Transmission Transfer Case Disengaged Solenoid Short to Ground
The solenoid that controls the transfer case disengagement has an electrical short to ground, like a wire touching metal where it shouldn't. The ECU can't properly control the transfer case engagement because it's losing electrical signal.
The ECU monitors the solenoid coil resistance and voltage when commanding the transfer case disengagement solenoid. It detects abnormally low or zero resistance readings that indicate current flowing directly to ground instead of through the solenoid circuit.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Solenoid coil resistance | 5-50 ohms (depends on solenoid type) | Less than 2 ohms or 0 ohms (short to ground) |
| Solenoid voltage drop | 11-13V when commanded on | 0V or excessive current draw detected |
Code P1901 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, P1901 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.