Pressure Control Solenoid "C" Open Circuit
The transmission's pressure control solenoid C has lost electrical connection, like a broken wire preventing a light switch from working. Without this signal, the transmission cannot properly regulate hydraulic pressure and shift smoothly.
The ECU monitors the solenoid coil resistance and continuity through a circuit check during key-on self-test. It expects a specific resistance range when the solenoid is de-energized and detects abnormal voltage signals when attempting to actuate the solenoid.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Solenoid Coil Resistance | 4-8 ohms (varies by model) | Open circuit (infinite resistance) or <0.5 ohms |
| Solenoid Control Voltage | 0V idle, 12V during command | No voltage change or unable to energize |
Code P1909 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, P1909 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.