Error Code P0649 is defined as Speed Control Lamp Control Circuit. This means 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 trouble code, meaning it applies to all vehicles equipped with the OBD-II system. This includes vehicle models from Chevrolet, Chrysler, Dodge, Ford, Jeep, Honda, Hyundai, Kia, etc. Specifications on the definition, troubleshooting, and repairs, of course, vary from make, model, and powertrain configuration.
Error Code P0649 means the PCM (powertrain control module, also known as ECM or engine control module in other vehicle makes), has detected a malfunction in the speed control lamp control circuit.
Contained in the instrument panel, the job of the speed control lamp is to alert drivers when the speed control system has been activated (when it lights up). The speed control system is also sometimes called cruise control.
The PCM usually monitors the continuity of the speed control lamp control circuit every time the ignition is turned on. Speed Control system uses input signals from the VSS (vehicle speed sensor) and wheel speed sensor to automatically regulate the vehicle (after the driver activates it).
Every time the ignition is turned on and the PCM is energized, various controller self-tests are performed simultaneously. Aside from running internal controller self-test, the CAN (controller area network) carries serial data from each individual module to make sure the onboard controllers are interfacing properly.
When a problem in monitoring the speed control lamp control circuit, then Error Code P0649 will be stored and Check Engine light will be activated.
This code renders the speed control system inoperative. Other stored (speed codes) code may be present as well.
There are many possible causes for this code, such as:
- Defective or programming error in PCM
- Open or shorted immobilizer lamp control circuit
- Failed immobilizer lamp bulb
How to Check
Checking this code requires a DVOM (digital volt ohmmeter), reliable vehicle information for TSB (technical service bulletin) that has the code stored, vehicle (year, make, model, engine), and exhibited symptoms. Finding the right TSB can lead you to more easy and successful diagnostic and repair procedure.
Start by connecting the scanner to the vehicle diagnostic port. Retrieve all the stored data including freeze frame data. Write down this information. Then, clear the codes and then take the vehicle for a test drive until the code reset, or the PCM enter readiness mode.
If the PCM enters readiness mode, that means the problem is intermittent. You may have to wait for the problem to get worse before you can successfully diagnose and repair. If the code resets on the other hand, then continue with the diagnosis.
Use the vehicle information to know the connector pin-out charts, face views, component locators, wiring diagrams, and diagnostic flow charts of the vehicle.
Use the diagram and DVOM to check the voltage of the battery on the speed control warning lamp circuit. If there’s none, check the system fuses and relays. Replace the defective parts as necessary.
If there is a voltage at the speed control warning lamp, then you can suspect a defective speed control warning lamp bulb.
If the speed control warning lamp bulb is working properly, and this code still resets, then use the DVOM to test the controller power supply fuses and relays. Replace as needed; fuses must be tested with the circuit loaded.
If all fuses and relays look good, then inspect the controller related wiring and harness. Check the chassis and engine ground junctions as well. Use your vehicle information source to locate ground for related circuits.
Also, check the system controllers; look for signs of damage, water intrusion, heat, or collision damage. Damaged controllers, especially those damaged by water, is considered defective and must be replaced.
If the controller power and ground circuits are in good shape, then you can suspect a defective controller or a programming error. In which case, controller replacement will be required, as well as reprogramming.
In some cases, you may want to purchase a re-programmed controller from aftermarket sources. Other vehicles/controllers require on-board reprogramming that may be done through a dealership, or by a qualified technician.
How to Fix
- 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
If the speed control lamp doesn’t light up during KOEO (key on engine off), then you can suspect a defective speed control warning lamp bulb.
It’s important to test the integrity of the controller ground by connecting its negative test lead of the DVOM to ground, and a positive test leads to the battery voltage.