Technical Description of OBD-II Trouble Code
The circuit that controls the cruise control servo is open.
What does that mean?
This is a generic diagnostic trouble code (DTC) that applies to OBD-II vehicles with cruise control. It can be found in various vehicle makes such as Dodge, Chrysler, Ram, Jeep, Hyundai, BMW, Vauxhall, Mini, Chevrolet, etc.
The cruise control feature offers several benefits. It allows you to maintain a constant speed on long trips without constantly monitoring and adjusting your speed according to traffic and traffic laws. By pressing a button or two, a combination of sensors, solenoids, and modules ensure that the vehicle maintains the specified speed until you deactivate or adjust it. Additionally, the cruise control system helps optimize fuel economy during the trip, as we often don’t consider all factors that can affect costs while driving.
The ECM (engine control module) is responsible for the smooth and efficient operation of the cruise control system. It relies on various sensors, switches, modules, and the vehicle speed sensor (VSS). One important component is the cruise control servo, which controls the throttle when you set or adjust the cruise control. By controlling the throttle, the ECM can easily and efficiently adjust the speed. These servos are often mechanically controlled through vacuum lines and throttle cables. The vacuum to the servo is controlled by solenoids, which are in turn controlled by the ECM. The ECM interprets various values before specifying the throttle position and speed. This fault refers to the circuit involved, so consult the service manual to determine the specific physical circuit you are diagnosing.
The P0594 Cruise Control Servo Control Circuit /Open and related codes (P0525, P0595, and P0596) are set when the ECM detects a malfunction in the cruise control servo control circuit. In the case of the P0594, it is set when a general malfunction or open electrical condition is detected in the circuit.
Example of a cruise control servo:
What is the severity of this DTC?
I classify the severity of this code as medium. While most cruise control OBD2 fault codes are relatively minor, this one is more severe because the cruise control servo is directly connected to the throttle control via a cable and is responsible for throttle position at times.
It is important to note that neglecting any active OBD2 fault can lead to not being aware of new fault codes. Even if there are no symptoms, leaving OBD2 fault codes unattended can result in more serious and expensive problems.
What are some of the symptoms of the code?
Symptoms of a P0594 diagnostic code may include:
What are some of the common causes of the code?
Possible causes for this P0594 cruise control code include:
What are some P0594 diagnostic and troubleshooting steps?
The first step in troubleshooting any malfunction is to research the Technical Service Bulletins (TSB) for known issues with the specific vehicle.
Advanced diagnostic steps are highly vehicle-specific and may require specialized equipment and knowledge for accurate performance. The following are basic steps, but refer to a vehicle year/make/model/powertrain specific repair guide for precise steps for your vehicle.
Basic Step #1
In most cases, cruise control servos are located in the engine compartment, usually near the throttle body since they control vehicle speed through it. Once located, perform a general visual inspection. Check the cable for fraying, kinks, corrosion, or any signs of malfunction. If the servo cable does not move freely when controlling the throttle, the ECM may interpret it as a performance issue.
Inspect the physical condition of the servo, as it is usually vacuum-controlled. Check all the vacuum lines connected to the servo. Replace any cracked or deteriorated lines to ensure the health of your vacuum system, as this can prevent future problems. Vacuum lines are inexpensive and can save you from potential issues.
Basic Step #2
On the electrical side, verify the connector on the servo. Repair any broken tabs or loose connections. Pay attention to any areas where the harness runs through tight spaces or moving parts, as chafing can cause issues. Repair any broken or damaged wires.
Basic Step #3
If your OBD2 scan tool allows it, you can potentially monitor the servo’s operation while it is connected to the vehicle. You may receive percentages or electrical values to compare with its physical operation under the hood with the wheels off the ground. However, this procedure is outdated and may cause issues, especially with modern vehicles. Always ensure that any diagnosing or repair procedures are recommended by your vehicle’s manufacturer. Neglecting to do proper research can lead to damage to other vehicle components.
