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What lights ? Exterior - interior or all lights ? Could be the light switch , dimmer switch , daytime running lamps module . To find out for sure would need to test the electrical circuits , Can you do that ? There are videos on you tube on basic electrical trouble shooting , how to read a wiring diagram , how to use a DVOM - digital volt ohm meter .
With a scan tool, observe the Head Lamp Switch parameter in the Dash Integration Module (DIM) Inputs data list.
Does the scan tool indicate that the Head Lamp Switch parameter is OFF?
I would think you probably don't have a GM Tech 2 factory scan tool , this is a very involved diagnostic flow chart an you can't diagnose it very easily without the factory scan tool . You could replace parts an get lucky . I would start with the high beam relay in the under hood fuse / relay center . Do you know what a relay is , not to talk down to you because I don't what you know . The relay is energized by the DIM - dash integrated module . You may want to take this to a ASE certified repair shop or the dealer !
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.
The power is not getting from the positive battery post to the brake light bulbs and back again to the negative battery post. I know this sounds stupid, but I have seen many people beat their heads against the wall trying to figure out a lighting problem when it really is as simple as "the power has to go from positive to negative"...Period!
So now, you could have a shorted brake lamp circuit that is causing the fuses to blow and cut the power from the circuit. You could have a bad brake lamp switch keeping the power from going where it is supposed to. It could be a bad turn signal switch. It could be burned out light bulbs. Last, but not least, it could be broken wires or a bad connector causing an open circuit.
the brake light switch has either moved out of the bracket just above the brake light pedal or the brake light switch itself is faulty.
look just above the brake pedal, there should be a switch on a bracket so that when you press the brake pedal the spring plunger piece of the switch moves out and the lights come on. make sure the plunger goes in when the brake pedal goes back to normal position.if the lights stay on when the plunger is pushed in the switch is faulty and needs changing, quite common. hope this helps