What does it mean?
OBD II fault code P0571 is a generic code that is universally defined as “Cruise/brake switch “A” -circuit malfunction”, and is set when the PCM (Powertrain Control Module) detects an abnormally high / low resistance or voltage in the cruise control/brake switch circuit(s). Note however that the way circuits/switches/sensors are labelled vary between applications and manufacturers.
For this reason, it is recommended that the manual for the affected application be consulted for details on exactly which circuit is labelled “A” on the cruise control system. While we need not delve into the complexities of modern cruise control systems here, it should be noted that code P0571 is not concerned with faults in the parts / components and subsystems that actually regulate and control the cruise control system.
This code is solely concerned with the circuit that links the brake -, and the cruise control switches both to each other, and to the PCM.
When the PCM detects an abnormal resistance or voltage in this circuit, it will initiate a series of self-diagnostic tests that checks the operation and condition of the entire cruise control system, and if the PCM determines that a condition exists in the brake/cruise control circuit that falls outside of pre-programmed parameters, it will disable the cruise control system if it is activated, which effectively restores control over the vehicles’ speed to the driver.
In terms of operation, once a speed is set via the cruise control switch, the PCM uses input data from several sensors and control modules to maintain the set speed without requiring any inputs from the driver. While design specifics vary between applications, the set speed can be increased /decreased by manipulating various controls on the main cruise control switch, or, the cruise control system can be deactivated by applying a momentary pressure on the brake pedal.
If the fault occurs when the cruise control system is not activated, the PCM will prevent activation of the cruise control system as a protective measure until the problem is resolved. Note however, that on some applications a code will be stored and a warning light be illuminated on the first failure cycle, while on others, up to three failure cycles need to occur before a warning light will be illuminated. In these cases, the code may be present as “pending”.
The image below shows the typical location of the brake / cruise control circuit switch. However, it should be noted that on many applications, the switch that operates the brake / taillights, is incorporated into the brake / cruise control circuit switch, while on many others, the two switches are separate units that are usually colored differently to aid identification.
It should also be noted that on many applications with manual transmissions, the brake / cruise control circuit switch might be located on the clutch pedal.