P0620
Control Module VSS Output "B" Malfunction
Powertrain Transmission Control Speed Sensor Output 🟡 Moderate — Fix within a week
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What This Actually Means
In plain language — no jargon

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.

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Symptoms You May Notice
3 known symptoms for this code
Check engine light illuminated
Transmission shifting erratically or staying in one gear
Speedometer reading incorrectly or not working
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Embedded Systems Insight
What the ECU/ECM is actually computing

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.

Voltage & Parameter Thresholds

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
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Diagnostic & DIY Fix Guide
Check these in order — from cheapest to most complex
1
VSS Connector and wiring harness
Inspect the speed sensor connector for corrosion, loose pins, or damaged wires; clean or reseat as needed.
2
Vehicle Speed Sensor B
Locate the secondary speed sensor on the transmission and replace if connector is secure but signal is still missing.
3
Transmission Control Module (TCM)
Have the TCM scanned and reprogrammed at a dealer if sensors and wiring are confirmed good.