Replace BCM and Passlock sensor

You may have a key with a chip in it so you will need to hold the old key that has the correct chip in it next to the ignition switch when you try to start the car.

If the thief damaged the 2 wires going down the column from the sensor ring those will have to be repaired. That is what picks up the signal from the key to tell the car to start.

DTC P1631 Theft Deterrent Fuel Enable Signal Not Correct

The powertrain control module (PCM) controls the fuel injector operation and the starter operation based on a vehicle theft deterrent (VTD) password from the vehicle body control module (BCM). When the ignition is first turned ON, the BCM sends a programmed theft deterrent password to the PCM. The PCM acknowledges the password and responds to the BCM that normal fuel injector and starter operation will continue. If the PCM detects an incorrect password, a theft deterrent system failure, or an attempted vehicle theft, DTC 1631 will set. The engine will not start or crank as long as the condition is present.

Diagnostic Aids
DTC P1631 indicates that the vehicle theft deterrent password that the PCM has learned does not agree with the password received from the VTD system. This condition can occur if the BCM has been replaced and the PCM Password Learn function has not been enabled. If the BCM has been replaced, the password must be relearned.


Step
Action
Yes
No
1
Did you perform the Diagnostic System Check - Vehicle?
Go to Step 2
Go to Diagnostic System Check - Vehicle in Vehicle DTC Information
2
Perform the Password Learn Procedure. Refer to Programming Theft Deterrent System Components .
Did you complete the procedure?
System OK

30-Minute Learn Procedure
Turn ON the ignition, with the engine OFF.
Attempt to start the engine, then release the key to ON; the vehicle will not start.
Observe the SECURITY telltale. After approximately 10 minutes, the telltale will turn OFF.
Turn OFF the ignition, and wait 5 seconds.
Repeat steps 1 through 4 two more times for a total of 3 cycles/30 minutes. The vehicle is now ready to relearn the Passlock™ Sensor Data Code and/or passwords on the next ignition switch transition from OFF to CRANK.
Important: The vehicle learns the Passlock™ Sensor Data Code and/or password on the next ignition switch transition from OFF to CRANK. You must turn the ignition OFF before attempting to start the vehicle.
Start the engine. The vehicle has now learned the Passlock™ Sensor Data Code and/or password.
With a scan tool, clear any DTCs if needed. History DTCs will self clear after 100 ignition cycles.

Passlock - anti-theft system ! Is the security light lit or flashing ? If light is on taking it to a qualified repair shop an having the DTC'S - diagnostic trouble codes checked would be one option . Do you know what DTC'S are for ? Plus there are diagnostic tests for finding the problem associated with each code .
DTC B2957
DTC B2958
DTC B2960
DTC B3031
DTC B3033
DTC P1626
DTC P1630
DTC P1631
These codes are all for the Passlock system . This is just one of the above codes .
DTC B2957 Security System Sensor Data Circuit Low
The Passlock System is provided in order to prevent vehicle theft if the ignition lock cylinder is forced to rotate. The Passlock controller reads the coded Passlock data signal. The data consists of a voltage generated by a voltage divider network created between the Passlock assembly and the Passlock controller. If the data is correct , indicating rotation of the lock cylinder with the proper mechanically cut key, then the Passlock controller sends a password to the powertrain controller via serial data to indicate that engine starting should be allowed. The Passlock controller also interfaces with the driver displays subsystem to indicate, via the security indicator, the Passlock subsystems status and acute subsystem failures.
Test Description
The numbers below refer to the step numbers on the diagnostic table.
This step tests for proper operation of the 5-volt reference circuit from the BCM.
This step checks for a voltage drop through the Passlock resistor located in the ignition switch. Voltage in this step will vary due to unique differences in each individual Passlock resistor.
Step
Action
Values
Yes
No
1
Did you perform the Diagnostic System Check - Vehicle?
--
Go to Step 2
Go to Diagnostic System Check - Vehicle in Vehicle DTC Information
2
Turn OFF the ignition.
Disconnect the ignition switch connector.
Back the 5 V reference circuit terminal from the ignition switch harness connector.
Reconnect the ignition switch harness connector.
Turn ON the ignition, with the engine OFF.
With a DMM, measure the voltage at the 5 V reference circuit terminal.
Is the 5 V reference circuit signal circuit voltage approximately equal to the specified value?
5 V
Go to Step 3
Go to Step 4
3
Disconnect the ignition switch connector.
Resecure the 5 V reference circuit terminal into the harness connector.
Reconnect the ignition switch harness connector.
Disconnect the body control module (BCM) connector containing the off/run/crank voltage circuit.
Back the off/run/crank voltage circuit terminal from the BCM harness connector.
Reconnect the BCM harness connector.
Connect a DMM between the off/run/crank voltage circuit terminal and a good ground.
Important: Do not start the vehicle.
Momentarily switch the ignition to the CRANK position. Do not start the vehicle.
Is voltage measured at the off/run/crank voltage circuit?
--
Go to Step 6
Go to Step 5
4
Inspect the 5 V reference circuit for an open or short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.
Did you find and correct the condition?
--
Go to Step 11
Go to Step 6
5
Inspect the Off/Run/Crank voltage circuit for an open or short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.
Did you find and correct the condition?
--
Go to Step 11
Go to Step 7
6
Inspect for poor connections at the BCM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.
Did you find and correct the condition?
--
Go to Step 11
Go to Step 8
7
Inspect for poor connections at the ignition switch. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.
Did you find and correct the condition?
--
Go to Step 11
Go to Step 9
8
Replace the BCM. Refer to Control Module References in Computer/Integrating Systems for replacement, setup, and programming.
Did you complete the replacement?
--
Go to Step 11
--
9
Replace the ignition switch. Refer to Ignition and Start Switch Replacement in Steering Wheel and Column.
Did you complete the replacement?
--
Go to Step 10
--
10
Perform the Programming Theft Deterrent System Components procedure.
Is the repair complete?
--
Go to Step 11
--
11
Use the scan tool in order to clear the DTCs.
Turn OFF the ignition
Turn ON the ignition, with the engine OFF.
Select the BCM display DTCs function on the scan tool.
Does the DTC reset?
--
Go to Step 2
System OK


Or if you wish you can try a bypass of the system . There are a lot of videos on youtube showing different thing's . I 'am a GM Tech ,have been for over thirty years . I see a lot of the end result of people that have no clue how these systems work . When they can't fix it ( get the vehicle to start an run ) They bring them to the dealer. Then they want to complain how much it costs . If they would have brought it to us first ,wouldn't have been that way . Plus if the BCM is the trouble it needs to come to the dealer programming any way .

The most common repair is to replace the ignition lock cylinder due to broken wires inside the steering column. Most common problems related to the Passlock family involve the Passlock sensor. The sensor is part of the ignition lock cylinder on Passlock I and Passlock II systems. ( Where the key goes ), Good-Day!

You don't need to program key's with this anti-theft system , there is no chip inside . Did you get a correct lock cylinder. . On this system there is a pass lock sensor in side an the lock clinder has a magnet !
Passlock™ Sensor
The passlock™ sensor is inside the upper right side of the steering column. The passlock™ sensor contains 2 hall effect sensors. The tamper hall effect sensor is on the top. The security hall effect sensor is under the tamper hall effect sensor. Both of the hall effect sensors monitor the magnet of the lock cylinder through an opening. The tamper hall effect sensor is physically placed on top of the security hall effect sensor. This arrangement enables the tamper hall effect sensor to engage first if an intruder attempts to bypass the passlock™ sensor by placing a large magnet around that area of the steering column. There is a tamper resistor inside the passlock™ sensor in order to help prevent tamper to the system. Passlock™ equipped vehicles have a selection of 10 different security resistors ranging from 0.5K ohms up to 13.6K ohms. Installing one of the security resistors inside the passlock™ sensor will generate a unique passlock™ code. All 10 combinations of the passlock™ sensor have the same part number. However, you cannot simply replace the passlock™ sensor and expect the system to operate properly. Always start by performing the Diagnostic System Check first and following the instructions.
Body Control Module
The Body Control Module (BCM) contains the theft deterrent system logic. The BCM reads the passlock™ data from the passlock™ sensor. If the passlock™ data is correct, the BCM will pass theft. The BCM will then transmit the code password to the VCM/PCM.
During the tamper mode the vehicle may start. The vehicle will then stall. If the BCM receives the wrong passlock™ data, the VTD will immediately go into the tamper mode. The tamper mode will lock-out the vehicle fuel injectors for 10 minutes. The SECURITY indicator will flash while the VTD is in the tamper mode.
If the passlock™ sensor sends a correct password to the BCM when the ignition is in the ON position, the BCM will send a fuel enable signal to the VCM/PCM. The VCM/PCM will not disable the fuel due to any BCM message for the remainder of the ignition cycle.

You only need to do a relearn if one of these are replaced
Use this procedure after replacing:
• Passlock™ Sensor
• BCM
• VCM
• PCM

Did you replace any of these ?

30 Minute Re-Learn Procedure
Turn ON the ignition, with the engine OFF.
Attempt to start the engine, then release the key to ON (vehicle will not start).
Observe the SECURITY telltale, after approximately 10 minutes the telltale will turn OFF.
Turn OFF the ignition, and wait 5 seconds.
Repeat steps 1 through 4 two more times for a total of 3 cycles/30 minutes ( the vehicle is now ready to relearn the Passlock™ Sensor Data Code and/or passwords on the next ignition switch transition from OFF to CRANK).
Important: The vehicle learns the Passlock™ Sensor Data Code and/or password on the next ignition switch transition from OFF to CRANK. You must turn the ignition OFF before attempting to start the vehicle.
Start the engine (the vehicle has now learned the Passlock™ Sensor Data Code and/or password).
With a scan tool, clear any DTCs if desired (history DTCs will self clear after 100 ignition cycles).

Do you have a second key ?
Passlock™ System
The theft deterrent system has been incorporated into the body control module (BCM). The theft deterrent is provided in order to prevent vehicle operation if the correct key is not used in order to start the vehicle. The ignition key turns the lock cylinder. The cylinder rotation produces an analog voltage code in the Passlock™ sensor. This voltage code is received by the BCM. The BCM compares the voltage code to the previously learned voltage code. If the codes match, a class 2 message is sent from the BCM to the powertrain control module (PCM)/vehicle control module (VCM). The message enables the fuel injectors.
The design of the Passlock™ system is to prevent vehicle theft by disabling the engine unless the ignition lock cylinder rotates properly by engaging the correct ignition key. The system is similar in concept to the PASS-Key® system. However, the Passlock™ system eliminates the need for a key mounted resistor pellet. The components of the Passlock™ system are as follows:
• The ignition lock cylinder
• The ignition switch
• The Passlock™ sensor
• The BCM
• The security telltale
• The VCM/PCM

Your best bet , take it to a qualified repair shop . Security light on - DTC'S - diagnostic trouble codes are stored . Having codes check would point in a diagnostic procedure to fix the issue .

DTC B2947
DTC B2948
DTC B2957
DTC B2958
DTC B2960
DTC B3031
DTC B3033
DTC P1626
DTC P1631
Circuit Description
The Passlock™ system is provided in order to prevent vehicle theft if the ignition lock cylinder is forced to rotate or the ignition switch is operated while separated from the ignition lock cylinder case. The body control module (BCM) provides security system sensor power and low reference for the Passlock™ sensor. The BCM also measures the security system sensor voltage.
When the correct key is used to start the vehicle, a magnet on the lock cylinder passes close to the Passlock™ sensor within the ignition lock cylinder case. The magnet activates the security hall effect sensor in the Passlock™ sensor which completes a circuit from the security sensor signal circuit through a resistor to the security sensor low reference circuit. The resistance value will vary from vehicle to vehicle. The BCM will measure the voltage on the security sensor signal circuit and compare this voltage to a previously learned voltage. If the voltage measured is within the valid range, the BCM will send a class 2 message to the powertrain control module (PCM) to enable vehicle starting. If the voltage measured is not within the valid range, a class 2 message will be sent to the PCM to disable starting of the vehicle.

I have a 2002 Silverado 1500 My truck Cranks but will not run . With security light flashing . I've done the 10 minute method It works . for a week or two . What I've done is put my key in the ignition with very little pressure upward works every time at this point I think its a worn key. I'm waiting for it to do it again so I can try it with my original spare key. I've read it could be BCM connection / lose wire connection that inserts to pass lock sensor / or the pass lock sensor is dirty between sensor and key cylinder . I hope this helps Good Luck .

Read the following paragraph from GM, this will give you some insight on the passlock system on your vehicle.

The electrical switching portion of the ignition lock cylinder assembly is separate from the key and lock cylinder. The electrical switch portion and the key and lock cylinder synchronize and work in conjunction through the action of the mechanical assembly between the two components.

PASSLOCK(TM) SENSOR
The Passlock(TM) sensor contains 2 hall effect sensors. The tamper hall effect sensor is on the top. The security hall effect sensor is under the tamper hall effect sensor. Both of the hall effect sensors monitor the magnet of the lock cylinder through an opening. The tamper hall effect sensor is physically placed on top of the security hall effect sensor. This arrangement enables the tamper hall effect sensor to engage first if an intruder attempts to bypass the Passlock(TM) sensor by placing a large magnet around that area of the steering column. There is a tamper resistor inside the Passlock(TM) sensor in order to help prevent tamper to the system. Passlock(TM) equipped vehicles have a selection of 10 different security resistors ranging from 0.5K ohms up to 13.6K ohms . Installing one of the security resistors inside the Passlock(TM) sensor will generate a unique Passlock(TM) code. All 10 combinations of the Passlock(TM) sensor have the same part number.

BODY CONTROL MODULE
The body control module (BCM) contains the theft deterrent system logic. The BCM reads the Passlock(TM) data from the Passlock(TM) sensor. If the Passlock(TM) data is correct, the BCM will send a class 2 message to the PCM to enable the fuel injectors and crank relay.

During the tamper mode the vehicle may start. The vehicle will then stall. If the BCM receives the wrong Passlock(TM) data, the VTD system will immediately go into the tamper mode. The tamper mode will lock-out the vehicle fuel injectors and crank relay for 10 minutes . The SECURITY indicator will flash while the VTD is in the tamper mode.

If the Passlock(TM) sensor sends a correct password to the BCM when the ignition is in the ON position, the BCM will send a fuel enable and crank relay enable signal to the PCM.

SECURITY INDICATOR
The SECURITY indicator appears on the message center inside the instrument cluster. If the SECURITY indicator flashes or if the SECURITY indicator lights continuously during vehicle operation, refer to the system diagnosis.

POWERTRAIN CONTROL MODULE
The PCM communicates with the BCM over the Class 2 serial data line. When the BCM determines an incorrect password, it sends a Class 2 serial data password to the PCM in order to disable the crank relay and fuel injection system. If the BCM receives the expected voltage from the Passlock(TM) sensor, the BCM sends a class 2 serial data password to the PCM in order to enable the crank relay and fuel injection system. The PCM then allows the vehicle to start normally.

FUEL LOCKOUT CYCLE
The Passlock(TM) system has a lockout cycle of approximately 10 minutes . Once the vehicle is in the lockout cycle, the vehicle remains in the lockout cycle for 10 minutes , even if the ignition switch is turned from the ON position to the OFF position.

The Passlock(TM) system uses the lockout cycle in order to synchronize all of the Passlock(TM) components when any Passlock(TM) related part is replaced. The Passlock(TM) system requires 3 consecutive lockout cycles in order to complete the 30 minute learn procedure for a changed component.

CHANGING THE PASSLOCK(TM) COMPONENTS
The design of the Passlock(TM) system is to prevent theft even if the various theft deterrent parts change. The parts that can no longer be changed without the possibility of going into a tamper mode are:

* The Passlock(TM) sensor
* The BCM
* The PCM

If you replace any of these parts the vehicle may not start for 10 minutes . This is the long tamper mode. If this occurs, the system must go through a long tamper mode cycle. During this time the SECURITY indicator will be flashing for the full 10 minutes and DTC B3031 will be set. The BCM and the PCM require the full 10 minutes in order to complete a learn cycle. The ignition switch must remain in the ON position until the SECURITY indicator stops flashing. You will need to repeat the cycle if the ignition switch does not remain in the ON position. When replacing any of the above parts, perform the Programming Theft Deterrent System Components procedure.

The design of the Passlock(TM) system is to prevent the vehicle operation if the proper ignition key is not used in order to start the vehicle. The mechanical key, in normal operation, will turn the ignition lock cylinder. The Passlock(TM) sensor monitoring the position of the lock cylinder will relay the Passlock(TM) data to the body control module (BCM). The BCM will determine the validity of the Passlock(TM) data. The BCM will send a password to the powertrain control module (PCM). When the PCM receives the correct password the PCM allows the crank relay and fuel injectors to operate normally. The Passlock(TM) system requires the PCM and the BCM to communicate the various functions in order to operate. These functions transmit over the class 2 serial data line. For an explanation of the class 2 serial data description and operation refer to Data Link Communications Description and Operation in Data Link Communications.

Yes you can install and new ignition switch in your Blazer, once you have installed the new switch, your going to have to do a PASSLOCK II relearn for the new ignition key's to start the Blazer.
PASSLOCK - Reprogramming Auto Learn

IMPORTANT:

* The Body Control Module (BCM) must be programmed with the proper RPO configurations before performing learn procedures. See BCM Programming/RPO Configuration.
* If replacing the BCM with a GM Service Parts Operations (SPO) replacement part, the module will learn PASSLOCK sensor data code immediately. The existing VCM/PCM however, must learn the new fuel continue password when the BCM is replaced.
* If replacing a VCM/PCM with a GM Service Parts Operations (SPO) replacement part, after programming, these modules will learn the incoming fuel continue password immediately upon receipt of a password message. Once a password message is received, and a password is learned, a learn procedure must be performed to change this password again. A VCM/PCM which has been previously installed in another vehicle will have learned the other vehicle's fuel continue password and will require a learn procedure after programming to learn the current vehicle's password.

CONDITIONS
Use this procedure after replacing:

* Passlock Sensor
* BCM
* VCM
* PCM

30 MINUTE RE-LEARN PROCEDURE

1. Turn ON the ignition, with the engine OFF.
2. Attempt to start the engine, then release the key to ON (vehicle will not start).
3. Observe the SECURITY telltale, after approximately 10 minutes the telltale will turn OFF.
4. Turn OFF the ignition, and wait 5 seconds .
5. Repeat steps 1 through 4 two more times for a total of 3 cycles/30 minutes (the vehicle is now ready to relearn the PASSLOCK Sensor Data Code and/or passwords on the next ignition switch transition from OFF to CRANK). IMPORTANT: The vehicle learns the PASSLOCK Sensor Data Code and/or password on the next ignition switch transition from OFF to CRANK. You must turn the ignition OFF before attempting to start the vehicle.
6. Start the engine (the vehicle has now learned the PASSLOCK Sensor Data Code and/or password).
7. With a scan tool, clear any DTCs if desired (history DTCs will self clear after 100 ignition cycles).
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if it stalls while driving it is not a problem with passlock. I would have a look at the camshaft and crankshaft sensors, if those are faulty the PCM would shutdown the engine. It is a typical problem in GM cars.