Principles of Operation

Anti-Theft

Principles of Operation

NOTE: Replacement of the Passive Anti-Theft System (PATS) transceiver does NOT require the PATS keys to be programmed into the Instrument Cluster (IC) again.

NOTE: A minimum of 2 PATS keys must be programmed into the IC before the vehicle starts.

NOTE: Make sure any aftermarket remote start systems have been removed from the vehicle before any PATS-related no-start concerns are investigated. If the remote start system is a Power Code(TM) system, make sure it is not the cause of the no-start. Removal of the system may be necessary.

NOTE: The Smart Junction Box (SJB) is also known as the Generic Electronic Module (GEM).

Anti-Theft Indicator

PATS uses a visual anti-theft indicator located in the IC. The anti-theft indicator proves out for 3 seconds when the ignition key is in the ON or START position under normal operation. If there is a PATS concern, this indicator either flashes rapidly or glows steadily when the ignition key is turned to the ON or START position. PATS also flashes the anti-theft indicator every 2 seconds when the ignition key is in the OFF position to act as a visual theft deterrent. Refer to Instrumentation, Message Center, and Warning Chimes Testing and Inspection.

PATS is active only for a few seconds when the vehicle is starting. It is not a PATS concern if the vehicle stalls after it has been running for a minimum of 3 seconds. PATS cannot disable a running vehicle.

A PATS no-start can involve a vehicle no-start due to either the fuel injectors not operating or the starter not operating (or both). If the PATS anti-theft indicator does not prove out and one (or both) of the previous conditions are present, it may be due to a PATS concern. If the anti-theft indicator proves out, and the vehicle does not start, it is probably not a PATS concern. Refer to Computers and Control Systems Information. If the anti-theft indicator does not illuminate at all, it may be an IC concern. GO to Symptom Chart. Anti-Theft - Passive Anti-Theft System A low battery voltage may cause PATS to allow starter operation, but prevent the fuel injectors from operating.

Encoded Passive Anti-Theft System (PATS) Keys

PATS uses a special ignition key that is larger than a conventional ignition key because it contains a permanently-installed electronic device called a transponder. Each transponder contains a unique encrypted identification code which is one of a very large number of combinations. The addition of this transponder to the key makes it an "encoded" key. The PATS key does not require batteries and should last the lifetime of the vehicle. Each PATS key must be programmed into the IC before it can be used to start the vehicle. There are specific general procedures described that must be carried out if a new PATS key is necessary. Refer to Key Programming Using Diagnostic Equipment Using Diagnostic Equipment or Key Programming Using Two Programmed Keys Using Two Programmed Keys.

Passive Anti-Theft System (PATS) Transceiver

The PATS transceiver is located under the steering column shroud and communicates with the encoded ignition key. During each vehicle start sequence, the PATS transceiver reads the encoded ignition key identification code and sends data to the IC. The IC validates the code, and if it is the correct code, sends a message to the PCM to ground the starter relay solenoid coil and allow the fuel injectors to operate.

Passive Anti-Theft System (PATS) Operation

The PATS function is controlled by the IC. With this type of PATS, there are parameters that need to be reset if either the IC or the PCM (or both) are replaced. Refer to Passive Anti-Theft System (PATS) Parameter Reset Passive Anti-Theft System (PATS) Parameter Reset. When the PATS key is turned to the ON or START position, the IC initiates the key interrogation sequence by sending a voltage signal to the PATS transceiver. The transceiver then uses its antenna to communicate with the transponder in the PATS key. This process "reads" the PATS key identification code and sends the key identification code back to the IC, which interprets it and determines if it matches one of the stored key codes. If it does match one of the stored key codes, the IC sends a message to the PCM to ground the starter relay solenoid coil and allow the fuel injectors to operate. If it does not match one of the stored key codes, or it is only a partial key read or no key read, the IC sends a message to the PCM to not ground the starter relay solenoid coil and not allow fuel injector operation. The anti-theft indicator in the IC flashes (or may glow steadily) and the IC stores one or more DTCs.

All elements of PATS must be functional before the vehicle starts. If any of the components are not working correctly, the vehicle cannot start. If the IC must be replaced for any reason (PATS or IC concerns), the PATS keys must also be programmed into the new IC. Refer to Key Programming Using Diagnostic Equipment Using Diagnostic Equipment.

PATS disables the vehicle from starting if there is:

- a damaged PATS key.
- a non-programmed PATS key.
- a non- PATS key (key has no electronics).
- damaged wiring.
- a damaged transceiver.
- a damaged IC.
- a damaged PCM.

Passive Anti-Theft System (PATS) PIDs

Monitoring the PATS PIDs can be very useful in determining which diagnostic steps to follow. Viewing the MASTERKEY (verifies if the key is programmed) PID (with both keys) determines if the key is a programmed key and also used to prove out the transceiver, circuitry and the IC. A master key is any key that is programmed into the IC. Viewing the MIN_KEYS (minimum number of keys) PID (this PID does not change) determines the minimum number of keys that must be programmed into the IC. There must be at least 2 keys programmed into the IC in this type of PATS before the vehicle starts. Viewing the N_KEYCODE (number of keys programmed) PID determines if the minimum number of keys have been programmed into the IC. If the N_KEYCODE PID reads 0 or 1, additional key(s) needs to be programmed into the IC in order to meet the minimum of 2 keys. If the N_KEYCODE PID reads 0 or 1, and the MASTERKEY PID reads Not Present, that particular key must be programmed into the IC. If the N_KEYCODE PID reads 1, and the MASTERKEY PID reads Present, that particular key is already programmed into the IC. If the IC was replaced, the parameters in the IC, and the parameters in the PCM need to be reset.

When the parameters in the IC are reset, it clears (erases) the PCM ID from the IC. A parameter reset of the PCM sends a PCM ID to the IC that is necessary for the system to operate. Make sure to cycle the key at least once, then turn the key ON and make an attempt to start the vehicle for 3-5 seconds before attempting more procedures.

If the PCM_ID (PCM id) PID reads Stored, the IC has a PCM ID stored. If it reads Not Stored, a parameter reset of the PCM may resolve this concern. If the PCM_VFY (PCM verify status) PID reads No, and the N_KEYCODE reads 2 or more and the MASTERKEY reads Present, the IC, and the PCM needs to have their parameters reset. Refer to Passive Anti-Theft System (PATS) Parameter Reset Passive Anti-Theft System (PATS) Parameter Reset. In this particular scenario, the PATSENABL (vehicle enable status) PID read Disabled.

If the PCM_VFY PID reads No, and the N_KEYCODE PID reads 1 and the MASTERKEY PID reads Present, or if the N_KEYCODE PID reads 0 and the MASTERKEY PID reads Not Present, a minimum of 2 keys need to be programmed into the IC. Refer to Key Programming Using Diagnostic Equipment Using Diagnostic Equipment.

If the MASTERKEY PID continues to read Not Present after programming, this may indicate a defective key.

The SPAREKEY (spare key) PID is defaulted to ENABLE. With the SPAREKEY PID displaying ENABLE, the IC accepts more than 2 keys (up to a maximum of 8) being programmed into the IC using the Key Programming Using Two Programmed Keys Using Two Programmed Keys procedure. It can be toggled to DISABLE if the customer wants to disable the customer spare key programming function. Refer to Key Programming Switch State Control Switch State Control. This switch state control does not affect the Key Programming Using Diagnostic Equipment procedure.

In summary, for the PATS PIDs and their correct state in order for the vehicle to start:

- N_KEYCODE must read 2 or more
- MASTERKEY must read Present
- PCM_ID must read Stored
- PCM_VFY must read Yes
- PATSENABL must read Enabled

Unlimited Key Mode

PATS contains a feature called unlimited key mode (UNL_KEY_MODE). This feature allows a customer to program more than 8 vehicle keys, if requested. Each vehicle in unlimited key mode is set up with a special unlimited transponder security key code. This allows all 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. Refer to Spare Key Programming - Unlimited Key Mode Unlimited Key Mode.