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Error Code P0594 is defined as Cruise Control Servo Control Circuit/Open, meaning there’s an ongoing malfunction in the electrical components of the cruise control servo circuit, a fault likely caused by blown fuses or liquid spills.
This error code is a generic trouble code, meaning it applies to all vehicles equipped with the OBD-II system. It’s particularly common among certain brands like BMW, Chevrolet, Chrysler, Dodge, Hyundai, Jeep, Ram, Vauxhall, etc. Specifications on the definition, troubleshooting, and repairs, of course, vary from one make and/model to another, and powertrain type.
Cruise control system has lots of uses; for long trips, drivers are given the convenience to not have to monitor and adjust speed to traffic and traffic laws consistently. There are buttons(s) and a combination of sensors, modules, solenoids, etc. that will ensure the vehicle is moving at a specific speed for easy and safe driving. This feature helps greatly in terms of fuel economy.
The PCM (powertrain control module, also known as ECM or engine control module in other vehicle makes) is responsible for the efficient operation of the cruise control system. The proper function of the system relies on many factors, such as switches, sensors, modules, VSS, etc. One of which is the cruise control servo; which is usually responsible for adjusting the throttle when the cruise control is set and adjusted. By controlling the throttle and adjusting accordingly, with this servo, the PCM can adjust the speed easily and efficiently. In many cases, the servo is controlled by the PCM, which can interpret every other value before specifying throttle position, speed, etc.
Error Code P0594 refers to the circuit involved. Thus, it’s important to refer to the service manual to determine what type of physical circuit the vehicle has and its specific diagnosis.
Error Code P0594 is set when the PCM monitors a malfunction within the cruise control servo control circuit. It’s a general malfunction, or there’s an open electrical condition problem determined in the said circuit.
Other related error codes include:
- Error Code P0525
- Error Code P0595
- Error Code P0596
As with other cruise control related error codes, cruise control function is inoperable if this condition is present. Other common symptoms include:
- Erratic cruise control operation
- Stuck vehicle speed
- Vehicle speed limit with cruise control active
- Inaccurate vehicle speed setting (compared to desired or set speed)
- Other functions of cruise control are erratic
The most common cause for this code is failed cruise control servo control unit, as it results to abnormal voltage or resistance levels. This is particularly common in older vehicle models that integrate the servo with cruise control mode.
Other possible causes include:
- Shorts or open circuit in cruise control button
- Short or open wiring
- Dirt, debris buildup, liquid spills on cruise control components
- Damaged connectors in the cruise control system
- Blown fuses (may indicate a more serious underlying problem)
How to Check
As with most codes, the best thing to do before the diagnosis is to check with the TSB (technical service bulletin) for known issues with the specific vehicle. Some vehicles may require advanced diagnostic steps and special tools, which differs from one type of vehicle to another. Here are some basic steps for diagnosis.
The cruise control servo is usually mounted in the somewhere in the engine compartment. Usually, they’re in close to the throttle body, as they control vehicle speed. After locating the component, check it thoroughly. Pay close attention to the cable and look for any signs of corrosion, fraying, kinks, or any signs or malfunctions. If the servo cable doesn’t freely move when controlling the throttle, the PCM could see it as a problem.
Also, check the physical condition of the servo itself, as it could be a tell-tale sign of a problem. Make sure you check all the vacuum lines that run to the servo. Any cracks or deterioration in the lines should be a tell-tale sign of the state of the vacuum system. Thus, it’s highly recommended to replace vacuum lines. They’re pretty inexpensive and could save you from further problems in the future.
Concerning the electrics, check the connector on the servo. Repair any broken tabs or loose connections. The harness could be running through tight areas and moving parts, so look for any chafing areas that could cause a problem. Repair or replace broken or damaged wires.
Depending on the capabilities of your OBD-II scanner, you can monitor the operation of the servo through it while connected to the vehicle. It can display percentage or electrical values. Either way, you can use these values to check the physical operation by watching it under the hood, with wheels off the ground. Note that this is procedure usually applies to older vehicles.
How to Fix
Depending on your diagnosis, common repairs for this code include:
- Replacement of vacuum supply hoses
- Repair or replacement of broken wires
- Replacement of cruise control switches
Make sure the vehicle’s manufacturer advises any procedure you do in your diagnosis. Thus, make sure you research thoroughly.