Transmission Transfer Case Contact Plate 'B' Open Circuit
The transmission's transfer case solenoid contact plate B has an electrical break or poor connection, like a frayed wire in a light switch that prevents current from flowing. The transmission control module can't energize the solenoid to engage or disengage the transfer case properly.
The ECM monitors the solenoid coil resistance and continuity by applying a test voltage across the contact plate B circuit. It expects a specific resistance range when energized and de-energized. An open circuit causes zero current flow, triggering a fault.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Solenoid Coil Resistance | 5-15 ohms | Infinite resistance (open circuit) |
| Solenoid Current Draw | 0.5-2.0 amps when commanded | No current detected |
Code P1851 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, P1851 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.