P1568

What Does Code P1568 Mean?

The OBD-II diagnostic trouble code P1568, often translated as “Speed Control Unable to Hold Speed,” indicates that the Powertrain Control Module (PCM) or Engine Control Module (ECM) has detected an anomaly in the vehicle’s cruise control system. Specifically, this code signifies that while the cruise control system was engaged or attempting to engage, the actual vehicle speed significantly deviated from the driver-set target speed for a prolonged period or multiple attempts. The PCM monitors various inputs, including Vehicle Speed Sensor (VSS) data, Throttle Position Sensor (TPS) data, and brake/clutch switch statuses, to control the cruise control actuator (either vacuum or electronic). When the PCM commands the actuator to maintain a specific throttle position to hold speed, but the VSS input indicates a failure to maintain that speed, or an inability to achieve it within acceptable parameters, code P1568 is stored. This often points to a malfunction within the mechanical or electrical components directly responsible for throttle manipulation under cruise control.

Common Symptoms

• Cruise control fails to engage when activated.
• Cruise control engages but is unable to maintain the set vehicle speed, often resulting in speed fluctuations or gradual deceleration/acceleration.
• Cruise control disengages unexpectedly or intermittently while driving.
• The Malfunction Indicator Lamp (MIL), commonly known as the Check Engine Light, illuminates on the dashboard.
• No cruise control operation whatsoever.

What Causes the Code P1568?

• Faulty Cruise Control Servo/Actuator: This component, whether vacuum-operated or electronic, is directly responsible for controlling the throttle position. Internal mechanical failure, vacuum leaks (for vacuum systems), or electrical motor failure can prevent it from holding speed.
• Vacuum Leaks in Cruise Control System: For older vacuum-actuated systems, a compromised vacuum line, check valve, or a perforated diaphragm within the servo itself will prevent adequate vacuum pressure from being applied, making the system unable to hold the throttle.
• Sticking or Binding Throttle Body/Cable: Physical impediments such as a dirty, carbon-laden throttle body, a frayed or misrouted throttle cable, or a binding throttle linkage can prevent the cruise control actuator from smoothly and accurately adjusting the throttle.
• Faulty Vehicle Speed Sensor (VSS): An erratic, intermittent, or inaccurate VSS signal can cause the PCM to misinterpret the actual vehicle speed, leading to incorrect cruise control commands or an inability to regulate speed effectively.
• Faulty Brake Pedal Position (BPP) Switch or Clutch Pedal Position (CPP) Switch: These switches are critical safety interlocks. If either switch provides a constant “on” signal (indicating the brake or clutch is always depressed) or an intermittent false signal, the PCM will either disable cruise control or struggle to maintain speed as it constantly receives disengage commands.
• Wiring and Connector Issues: Open circuits, shorts to ground or power, or high resistance within the wiring harness leading to the cruise control servo, BPP switch, CPP switch, VSS, or cruise control buttons can disrupt communication and functionality.
• Faulty Cruise Control Switch Assembly: A malfunctioning set of cruise control buttons on the steering wheel or stalk can send erroneous signals to the PCM, leading to activation issues or an inability to maintain speed.

How to Diagnose and Troubleshoot

Diagnosis of P1568 requires a methodical approach, often utilizing an OBD-II scanner and a digital multimeter (DMM).

1. Initial Scan and Freeze Frame Data: Connect an OBD-II scanner to verify the presence of P1568. Check for any other related codes, particularly those related to the VSS, TPS, or brake/clutch switches. Review freeze frame data to understand the operating conditions (vehicle speed, engine RPM, load) at the moment the code was set.
2. Visual Inspection:
   • Inspect all vacuum lines connected to the cruise control servo for cracks, kinks, disconnections, or dry rot. Check the integrity of the vacuum reservoir and any one-way check valves.
   • Examine the throttle cable (if applicable) from the cruise control servo to the throttle body. Ensure it is not frayed, binding, or improperly adjusted. Verify that the throttle plate moves freely without sticking when manually actuated.
   • Check all electrical connectors associated with the cruise control system (servo, VSS, BPP switch, CPP switch, steering wheel controls) for corrosion, damage, or loose pins.
3. Brake and Clutch Switch Verification:
   • Using the OBD-II scanner’s live data function, monitor the status of the BPP switch and CPP switch (for manual transmissions). Verify that the scanner accurately reflects the pedal positions (on/off, pressed/released).
   • If live data is not available, use a DMM to test continuity or voltage across the switch terminals while actuating the pedal. The switch should show open/closed or voltage change as per the vehicle’s wiring diagram. Ensure proper adjustment of the switch plunger or sensor.
4. Vehicle Speed Sensor (VSS) Verification:
   • With the OBD-II scanner connected, monitor live VSS data while driving. Compare the scanner’s speed reading with the vehicle’s speedometer. Look for erratic readings, sudden drops, or discrepancies.
   • Inspect the VSS wiring and connector for damage. If accessible, remove the VSS and check for debris or damage to the sensor tip or its reluctor wheel.
5. Cruise Control Servo/Actuator Testing:
   • For Vacuum Systems: Disconnect the vacuum line from the servo and apply vacuum using a hand-held vacuum pump (e.g., Mityvac). The servo should hold vacuum without leakage. With vacuum applied, attempt to move the throttle cable from the servo; it should extend and retract smoothly.
   • For Electronic Systems: Consult the vehicle’s specific service manual for testing procedures. This often involves checking for power and ground at the actuator’s connector and potentially testing internal resistance, though full functional testing usually requires specialized equipment or replacement.
6. Wiring Integrity Test: Using a DMM, perform continuity checks and voltage drop tests on the wiring harness connecting the PCM to the cruise control servo, BPP switch, CPP switch, and VSS. Refer to the vehicle’s specific wiring diagrams to identify correct pinouts and expected values. Look for opens, shorts to ground or power, or excessive resistance, especially on signal lines.

Recommended Repairs and Solutions

Once the diagnostic steps have isolated the root cause, the following repairs are commonly indicated:

• Replace Faulty Cruise Control Servo/Actuator: If internal failure or vacuum leaks within the servo are confirmed, replacement is the primary solution. Ensure the replacement part is compatible with your vehicle’s specific system (vacuum vs. electronic).
• Repair or Replace Vacuum Lines and Components: If vacuum leaks are identified, replace all compromised vacuum lines, check valves, and fittings. Verify the integrity of the vacuum reservoir if present.
• Clean or Repair Throttle Body and Cable: Thoroughly clean the throttle body bore and plate to remove carbon buildup, ensuring smooth operation. If the throttle cable is frayed or binding, replace it and ensure it’s routed correctly without sharp bends. Lubricate appropriate pivot points.
• Replace Faulty Brake Pedal Position (BPP) Switch or Clutch Pedal Position (CPP) Switch: If either switch is found to be faulty or improperly adjusted, replace it. After installation, ensure correct adjustment according to manufacturer specifications to prevent future issues.
• Replace Vehicle Speed Sensor (VSS): If the VSS is determined to be providing inaccurate or intermittent signals, replace the sensor. Inspect the VSS reluctor wheel for any damage or debris that might interfere with its operation.
• Repair Wiring and Connectors: If an open, short, or high resistance is found in the wiring harness or at a connector, perform precise repairs. This may involve splicing in new sections of wire, replacing damaged terminals, or cleaning corroded connections. Always use appropriate automotive-grade connectors and heat-shrink tubing for durable repairs.
• PCM Software Update or Replacement: In very rare cases, if all other components and wiring check out, a PCM software update or even replacement might be considered. This should only be pursued after exhaustive testing of all other potential causes and typically requires dealership-level tools for programming.

Mechanic’s Tip: After any repair, always clear the DTCs with an OBD-II scanner and perform an extended test drive under conditions where the cruise control would normally be used. This ensures the repair has been successful and the code does not return. Pay close attention to vehicle speed stability once cruise control is engaged.