P0649 OBD-II Trouble Code: Speed Control Lamp Control Circuit

P0649 code definition

A P0649 trouble code indicates that the Powertrain Control Module (PCM) or one of the other control modules has detected a problem with the speed control lamp circuit. In addition to the PCM, other control modules that might detect this fault include the alternative fuel control module, anti-theft control module, anti-lock brake control module, body control module, climate control module, cruise control module, fuel injection control module, instrument panel control module, proximity alert module, traction control module, and turbo control module. Some related codes include the P0648 and P0650 trouble codes.

What the P0649 code means

When a problem with the speed control lamp circuit is detected, a code is stored on the PCM and the Check Engine Light illuminates. Some vehicle models require multiple drive cycles, up to eight, with a failure before the Check Engine Light illuminates.

What causes the P0649 code?

Some common causes of a P0649 trouble code include:

• A faulty speed control lamp bulb or circuit
• A defective CAN Bus
• A loose/broken control module ground strap/wire
• Shorted, corroded, or damaged wiring and connectors in the Controller Area Network (CAN) Bus harness

What are the symptoms of the P0649 code?

In addition to a stored P0649 trouble code, symptoms of this code include an illuminated or flashing speed control lamp, a speed control lamp that fails to light, and an illuminated Check Engine Light.

How does a mechanic diagnose the P0649 code?

To diagnose a P0649 trouble code, a mechanic needs an OBD-II scanner or code reader, digital volt/ohmmeter, a specialized diagnostic CAN scanner, a CAN Bus system wiring diagram, and an auxiliary ground cable. In addition, the mechanic must perform the following steps:

• The mechanic should start by carefully checking the wiring, connectors, and components for damage or fault.
• Clear the trouble code and retest the system to see if the code persists. The mechanic should also test drive the vehicle at this point.
• If the code returns, the mechanic should use a specialized scanner to check the pins of the CAN Bus harness. The mechanic should first install a memory saving device to prevent control module memory loss.
• Operate the vehicle while testing the CAN Bus harness pins to more accurately determine where the problem lies.
• Using the digital volt/ohmmeter, test the control module system circuitry, connectors, and fuses for continuity, using an auxiliary ground cable to help with diagnosis.
• With the CAN Bus system wiring diagram in hand, the mechanic should check the continuity between each individual control module connectors, comparing their findings with the manufacturer's specifications.
• Using the digital volt/ohmmeter, check the speed lamp control circuit for continuity. The mechanic should also inspect the speed control lamp bulb to make sure it is not bad.
• Clear the trouble code and retest the system.

Common mistakes when diagnosing the P0649 code

Mechanics often make the mistake of repairing stored trouble codes that are present in response to a P0649 trouble code as opposed to the real issue. This leads to an unsuccessful repair and the trouble code returning.

How serious is the P0649 code?

Usually the worst that happens when this trouble code is stored are an illuminated or non-working speed control lamp, as well as a lit Check Engine Light.

What repairs can fix the P0649 code?

• Repairing a P0649 trouble code requires a mechanic to perform the following repairs:
• Replace any damaged or faulty wiring, connectors, or components.
• Replace any blown fuses. The mechanic should also test the system to see what is making the fuses blow in the first place and repair that as well.
• Replace any defective wiring within the CAN Bus harness, though it is sometimes easier to replace a section of defective wiring completely as opposed to replacing individual wires and connectors.
• Replace the speed control lamp bulb if blown. If the speed control lamp circuit is at fault, replace that component.
• The mechanic should perform an inspection to ensure that all engine and transmission ground cables, straps, and wires are properly connected.

Need help with a P0649 code?

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