Transmission Automatic 4-Wheel Drive Indicator (Lamp) Circuit Failure
The transmission's 4-wheel drive indicator light circuit is broken or not communicating properly with the engine computer. Think of it like a dashboard warning light that's stuck or the wire connecting it to the brain is severed.
The ECU monitors the voltage signal from the 4WD indicator lamp circuit, expecting a clean on/off signal when 4WD mode is selected or disengaged. The system checks for proper circuit continuity and voltage levels to confirm the lamp and wiring are functioning. If voltage is absent, shorted, or out of range, the fault is triggered.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Indicator lamp voltage | 12V when active, 0V when inactive | Open circuit or <2V / >14V |
| Circuit resistance | <5 ohms when lamp is on | >100 ohms or infinite resistance |
Code P1873 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, P1873 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.