Passive Anti-Theft System (PATS)
The PATS function is controlled by the BCM. There are 2 main checks the PATS performs before allowing the vehicle to start. If either of these checks fail, the PATS does not allow the vehicle to start and Starting System Fault displays in the message center. These 2 checks are the BCM verifying the PCM identification to make sure it matches the PCM identification stored in memory and to verify that a valid programmed key was used to change the ignition to RUN or START.
The first check is initiated by the BCM wakeup the PCM by supplying voltage on the wake up control circuit. The BCM only activates the wake up circuit 5 times. After that, the wakeup circuit remains off until the ignition is cycled to RUN. The BCM activates the wakeup control circuit when any of the following occur:
- The driver door is opened
- A remote start request is received (if equipped with factory remote start)
- The brake pedal is applied
- A key is inserted into the ignition key cylinder
- The ignition transitions to RUN or START
Once the PCM is awake, the BCM sends the PCM a challenge message over the HS-CAN. When the PCM receives the challenge message, it generates a response and sends it back to the BCM. If the response from the PCM does not match the response in the BCM memory, this first check fails and the vehicle does not start.
Once the PCM identification has been verified, the PATS performs the second check to make sure a programmed key is in the ignition lock cylinder. When the BCM determines the ignition changes to RUN or START, it generates a challenge message. It sends the challenge message to the PATS transceiver on the transmit (TX) circuit. The transceiver in turn reads the key and generates a response message that is sent back to the BCM on the receive (RX) circuit. If the message received from the transceiver does not match a key stored in the BCM memory, the vehicle does not start.
If both the PCM identification and key verification pass, the PATS allows the vehicle to start. The PATS cannot disable or stall a vehicle that has already been started.
PATS PIDs
In conjunction with DTCs, the PATS PIDs are a useful tool when diagnosing PATS concerns.
BCM Module PID Chart
| Acronym | Name | Description |
|---|---|---|
| MIN_KEYS_RQD | Minimum number of keys required | Minimum number of programmed keys required. This PID always reads 2. |
| KEYS_PROGMD | PATS number of ignition key codes supported | Number of keys currently programmed into the BCM (If unlimited key mode is enabled, this PID only reads 2 regardless of how many keys are programmed). |
| START_ENABLD | Vehicle enable status | Indicates if PATS allows the vehicle to start. Must read enabled for the vehicle to start. |
| PCM_ID | PCM ID status stored | Indicates if there is a PCM identification currently stored in the BCM. Must read stored for the vehicle to start. |
| PCM_VERIFY | PCM verify OK | Indicates if the PCM identification received by the BCM matches the PCM identification stored in the BCM memory. Must read yes for the vehicle to start. |
| UNL_KEY_MODE | PATS unlimited key mode | Eight-digit unlimited transponder security key code that is selected by the customer. This can only be viewed while performing the unlimited key mode programming. |
Unlimited Key Mode
The PATS contains a feature called unlimited key mode. This feature allows a customer to program more than 8 keys to their vehicle, if requested. Each vehicle in unlimited key mode is set up with a special unlimited transponder security key code. This allows all of the customer vehicles to share the same mechanically cut keys. For an individual customer, any randomly selected security key that has been previously mechanically cut and electronically programmed to the vehicle is acceptable. To enable/disable the unlimited key mode, refer to Key Programming - Unlimited Key Mode .
When in the unlimited key mode, the KEYS_PROGMD PID in the BCM always reads 2, no matter how many keys are programmed to the vehicle (after the first 2 keys have been programmed to the vehicle).