Gear Shift Module Communication Circuit Low
The transmission control module isn't receiving a clear signal from the gear shift module, like a phone line with poor connection. This prevents proper communication between components that control gear selection and shifting.
The ECM monitors the voltage signal from the Gear Shift Module (GSM) communication circuit. It expects a steady voltage within a normal operating range. When voltage drops below the minimum threshold, the ECM detects a circuit low condition and triggers the fault code.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| GSM Signal Voltage | 4.5V - 5.5V | Below 3.0V or open circuit |
| Signal Response Time | 50-200ms response | No response or timeout |
Code P0861 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, P0861 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.