Error Code P0608 is defined as Control Module VSS Output “A” Malfunction, meaning there’s a malfunction in the speed sensor A control module of the vehicle, often caused by a faulty speed sensor or shorted electrical wires.
This code is a generic trouble code, meaning it applies to all vehicles equipped with the OBD-II system, especially those made since 1996 up to present. It is more common among Chevrolet, Citroen, Dodge, Ducati, GMC, Peugeot, and Ram. Specifications on the definition, troubleshooting, and repairs, of course, vary from one make and model to another.
When Error Code P0608 is stored, that means the PCM (powertrain control module, also known as ECM or engine control module in other vehicle makes) has detected a malfunction in a specific VSS (vehicle speed sensor) output voltage signal.
If this is the case, a VSS output signal is expected to come from either the PCM itself or one of the many (other) onboard controllers. This particular VSS output circuit has been given the designation “A”. Consult a source of reliable vehicle information for the specific function of this circuit as it pertains to the vehicle in question.
Vehicles with OBD-II diagnostic system can use as many as seventy onboard controllers. These controllers communicate through the CAN (controller area network).
VSS is used (electromagnetic sensor) to complete a circuit that provides one or more controllers with an accurate signal that reflects the vehicle speed. Usually, VSS is positioned near the output shaft of the transmission (or 4WD transfer case). In some cases, it’s in the rear differential. A toothed, steel reluctor (permanently affixed to the output shaft or ring gear) passes close to the electromagnetic sensor and completes the circuit. The gaps between teeth result in circuit interruptions. The PCM along with other controllers, receive these circuit completions and interruptions as a waveform pattern of voltage. This wave pattern frequency is read as the speed of the vehicle.
VSS signal is usually input for a primary controller, like the PCM or TCM (transmission control module). As soon as the primary controller gets the VSS input, it’s automatically shared with other controllers through the CAN. If any of these secondary controllers fails to receive the VSS signal, or gets inconsistent VSS signal, then the Error Code P0608 will be stored.
Other modules that could also detect and store this code include the TCM, BCM (body control module), turbo control module, fuel injection control module, an anti-theft module, and antilock brake control module.
As with other error codes, this code registers to the memory system and activates the Check Engine light. In some cases where the code is pending, the Check Engine light will not illuminate yet. However, drivers may notice drivability issues with the vehicle when the code is present.
Other common symptoms include:
- Erratic or incorrect odometer/speedometer
- Abnormal transmission shifting (for automatic vehicles)
- Difficulty in shifting
- Poor acceleration
- Rough idling
Others codes that refer to the Output shaft speed sensor, input speed sensor, ABS or TCS (traction control system) codes may also be present.
Common causes of this code include:
- Defective speed sensor
- Open or shorted VSS (vehicle speed sensor) harness
- Broken, corroded, or loose electrical connectors
- Defective CAN bus
- Poor electrical connection in VSS
- Defective PCM, TCM or another controller
- Controller programming error
How to Check
If there are other VSS related codes stored in the vehicle, then they must be addressed first before attempting to diagnose P0608.
As with most codes, diagnosing this code requires a diagnostic scanner, a DVOM (digital volt/ohmmeter), and dependable vehicle information from TSB (Technical Service Bulletin). The information from the TSB such as diagnostic flow chart, connector pin-out charts, wiring diagrams, connector face views, and component testing procedures and specifications are all important.
Conduct a visual inspection of the controller connectors and visible CAN harness. Then, connect the scanner to the vehicle diagnostic port and get all stored data. If freeze frame data is available, get it too. Take note of all this information, as it can be very important for the diagnosis. Then, clear the codes and then take the vehicle for a attest drive to see if the code comes back, or the PCM enters readiness mode.
If the PCM gets into readiness mode without any codes stored, then you have an intermittent condition, which can get worse before you can have an accurate diagnosis.
If the code resets, then continue with your diagnosis.
Before you use a DVOM to test for continuity and circuit resistance, make sure you disconnect all related controllers. Failure to do this can result to controller damage.
With Key On Engine Off, test the individual VSS output circuit at the appropriate controller, use the DVOM. Then, compare your results to the VSS signal data. If you get no unacceptable output signal, then you can suspect a controller failure.
If there is an acceptable VSS output signal, then use the DVOM to test the individual circuits between each of the related controllers. Repair or replace system circuits or components are required, and then retest the VSS signal.
- Error Code P0609 (without other codes stored) will not be a result of a defective VSS
- In most cases, controller failure is highly unlikely. For this code, however, it’s more likely that the culprit of the problem is a failed controller or a programming error.
How to Fix
Common repairs for this code include:
- Repair or replacement of bad connections, shorted or open wires, or corrosion
- Replacement of defective VSS
- Replacement of defective PCM
This code can be a serious problem, as it can result in drivability issues. Thus, it’s important to have it fixed as soon as possible.