Transmission Transfer Case Rear Shaft Speed Sensor Circuit Failure
The transmission's rear shaft speed sensor isn't sending a proper signal to the engine computer, similar to a speedometer that stopped working. Without this signal, the transmission can't properly manage gear changes in all-wheel-drive or four-wheel-drive modes.
The ECU monitors voltage pulses from the rear shaft speed sensor in the transfer case to calculate rear axle speed. It compares this signal to front shaft speed to manage torque distribution and gear engagement. If the signal is absent, erratic, or outside expected frequency ranges, a circuit failure is detected.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Signal Frequency | Proportional to shaft RPM (typically 0-5 kHz) | No signal or irregular pulses below threshold |
| Voltage Output | 0.5-5V AC or digital logic levels | Stuck voltage, open circuit, or short to ground |
Code P1842 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, P1842 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.