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P codepowertrainMedium severity

P0608Control Module VSS Output "A"

The engine computer's vehicle speed output signal 'A' to other modules is faulty or missing.

Definition

On many vehicles the PCM receives the raw vehicle speed sensor signal and rebroadcasts a buffered speed output to other consumers such as the instrument cluster, ABS module, or speed-control system. P0608 sets when the PCM detects a fault on that VSS output 'A' circuit — typically a short, open, or implausible feedback. Symptoms can include an erratic or dead speedometer, inoperative cruise control, or odd shift behavior, and the MIL may illuminate. The engine itself usually runs normally.

Common causes

  • Short to ground or to power on the VSS output wire between the PCM and the receiving module
  • Open circuit or corroded connector terminal on the speed output circuit
  • Failed input circuit in the receiving module (cluster, ABS, or cruise module) loading down the signal
  • Aftermarket accessories (alarms, remote starts, trailer modules) tapped into the speed signal wire
  • Faulty PCM output driver

Common misdiagnoses

  • Replacing the vehicle speed sensor when the input signal is fine and the fault is on the output side
  • Replacing the instrument cluster before testing the signal wire from the PCM
  • Replacing the transmission output speed sensor for a code that concerns the PCM's rebroadcast circuit
  • Condemning the PCM without disconnecting aftermarket devices spliced into the speed wire

Troubleshooting steps

  1. 1.Confirm the PCM is receiving a good speed input

    Watch vehicle speed on the scan tool during a road test; a clean, accurate PID means the sensor and input circuit are good and the fault is on the output.

  2. 2.Scope the VSS output wire

    Backprobe the output circuit at the PCM while driving; you should see a clean square wave whose frequency rises with speed.

  3. 3.Check the circuit for shorts and opens

    With connectors disconnected, verify continuity from the PCM output pin to the receiving module and confirm no short to ground or voltage.

  4. 4.Isolate added accessories

    Unplug any alarm, remote start, or trailer brake module spliced into the speed signal and retest, since these commonly drag the circuit down.

  5. 5.Substitute the receiving module or PCM as a last step

    If the wire is proven good, disconnect the receiving module to see whether the signal returns before condemning the PCM driver.

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