Cruise control is a valuable feature in modern vehicles, allowing drivers to maintain a set speed on the open road, enhancing comfort and fuel efficiency. However, when the cruise control system malfunctions, it can be frustrating and potentially dangerous. Diagnostic Trouble Code (DTC) P0579 indicates a cruise control-related issue, specifically pointing to a problem with the cruise control multi-function input "A" circuit range/performance. Understanding the causes, symptoms, and potential solutions for this code is crucial for proper diagnosis and repair.
The P0579 code suggests that the vehicle's Powertrain Control Module (PCM) has detected an anomaly in the electrical signals coming from the cruise control switch or its associated circuitry. This could stem from a variety of issues, ranging from a faulty switch to wiring problems or even a malfunctioning PCM.
| Component/Aspect | Description | Potential Causes |
|---|---|---|
| Definition | Cruise Control Multi-Function Input "A" Circuit Range/Performance | The PCM has detected an out-of-range or illogical signal from the cruise control switch's "A" circuit. This means the voltage or resistance values being reported are not within the expected parameters. |
| Common Symptoms | - Cruise control inoperative or intermittent. - Cruise control engages unexpectedly. - Cruise control disengages unexpectedly. - Cruise control speed fluctuates. - Cruise control indicator light illuminates or flashes. - Other related codes may be present (e.g., U codes related to communication). | |
| Potential Causes | - Faulty Cruise Control Switch: The switch itself is defective, sending incorrect signals. - Wiring Issues: Damaged, corroded, or loose wiring or connectors in the cruise control circuit. - PCM Issues: Rare, but the PCM could be misinterpreting the signals or have an internal fault. - Clock Spring Issues: The clock spring, which maintains electrical connection to the steering wheel, may be damaged. - Brake Light Switch Malfunction: Though not directly a "cruise control" component, a faulty brake light switch can interfere with cruise control operation. - Throttle Position Sensor (TPS) Issues: The PCM uses TPS data to control the engine speed; a faulty TPS can cause cruise control problems. - Speed Sensor Issues: The PCM uses speed sensor data to maintain the desired speed; a faulty speed sensor can cause cruise control problems. - Aftermarket Accessories: Improperly installed aftermarket accessories can interfere with the cruise control system. | |
| Diagnostic Steps | - Visual Inspection: Thoroughly inspect all wiring, connectors, and the cruise control switch for damage, corrosion, or loose connections. - Scan Tool Diagnosis: Use a scan tool to read and clear the P0579 code, as well as any other related codes. - Live Data Monitoring: Monitor the cruise control switch signals using a scan tool to see if the signals are within the correct range when the switch is activated. - Circuit Testing: Use a multimeter to test the continuity, voltage, and resistance of the cruise control switch circuit. - Component Testing: Test the cruise control switch itself using a multimeter to ensure it is functioning correctly. - Brake Light Switch Test: Verify the brake light switch is functioning correctly, as a faulty switch can disable cruise control. - Clock Spring Inspection: Inspect the clock spring for damage or wear. - PCM Testing: As a last resort, the PCM may need to be tested or replaced. - Check for TSBs: Check for any Technical Service Bulletins (TSBs) related to the P0579 code for your vehicle make and model. | |
| Possible Solutions | - Replace Faulty Cruise Control Switch: If the switch is defective, replacing it is the most common solution. - Repair Wiring or Connectors: Repair or replace any damaged or corroded wiring or connectors. - Replace Clock Spring: If the clock spring is damaged, it needs to be replaced. - Replace Brake Light Switch: If the brake light switch is faulty, replacing it is necessary. - PCM Reprogramming or Replacement: If the PCM is the cause, it may need to be reprogrammed or replaced. - Throttle Position Sensor (TPS) Replacement: If the TPS is faulty, replacing it is necessary. - Speed Sensor Replacement: If the speed sensor is faulty, replacing it is necessary. | |
| Severity | Moderate. While cruise control is not essential for driving, a malfunctioning system can be frustrating and potentially dangerous if it engages or disengages unexpectedly. | |
| Cost to Repair | Varies widely depending on the cause. A faulty switch might cost $50-$200 to replace, while wiring repairs could range from $50-$300. PCM replacement could be significantly more expensive, potentially exceeding $1000. | |
| Related Codes | P0578, P0580, P0581, U0100, U0101, U0121, B codes related to body control module. |
Detailed Explanations
Definition: The P0579 code indicates that the PCM has detected an issue with the signal from the cruise control multi-function input "A" circuit. The "A" circuit is a specific wire or set of wires within the cruise control system that the PCM monitors. The code is triggered when the voltage or resistance on this circuit falls outside the expected range, suggesting a problem with the switch, wiring, or PCM itself.
Common Symptoms: The most common symptom is a non-functional cruise control system. This means the system will not engage at all when the cruise control button is pressed. Other symptoms include the cruise control engaging or disengaging unexpectedly, the set speed fluctuating erratically, and the cruise control indicator light illuminating or flashing on the dashboard. In some cases, other related trouble codes may also be present, providing further clues to the underlying issue.
Potential Causes:
- Faulty Cruise Control Switch: The cruise control switch itself is often the culprit. The switch contains multiple contacts that send different signals to the PCM depending on which button is pressed. Over time, these contacts can wear out, become corroded, or break, leading to incorrect signals.
- Wiring Issues: Damaged, corroded, or loose wiring and connectors are another common cause. The wiring harness that connects the cruise control switch to the PCM is exposed to vibration, heat, and moisture, which can cause the wires to fray, break, or corrode. Loose connectors can also disrupt the electrical signal.
- PCM Issues: While less common, a malfunctioning PCM can also trigger the P0579 code. The PCM is responsible for interpreting the signals from the cruise control switch and controlling the throttle accordingly. If the PCM has an internal fault, it may misinterpret the signals or fail to properly control the throttle.
- Clock Spring Issues: The clock spring is a coiled ribbon cable located in the steering column that allows the steering wheel to rotate while maintaining electrical connections to components like the airbag, horn, and cruise control switches. If the clock spring is damaged or worn, it can disrupt the electrical signals to the cruise control system.
- Brake Light Switch Malfunction: Although not directly part of the cruise control switch, a faulty brake light switch can interfere with cruise control operation. The cruise control system is designed to disengage when the brakes are applied. If the brake light switch is malfunctioning and sending an incorrect signal to the PCM, it can cause the cruise control to disengage unexpectedly or prevent it from engaging at all.
- Throttle Position Sensor (TPS) Issues: The PCM uses TPS data to control the engine speed; a faulty TPS can cause cruise control problems because the PCM is not getting accurate information.
- Speed Sensor Issues: The PCM uses speed sensor data to maintain the desired speed; a faulty speed sensor can cause cruise control problems because the PCM is not getting accurate information.
- Aftermarket Accessories: Improperly installed aftermarket accessories, especially those that tap into the vehicle's electrical system, can sometimes interfere with the cruise control system.
Diagnostic Steps:
- Visual Inspection: Begin by visually inspecting all wiring, connectors, and the cruise control switch for any signs of damage, corrosion, or loose connections. Pay close attention to the wiring harness that connects the cruise control switch to the PCM.
- Scan Tool Diagnosis: Use a scan tool to read and clear the P0579 code, as well as any other related codes. This will help you identify any other potential issues that may be contributing to the problem.
- Live Data Monitoring: Use the scan tool to monitor the cruise control switch signals in real-time while activating the switch. This will allow you to see if the signals are within the correct range. Look for any erratic or missing signals.
- Circuit Testing: Use a multimeter to test the continuity, voltage, and resistance of the cruise control switch circuit. This will help you identify any open circuits, short circuits, or high resistance connections. Consult the vehicle's wiring diagram for the correct testing procedures.
- Component Testing: Test the cruise control switch itself using a multimeter to ensure it is functioning correctly. Refer to the vehicle's service manual for the proper testing procedure and acceptable resistance values.
- Brake Light Switch Test: Verify the brake light switch is functioning correctly by checking if the brake lights illuminate when the brake pedal is pressed. Also, use a multimeter to test the switch's continuity.
- Clock Spring Inspection: Inspect the clock spring for any signs of damage or wear. Look for cracked or broken wires, or any signs of physical damage.
- PCM Testing: PCM testing is usually performed by a qualified technician using specialized equipment. This is typically only done as a last resort if all other possible causes have been ruled out.
- Check for TSBs: Consult your vehicle's manufacturer website or a reputable online database to check for any Technical Service Bulletins (TSBs) related to the P0579 code. TSBs often provide specific diagnostic and repair procedures for known issues.
Possible Solutions:
- Replace Faulty Cruise Control Switch: If the cruise control switch is found to be defective during testing, replacing it is the most common and often the most effective solution.
- Repair Wiring or Connectors: Repair or replace any damaged or corroded wiring or connectors. This may involve splicing wires, replacing connectors, or repairing the wiring harness.
- Replace Clock Spring: If the clock spring is damaged, it needs to be replaced. This is a more complex repair that may require specialized tools and knowledge.
- Replace Brake Light Switch: If the brake light switch is found to be faulty, replacing it is necessary.
- PCM Reprogramming or Replacement: If the PCM is determined to be the cause of the problem, it may need to be reprogrammed with the latest software or replaced altogether. PCM replacement is a more expensive repair that should only be done as a last resort.
- Throttle Position Sensor (TPS) Replacement: If the TPS is faulty, replacing it is necessary.
- Speed Sensor Replacement: If the speed sensor is faulty, replacing it is necessary.
Severity: The severity of the P0579 code is considered moderate. While a malfunctioning cruise control system is not essential for operating the vehicle, it can be frustrating and potentially dangerous if it engages or disengages unexpectedly.
Cost to Repair: The cost to repair the P0579 code can vary widely depending on the cause. A faulty switch might cost $50-$200 to replace, while wiring repairs could range from $50-$300. PCM replacement could be significantly more expensive, potentially exceeding $1000.
Related Codes: The P0579 code may be accompanied by other related codes, such as P0578 (Cruise Control Multi-Function Input "A" Circuit Low), P0580 (Cruise Control Multi-Function Input "A" Circuit High), P0581 (Cruise Control Multi-Function Input "A" Circuit Open), U0100 (Lost Communication With ECM/PCM "A"), U0101 (Lost Communication With TCM), U0121 (Lost Communication With ABS Control Module), and B codes related to the body control module. These codes can provide further clues to the underlying issue and help narrow down the diagnostic process.
Frequently Asked Questions
What does the P0579 code mean? The P0579 code indicates a problem with the cruise control multi-function input "A" circuit range/performance. It means the PCM has detected an out-of-range or illogical signal from the cruise control switch's "A" circuit.
Can I still drive my car with a P0579 code? Yes, you can usually still drive your car, but the cruise control system will likely be inoperative. Be aware that the system might engage or disengage unexpectedly in some cases.
What is the most common cause of the P0579 code? The most common cause is a faulty cruise control switch or damaged wiring/connectors in the cruise control circuit.
How can I diagnose the P0579 code myself? Start with a visual inspection of the wiring and connectors, then use a scan tool to read and clear the code. Monitoring live data from the cruise control switch can also be helpful.
Is it expensive to fix a P0579 code? The cost varies depending on the cause. A faulty switch is relatively inexpensive to replace, while PCM replacement can be very costly.
Conclusion
The P0579 code signals a problem within the cruise control system, specifically related to the multi-function input "A" circuit. Thorough diagnosis, starting with visual inspection and progressing to component testing, is essential for pinpointing the root cause. Addressing the issue promptly is recommended to restore cruise control functionality and prevent potential safety concerns.