|Trouble Code||Fault Location||Probable Cause|
|P0571||Cruise/brake switch A -circuit malfunction||Wiring, cruise/brake switch, ECM|
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What Does Code P0571 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.
What are the common causes of code P0571 ?
Common causes of code P0571 could include the following-
- Damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors. Note that liquid spills on the steering wheel often cause short circuits in the cruise control and other switches located in the steering wheel
- Defective brake / cruise control switch
- Blown fuses. Note that blown fuses can be either a cause, or a symptom of P0571
- Failed or failing PCM. Note that this is a rare event, and the fault must be sought elsewhere before any controller is replaced
What are the symptoms of code P0571 ?
Common symptoms of code P0571 could include the following-
- Stored trouble code and an illuminated warning light. Note however that on some applications, warning lights are not illuminated for cruise control related faults, while on others, a warning light may only be triggered after several failure cycles had occurred
- On most applications, the cruise control system will be deactivated by the PCM when this code is present
- Cruise control related fuses might be blown. Note that blown fuses could be either a symptom, or a cause of this code
- Several other cruise control related codes could be present along with P0571
- Note that code P0571 rarely, if ever, produces driveability issues
How do you troubleshoot code P0571 ?
NOTE: While it is possible to test fused circuits with a digital multimeter, performing this step with a dedicated fuse tester is often the safest way of doing it, since using this type of tester eliminates the possibility of causing accidental short circuits in confined fuse boxes. Refer to the image below for an example of a typical fuse tester that can test both continuity and resistance in a fused circuit. Note that test lights are useless in this context, since they cannot measure resistance.
Record all fault codes present, as well as all available freeze frame data. This information can be of use should an intermittent fault be diagnosed later on.
NOTE: Pay particular attention to any additional codes that may be present. On many applications, several unrelated circuits share a particular fuse, and it could happen that several circuits are disabled when a cruise control related fuse blows. Thus, if any additional codes are present that do not seem to be related to the cruise control system, checking for blown fuses should be the first step in diagnosing code P0571.
Assuming that no additional codes are present, refer to the manual for the application to locate all fuse boxes, and in particular, the fuse(s) that relate to the cruise control system. If any fuses are blown, do NOT replace any without first testing the circuits and switches for short circuits and other problems. Make repairs, or replace wiring as required to ensure that the problem is resolved definitively before replacing blown fuses.
If no fuses are blown, refer to the manual to locate, and identify the brake/cruise control switch. Note that gaining full access to this switch may require partial disassembly of the dashboard, and/or the removal of one or more trim panels. Be sure to follow the directions provided in the manual exactly to prevent causing damage to trim panels or their fasteners.
When the correct switch is fully accessible, inspect the switch itself and its electrical connector for signs of damage. Failure of the switch is a common cause of this code, so even if it appears to be undamaged, remove it from the vehicle for testing as per the instructions provided in the manual.
These switches are usually relatively simple “ON/OFF” switches, and testing one involves nothing more complicated than checking that a current passes through it, and that it interrupts a current when it is acted upon. However, when the switch passes a current, its resistance must conform to a specified value, so replace the switch if its resistance does not agree with the value stated in the manual.
If the switch checks out, perform a thorough visual inspection of all associated, wiring. Look for damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors. Repair or replace wiring as required.
Clear all codes after repairs are complete, and operate the vehicle at some length with the cruise control activated before scanning the system again to see if any codes return. Note however that if the fault is still present, it may not be possible to activate the cruise control system.
If no visible damage to wiring is found, perform resistance, ground connectivity (where applicable), and continuity tests on all associated wiring, but be sure to disconnect all relevant wiring from the PCM to prevent damage to the controller.
Compare all obtained readings with the values stated in the manual, and make repairs, or replace wiring as required to ensure that all electrical values conform to the ranges specified by the manufacturer.
If all electrical values check out, suspect a faulty cruise control switch, rather than a defective PCM. However, if the cruise control switch is located on the steering wheel itself, or if removing/working on the cruise control switch requires even partial disassembly of the steering wheel, take extreme care when attempting to diagnose open or short circuits in the cruise control switch.
The best course of action in these cases is to disable the steering wheel airbag(s) before doing anything since one mistake can accidentally deploy the airbag, which can cause serious personal injury. Refer to the manual for the application for full details on the correct procedure to follow to disable the airbag, but do NOT disconnect the battery unless the manual explicitly states that the battery must be disconnected at this point.
Also, be sure to install an approved memory saving device to keep critical systems powered up when battery power is removed.
Consult the manual on the correct procedure to follow to test the cruise control switch, but be sure to follow the directions EXACTLY in order to obtain accurate and reliable test results. Bear in mind that cruise control switches are NOT repairable, which means that the switch must be replaced if it fails any test at any point during the test procedure.
Reassemble the steering wheel after all tests / repairs are complete, clear all codes, and operate the vehicle at length with the cruise control system activated to see if any codes return. In the event that the code does not return, be sure to reactivate the airbag, and to clear the airbag related codes that may have set when the airbag was deactivated.
If the code does return, suspect either an intermittent fault, or a defective PCM or other control module. If this is suspected, the wiser option is to refer the vehicle to the dealer or other competent repair shop for professional diagnosis and repair, since generic code readers can generally not diagnose defects or programming errors in control modules.
Codes Related to P0571
- P0572 – Relates to “Cruise Control/Brake Switch “A” Circuit Low”
- P0573 – Relates to “Cruise Control/Brake Switch “A” Circuit High”