Technical Description of OBD-II Trouble Code
Circuit for Controlling the Speed Control Lamp
What does that signify?
This is a generic diagnostic trouble code (DTC) that is applicable to many OBD-II vehicles (1996-newer). This includes vehicles from various manufacturers such as Ford, Chevrolet, Chrysler, Dodge, Ford, Jeep, Hyundai, Honda, etc. Although it is a generic code, the specific repair steps may vary depending on the year, make, model, and powertrain configuration of the vehicle.
A stored code P0649 indicates that the powertrain control module (PCM) has detected a malfunction in the circuit that controls the speed control lamp.
The speed control lamp is located in the instrument panel. Its main purpose is to notify the driver when the speed control system, also known as the cruise control, has been activated (when it is illuminated). The speed control system uses input signals from the vehicle speed sensor (VSS) and/or wheel speed sensors to automatically regulate the vehicle speed after it has been activated by the driver.
Every time the ignition is turned on and the PCM is powered, multiple self-tests are performed by the controller. Along with running internal self-tests, the controller area network (CAN) transmits serial data from each individual module to ensure proper communication between on-board controllers.
If a problem is detected in monitoring the circuit that controls the speed control lamp, a code P0649 will be stored and a malfunction indicator lamp (MIL) may illuminate.
One type of cruise control lamp indicator:
What is the severity of this DTC?
All internal control module codes should be taken seriously. A stored code P0649 (with speed control lamp illumination) will likely result in the loss of speed control activation.
What are some of the symptoms of the code?
Symptoms of a P0649 trouble code may include:
What are some of the common causes of the code?
Possible causes for this code may include:
What are some P0649 troubleshooting steps?
To diagnose a code P0649, you will need a diagnostic scanner, a digital volt/ohmmeter (DVOM), and a reliable source of vehicle information.
Refer to your vehicle information source for technical service bulletins (TSB) that match the stored code, vehicle details (year, make, model, and engine), and exhibited symptoms. If you find the relevant TSB, it may provide helpful diagnostic information.
Start by connecting the scanner to the vehicle diagnostic port and retrieving all stored codes and freeze frame data. It is important to write down this information in case the code is intermittent. After recording all relevant information, clear the codes and test drive the vehicle until the code is reset or the PCM enters readiness mode.
If the PCM enters readiness mode, the code is intermittent and will be more challenging to diagnose. The condition that caused the P0649 to be stored may need to worsen before an accurate diagnosis can be made. If the code is reset, continue with the diagnosis.
Use your source of vehicle information to obtain connector face views, connector pin-out charts, component locators, wiring diagrams, and diagnostic flow charts related to the code and the specific vehicle.
Check if there is battery voltage on the circuit for the speed control warning lamp using the appropriate wiring diagram and your DVOM. If there is no voltage, check the system fuses and relays and replace any defective parts as necessary. If voltage is present at the speed control warning lamp, suspect a defective speed control warning lamp bulb.
If the speed control warning lamp bulb is functioning properly and the P0649 code continues to reset, use the DVOM to test the power supply fuses and relays for the controller. Replace any blown fuses as necessary. Fuses should be tested with the circuit loaded.
If all fuses and relays appear to be functioning properly, visually inspect the wiring and harnesses related to the controller. Also, check the chassis and engine ground junctions. Use your vehicle information source to find the ground locations for the relevant circuits.
Inspect the system controllers for any signs of water, heat, or collision damage. Any damaged controller, especially due to water, should be considered defective.
If the controller power and ground circuits are intact, suspect a defective controller or a controller programming error. Controller replacement will require reprogramming. In some cases, reprogrammed controllers may be available through aftermarket sources. Other vehicles/controllers may require on-board reprogramming that can only be done by a dealership or another qualified source.
