P0649 - OBD Trouble Code

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OBD II trouble code P0649 is technically defined as Speed Control Lamp Control Circuit. It indicates that a fault has been detected in the speed control lamp control circuit. The Powertrain Control Module (PCM) monitors the Speed Control Lamp Control Circuit. The PCM sets the OBDII code when the Speed Control Lamp Control Circuit is not to factory specifications. When a problem with the speed control lamp circuit is detected, a code is stored on the PCM and the Check Engine Light illuminates.

Error Code P0649 means that the PCM has detected a problem in the speed control lamp circuit, likely caused by a faulty speed control lamp bulb or circuit, or faulty wirings. This is a generic diagnostic trouble code (DTC) and applies to many OBD-II vehicles. Primary function of Speed Control Lamp is to alert the driver that the speed control system has been activated (when it is illuminated). The speed control system is also called the cruise control. The speed control system uses input signals from the vehicle speed sensor (VSS) and/or wheel speed sensors to regulate vehicle speed automatically (after the system is activated by the driver).

If a problem is detected in monitoring the speed control lamp control circuit, a code P0649 will be stored and a malfunction indicator lamp (MIL) may be illuminated.

What causes this problem with the Speed Control Lamp Control Circuit?

• Defective speed control lamp bulb
• Faulty Speed Control Lamp Circuit
• Corroded, open or shorted wiring or connectors
• Loose or broken control module ground strap or ground wire
• Defective CAN bus
• Faulty PCM

1. Speed Control Lamp Bulb - It is a bulb that illuminates and indicate when this trouble code appears, if it is inoperative or faulty can create confusions. So, always have a check on this and replace the faulty speed control lamp bulb immediately.
2. Fuses - Automotive fuses are a class of fuses used to protect the wiring and electrical equipment for vehicles. Fuses are found in your car as they protect car wires from too much current flowing through. If they were not there the wire would heat up and the insulation would melt, then the wire would melt and you would have an open circuit. Always replace the blown fuses to avoid any such situation.
3. PCM - The Powertrain Control Module PCM is an on-board electronic control unit that primarily controls the charging, emission and transmission systems of the vehicle. It works in conjunction with a network of sensors, using their inputs to control the functioning of the engine and powertrain. A Faulty PCM often lead to different trouble codes so repair or replace the faulty one in no time.

Common symptoms of this code include an illuminated or flashing speed control lamp and an illuminated Check Engine Light. Other symptoms may include:

• Non-functional speed control system
• Other stored codes

Note: If this code is stored, and a service engine soon lamp has not yet been illuminated, the code may be shown as pending.

Repairing a P0649 trouble code requires a mechanic to perform the following repairs:

• Replacement of faulty components, connectors, and wirings.
• Replacement of defective CAN bus harness or the whole harness.
• Replacement of faulty control modules (including PCM).
• Replacement of blown speed control lamp bulb.
• Replacement of faulty speed control lamp circuit.
• Replacement or reprogramming of PCM.
• Repair or replacement of engine and transmission straps, wires, and ground cables that are not properly connected.

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Trouble code P0649 should be diagnosed and rectified at earliest to avoid any major issues in near future. Below mentioned are some common steps to diagnose this code:

• Begin with a visual inspection of all wiring and connectors.
• Repair or replace damaged, disconnected, shorted, or corroded wiring, connectors, and components as necessary.
• Always retest the system after repairs are completed to ensure success. If all system wiring, connectors, and components (Including fuses) appear to be in normal working order, connect the scanner (or code reader) to the diagnostic connector and record all stored codes and freeze frame data.
• Continue by clearing the code and operating the vehicle to see if it returns.
• This will help to determine whether or not the malfunction is intermittent.
• After the codes are cleared, test drive the vehicle to see if the code returns.
• If the code fails to immediately return, you may have an intermittent condition.
• Replace blown fuses as required. Fuses should be tested with the circuit loaded.
• Visually inspect controller related wiring and harnesses is in order.
• Visually inspect system controllers for signs of water, heat, or collision damage.
• If controller power and ground circuits are intact, suspect a defective controller or a controller programming error.

Common Mistakes When Diagnosing the P0649 Code

Symptoms and stored codes that are present as a reaction to a communication failure are often misdiagnosed and repaired as the cause of the problem. Obviously, this leads to an unsuccessful repair.

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