My brake lights will not engage when i press the brake pedal

you replaced brake light sensor, not switch. Did you recalibrate it ?

Brake Pedal Position Sensor Calibration Description and Operation
Calibration Criteria
Brake pedal position sensor calibration must be preformed after the brake pedal position sensor or electronic brake control module (EBCM) have been serviced. The calibration procedure will set the brake pedal position sensor "home" value. This value is used by the EBCM to determine the action of the driver applying the brake system and to provide this information to the vehicle subsystems via the class II communication bus.
Calibration Procedure
• Install a scan tool.
• Turn ON the ignition, with the engine OFF.
• Select Chassis.
• Select DRP/ABS/TCS/VSES.
• Select Special Functions.
• Select the BPP Sensor Calibration and follow the directions displayed on the screen.


Stop Lamps
The brake pedal position sensor is a resistive device used to sense the action of the driver's application of the brake pedal. The brake pedal position sensor provides an analog signal that will increase as the brake pedal is pressed.
The brake pedal position sensor receives both a 5-volt reference voltage and a low reference signal from the electronic brake control module (EBCM). When the brake pedal is pressed, a variable signal is applied from the brake pedal position sensor to the EBCM through the brake sensor signal circuit. The fuse block left rear supplies battery positive voltage to the bass relay switch through the BASS fuse. One side of the bass relay coil is permanently grounded at G402. The EBCM supplies voltage to the bass relay coil upon receiving the signal from the brake pedal position sensor. The switch contacts of the bass relay close and battery positive voltage is applied through the bass relay switch to the center high mounted stop lamp (CHMSL), and to the turn signal/hazard flasher module through the stop lamp switch signal circuit. The ground for the CHMSL is applied from G402. The turn signal/hazard flasher module then supplies voltage to the left and right stop lamps in the tail lamp assemblies that have ground at G401 and G402. For export vehicles the stop lamps receive battery positive voltage directly from the bass relay.


your best bet, have a qualified repair shop check it.

Your best bet , take your vehicle to a qualified repair shop that has a professional scan tool an service repair info . So it can be diagnosed correctly . The Stop lamp switch is not a old school conventional type . Has three wires , five volt ref. voltage from the BCM ,
Stop Lamps
The brake pedal position sensor is used to sense the action of the driver application of the brake pedal. The brake pedal position sensor provides an analog voltage signal that will increase as the brake pedal is applied. The body control module (BCM) provides a low reference signal and a 5-volt reference voltage to the brake pedal position sensor. When the variable signal reaches a voltage threshold indicating the brakes have been applied, the BCM will apply battery positive voltage to the stop lamps, center high mounted stop lamp (CHMSL), transmission control module (TCM), and engine control module (ECM). Ground for the right rear stop lamp and CHMSL is applied at G402. Ground for the left rear stop lamp is applied at G400. The stop lamps on this vehicle will not illuminate unless the ignition is in the accessory, run, or crank positions. When the ignition is in the OFF position the stop lamps will not illuminate when the brake pedal is applied.

Checking for DTC's - diagnostic trouble code's in the BCM would be my first diagnostic step , not guessing an replacing parts .

DTC B3903 02: Stop Lamp Relay Circuit Short to Ground
DTC B3903 05: Stop Lamp Relay Circuit Open or Short to Voltage

DTC C0277 06: Brake Pedal Position Sensor Circuit Short to Ground or Open
DTC C0277 07 : Brake Pedal Position Sensor Circuit Voltage Above Threshold
DTC C0277 09 : Brake Pedal Position Sensor Circuit Rate of Change Above Threshold

DTC C0278 00: Brake Pedal Position Sensor Not Calibrated

DTC C0870 03 : Device Voltage Reference Output 1 Circuit Voltage Below Threshold
DTC C0870 07 : Device Voltage Reference Output 1 Circuit Voltage Above Threshold
As you can see a number of DTC'S can be set , so do yourself a favor an let a qualified technician check it

Circuit/System Description
The brake pedal position sensor is used to sense the action of the driver application of the brake pedal. The brake pedal position sensor provides an analog voltage signal that will increase as the brake pedal is applied. The body control module (BCM) provides a low reference signal and a 5-volt reference voltage to the brake pedal position sensor. When the variable signal reaches a voltage threshold indicating the brakes have been applied, the BCM will apply battery positive voltage to the stop lamps, center high mounted stop lamp (CHMSL), transmission control module (TCM), and engine control module (ECM). The stop lamps will not operate unless the ignition is in the ON position.

DTC P0572 : Brake Switch Circuit 1 Low Voltage
DTC P0573 : Brake Switch Circuit 1 High Voltage

It's hard to fix something when you don't have a clue how it works . The CC is also a controlled function of the BCM .
The BCM monitors the signal circuit of the cruise control switches. The BCM relays the cruise control switch status to the ECM via the GMLAN serial data circuit. The ECM uses the status of the cruise control switch to determine when to capture and maintain the vehicle speed. The ECM monitors the vehicle speed signal circuit in order to determine and the desired vehicle speed. The ECM uses the TAC motor in order to maintain the vehicle speed. For further review of the TAC system, refer to Throttle Actuator Control (TAC) System Description for the 2.0L engine or to Throttle Actuator Control (TAC) System Description for the 2.4L engine. Ignition voltage is supplied to the cruise control switch from the 2-amp CRUISE fuse located in the integrated BCM fuse block. The cruise control switches are located on the steering wheel. The cruise control function switches are arranged in a resistive ladder design, with each cruise control function switch having a different resistance value. The BCM detects a specific voltage value that is associated with any cruise control function switch being activated. When the normally open cruise control on/off switch is turned ON, the switch closes and the BCM detects a predetermined voltage signal on the cruise control set/coast and resume/accel switch signal circuit. The BCM sends a GMLAN serial data message to the ECM indicating that the On/Off switch is active. Similarly, when the + RES switch or the - SET switch are pressed, the BCM detects the predetermined voltage signal on the cruise control set/coast and resume/accel switch signal circuit. To engage the cruise control system, ensure that the vehicle speed is above 40.2 km/h (25 mph), turn the cruise On/Off switch ON and momentarily press the - SET switch. The ECM will engage the cruise control system and record the vehicle speed. The ECM sends a GMLAN serial data message via the BCM to the driver information center (DIC) in order to display the CRUISE ENGAGED message. The - SET switch or the + RES switch will remain inactive when the BCM has not received the predetermined voltage signal from the On/Off switch. Pressing the accelerator pedal, while the cruise control system is engaged, will allow the driver to override the cruise control system in order to accelerate the vehicle beyond the current set vehicle speed. When the accelerator pedal is released, the vehicle will decelerate and resume the current set vehicle speed. The driver can also override the current set vehicle speed via the - SET switch and the + RES switch. When the cruise control system is engaged, pressing and holding the - SET switch will allow the vehicle to decelerate from the current set vehicle speed without deactivating the cruise control system. When the - SET switch is released, the ECM will record the vehicle speed and maintain the vehicle speed as the new set vehicle speed. When the cruise control system is engaged, momentarily pressing the - SET switch will allow the vehicle to decelerate at 1.6 km/h (1 mph) increments for each time that the - SET is momentarily pressed, with a minimum vehicle speed of 37 km/h (23 mph). Pressing and holding the + RES switch, when the cruise control system is engaged, will allow the vehicle to accelerate to a greater vehicle speed than the current set vehicle speed. When the + RES switch is released, the ECM will record the vehicle speed and maintain the vehicle speed as the new set vehicle speed. When the cruise control system is engaged, momentarily pressing the + RES switch will allow the vehicle to accelerate at 1.6 km/h (1 mph) increments for each time that the + RES switch is momentarily pressed, with the maximum acceleration total of 16 km/h (10 mph) over the current set vehicle speed. Momentarily pressing the + RES switch after the cruise control system has been disengaged by pressing the brake pedal, will recall the previous set vehicle speed that is recorded in the ECM.

You have battery voltage at the fuse , not current . You won';t have current flow until the light's work . The brake switch does not have the voltage value's you would think. An it's called a brake pedal position sensor , it is a low voltage sensor powered by the BCM - body control module .
Stop Lamps
The body control module (BCM) is responsible for controlling the stop lamps. When the driver presses the brake pedal, the BCM will receive a signal from the brake pedal position sensor. The BCM then supplies battery positive voltage to both left and right stop lamp assemblies and to the center high mounted stop lamp (CHMSL). The stop lamps are grounded at G401 and G403. The CHMSL is grounded at G302.

Your best bet , take it to a qualified repair shop before you short out the BCM.

How do you know brake pedal positioning sensor needs to be replaced ? Traction light on ? DTC'S - diagnostic trouble code's stored . That's why cruise won't engage . Taking your vehicle to a qualified repair shop an having it diagnosed is what you should do.
Cruise Control Inhibited
The engine control module (ECM) inhibits the cruise control operation when any of the following conditions exist:
• The ECM has not detected a brake pedal activation from the body control module (BCM).
• A Cruise Control System DTC has been set.
• The vehicle speed is less than 40.2 km/h (25 mph).
• The vehicle speed is too high.
• The vehicle is in PARK, REVERSE, NEUTRAL, or 1st gear.
• The engine RPM is too low.
• The engine RPM is too high.
• The system voltage is not between 9-16 volts.
• The Antilock Brake System (ABS)/Traction Control System (TCS) is active for more than 2 seconds.

Do the stop lamps work ?
DTC P0703 Brake Switch Circuit 2
The body control module (BCM) monitors the brake pedal position sensor. When the brake pedal is applied, the BCM detects a predetermined voltage signal. The BCM sends a GMLAN serial data message to the powertrain control module (PCM) indicating the status of the stop lamps.

DTC P1574 Stop Lamp Switch Circuit
The stop lamp switch signal circuit is a direct hardwire input to the throttle actuator control (TAC) module from the body control module (BCM). The TAC module monitors the stop lamp switch signal circuit in order to detect when the brake pedal has been applied. When the brake pedal is pressed, the BCM sends a GMLAN serial data message to the powertrain control module (PCM) indicating that the brake pedal has been applied. The BCM also sends a high voltage signal on the stop lamp switch signal circuit to the TAC module. The TAC module sends a universal asynchronous receiver transmitter (UART) serial data message to the PCM indicating that a brake pedal application has occurred. The PCM will compare the UART serial data message and the GMLAN serial data message in order to confirm that both serial data messages match.

Kevin , this is a very complex way of stoplamp control , I will post GM service Description and Operation of how they work an let you deside if you want to tackle this ! Do the center high mount brake lights work ?
The brake pedal position sensor is a resistive device used to sense the action of the driver's application of the brake pedal. The brake pedal position sensor provides an analog signal that will increase as the brake pedal is pressed.
The brake pedal position sensor receives both a 5-volt reference voltage and a low reference signal from the electronic brake control module (EBCM). When the brake pedal is pressed, a variable signal is applied from the brake pedal position sensor to the EBCM through the brake sensor signal circuit. The fuse block left rear supplies battery positive voltage to the bass relay switch through the BASS fuse. One side of the bass relay coil is permanently grounded at G402. The EBCM supplies voltage to the bass relay coil upon receiving the signal from the brake pedal position sensor. The switch contacts of the bas relay close and battery positive voltage is applied through the bas relay switch to the center high mounted stop lamp (CHMSL), and to the turn signal/hazard flasher module through the stop lamp switch signal circuit. The ground for the CHMSL is applied from G402. The turn signal/hazard flasher module then supplies voltage to the left and right stop lamps in the tail lamp assemblies that have ground at G401 and G402. For export vehicles the stop lamps receive battery positive voltage directly from the bass relay.

There could be a code stored in the EBCM - electronic brake control module - DTC B3903 Stop Lamp Relay Circuit
DTC C0278 Brake Pedal Position Sensor Not Calibrated
there are several other codes that pertane to the brake lights . you could try Replaceing the bas relay !
Fuse Block - Left Rear
In the passenger compartment, under the left side of the rear seat at carpet.

Try checking the small switch at the top of the brake pedal. There may be two one for brake lights and one for the cruise control. To ascertain what is what, pull one wire off and see if brake lights work. If not, then the other switch controls the cruise control disconnect and engage..I you have a DC test light check to see if power is on on both side of the switch with brake pedal up. If so, depress pedal and check power on the other side there should be none, if there is the switch is defective.

I'm assuming you're talking about an automatic transmission. When you press on the brake pedal, you engage a switch under the pedal which turns your brake lights on, and also engages a solenoid in the shifter which allows you to shift the car out of Park.

That said, are your brake lights working? If your brake lights come on when you press the brake pedal, chances are the shifter solenoid/actuator is broken. If your brake lights DO NOT come on when you press the brake pedal, chances are you have a faulty brake light switch.

Regardless which part is to blame, these are pretty simple and inexpensive repairs.

Good luck, and if this helped, please rate my answer!

In all probability the brake sensor on the pedal is starting to work erratically. This sensor sends a signal to the shifter gate, and the gate will not open unless the brake pedal is depressed. I had it happen to me, and the only solution until i replaced the sensor was to hit the brake over and over until the stop light worked (You can check it while looking at the 3rd stop light in the rear view mirror). If the stop lights work, the shifter gate should allow the shifter to move out of park. This sensor/ switch is located about 3 inches above the brake pedal, and it is easily recognizable by the cable attached to it.

at the brake pedal the is a switch, when you press the pedal moves away from it which turns the lights on

probably the switch at the top of the pedal is already defective. that switch turns on the brake lights when you depress the pedal. you may have to just replace that switch.