Passenger's Front Seat Down Switch Circuit Short to Battery
The passenger seat's down switch is electrically stuck in the "on" position, like a light switch that's shorted and always thinks it's being pressed. The ECU detects excessive voltage where there should be a normal signal.
The ECU monitors the seat switch circuit for voltage levels that indicate switch position and operation. When the circuit shorts to battery voltage, the ECU reads a constant high signal instead of the expected switching pattern. A persistent battery-voltage reading at the switch input triggers the fault code.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Switch signal voltage | 0-5V with switching transitions | Continuous 12V (battery voltage) |
| Circuit resistance | Variable 0-1kΩ (switch open/close) | Near 0Ω (short to battery) |
Code B1978 is a low-severity fault. Your vehicle is generally safe to drive to a workshop for diagnosis. However, do not ignore it indefinitely — low-severity codes often indicate developing problems that become expensive if neglected. Book a diagnostic appointment within 2–4 weeks. If you notice any additional symptoms (rough running, power loss, unusual smells), treat it as higher priority.
Once the fault is repaired, B1978 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.