Transmission Automatic Hall Effect Sensor Power Circuit Failure
The transmission's Hall Effect sensor, which detects gear position and speed, has lost electrical power or the power circuit is broken. It's like a speedometer that's unplugged—the transmission can't read what gear it should be in.
The ECU monitors the 12V power supply and ground circuit to the Hall Effect sensor. It expects a stable voltage signal during operation. If voltage drops below threshold or the circuit opens, the sensor cannot provide gear detection signals to the transmission control module.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Sensor Power Supply Voltage | 11.5-14.5V | Below 10V or 0V detected |
| Sensor Signal Output | 0-5V switching pattern | No signal or constant voltage |
Code P1879 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, P1879 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.