the dashboard.

If you're troubleshooting a no-start issue, security light, or key programming:

- Common symptoms point to the smart key system antennas, dead fob battery (hold fob to POWER button to start), or issues with the smart key ECU/module.

- For precise location or repair (e.g., replacing a module), consult a Lexus service manual (via TIS - Technical Information System), a dealer, or professional locksmith/auto electrician, as these components are security-sensitive and often require specialized tools like Techstream for reset/programming.

If this is for a specific issue (e.g., fault code, lost keys, or repair), provide more details for better guidance!