2008 chevy pick up truck silverado po573 code cruise control brake switch open

There are some ideas in the link, but because of the Cruise Control issue, I think you need an auto electrician as things like this are controlled by ECM and BCM.

https://www.google.com/search?q=2006+chevy+silverado+brake+lights+not+working

https://www.google.com/search?q=auto+electrician+near+me

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There are several possible reasons why the 3rd brake light on your 2005 Chevy Malibu works only with the key in the accessory position and not when the car is running.

1. The brake light switch: The brake light switch is responsible for turning on the brake lights when the pedal is depressed. If the switch is malfunctioning, it may not be sending the signal to the brake light when the key is in the "on" or "start" position.
2. Electrical issue: An electrical issue with the wiring or a connection in the brake light circuit could be preventing the brake light from turning on.
3. The Body Control Module (BCM): The BCM controls various electrical functions in the car, including the brake lights. A malfunctioning BCM could prevent the brake light from turning on.
4. The Brake Light Relay: The relay could be defective and not allowing power to reach the brake light when the key is in the "on" or "start" position.
5. The code PO573 that is coming up is related to the brake light switch, it could be a symptom of the problem.
6. The cruise control not working can also be related to a problem with the brake light switch.

No fuse
The body control module (BCM) monitors the signal circuit of the cruise control switches. The BCM relays the cruise control switch status to the engine control module (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 the desired vehicle speed. The ECM uses the throttle actuator control (TAC) motor in order to maintain the vehicle speed. For further information on the TAC System, refer to Throttle Actuator Control (TAC) System Descriptionfor the 4.3L engine or Throttle Actuator Control (TAC) System Description for the 4.8L, 5.3L, 6.0L, 6.2L or 7.0L engines.

Voltage is supplied to the cruise control switch via the steering wheel control switch reference voltage circuit supplied by the BCM.

Does the light come on ?

DTC P0575: Cruise Control Switch Signal Circuit
DTC B3794 08: Cruise Control Function Request Circuit Signal Invalid
DTC B3794 61: Cruise Control Function Request Circuit Actuator Stuck
DTC B0835 00: Cruise Control System Active Indicator Circuit
The cruise control switch contains an LED that illuminates to indicate the cruise control system has been switched ON. Voltage is applied through the ON/OFF switch to the LED when the switch is pressed. The body control module (BCM) provides the ground path for the LED through a dedicated LED control circuit. Pressing the switch again opens the switch and the lamp turns OFF.
DTC P0571: Cruise Control Brake Switch Circuit
DTC P0703: Brake Switch Circuit

Your best bet , take it to a qualified repair shop .

brake switch.located on the brake pedal.

Its your brake light switch which also kills your cruise control

I've seen lots of problems with brake switches on these style pickups. Usually they stick on. That also prevents you from being able to set your cruise control, because the engine computer sees that the brakes are on, when they're really not. If you have access to a scan tool that can read codes and data, there should be a Diagnostic Trouble Code stored in the engine computer related to the brake switch. You can also verify by looking at data in the scan tool that the switch is actually staying "applied", when it really should be "released".
If you don't have a scan tool, reach under the dash by the top of the brake pedal and unplug the brake switch. If your brake lights turn off (ignition key has to be on for brake lights to work), then you've found your problem.

check cruise control fuse and check the brake lights see if brake light working because the brake light switch will deactivate cruise control when brakes is applied.if brake lights dont work either fuse blowed or switch bad.if all is good have vechicle code scan because cruise control control by PCM and vechicle speed sensor and thottle actuator control motor.

having same problem. narrowed it down to brake light switch. going down road and cruise wont set, messed with brake pedal and it will arm have even put foot under pedal and lifted a little and it would arm. have a switch coming to replace.

P0483 Cooling Fan Rationality Check Malfunction
This code is set when the Engine Coolant Temperature (ECT) sensor signal fails to rise above 15 mV (~1°F) after 2.5 minutes of operation. You probably need a new ECT sensor.

P0573 Cruise Control/Brake Switch A Circuit High
This can be caused when the brake switch is set too high. Possibly a faulty brake switch.

Code po573 is cruise control / brake switch circuit A circuit voltage high PO420 catalyst system below threshold, po573 is probably a bad brake circuit / switch problem, PO420 is a bad catalatic convertor for bank 1