P0609Control Module VSS Output "B"
The engine computer's second vehicle speed output signal ('B') to other modules is faulty or missing.
Definition
Vehicles with more than one buffered speed output use circuit 'B' to feed additional consumers such as the speedometer, cruise control, or a body module, and the PCM monitors that output for electrical faults. P0609 sets when the 'B' output is shorted, open, or reads back an implausible level. Expect symptoms in whatever system that output feeds — commonly an erratic speedometer or inoperative cruise — while engine operation stays normal; the MIL may illuminate depending on the manufacturer.
Common causes
- Short to ground or power on the VSS output 'B' wire
- Open circuit or damaged terminal between the PCM and the receiving module
- Receiving module input failure loading the circuit
- Aftermarket electronics spliced into the speed signal wire
- Faulty PCM output driver
Common misdiagnoses
- Replacing the vehicle speed sensor when the PCM's speed input reads correctly
- Replacing the instrument cluster without testing the wire from the PCM
- Replacing the ABS module for a fault on the PCM's rebroadcast circuit
- Condemning the PCM before removing aftermarket taps on the circuit
Troubleshooting steps
1.Verify the speed input PID
Road-test while watching vehicle speed on the scan tool; an accurate reading rules out the sensor side and points at the output circuit.
2.Identify what circuit 'B' feeds
Use the wiring diagram to find the module served by output 'B' so testing targets the correct wire and connector.
3.Scope the output while driving
Confirm a square-wave signal at the PCM pin that scales with speed; a flatlined or clipped waveform localizes the fault.
4.Test the harness between modules
Check the wire for continuity, short to ground, and short to power with both ends disconnected.
5.Isolate module versus wiring
Disconnect the receiving module and re-scope; if the signal returns, the receiver is at fault rather than the PCM.