Control Module VSS Output "B" Malfunction
Your car's computer can't properly read the vehicle speed signal from the secondary speed sensor, similar to a radio losing one of its antenna signals. This causes the transmission control module to lose critical feedback about how fast the car is moving.
The ECM monitors the Vehicle Speed Sensor output signal, which provides real-time transmission speed data. It checks for proper signal frequency and voltage levels within expected ranges. If the signal is absent, below threshold, or erratic, the fault is triggered.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| VSS Output Frequency | Proportional to vehicle speed (typically 0-8000 Hz) | Signal missing, inverted, or consistently below minimum threshold |
| VSS Signal Voltage | 0.2V to 4.8V (varies by sensor type) | Stuck low, stuck high, or no signal detected |
Code P0620 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.
Once the fault is repaired, P0620 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.