Fuel Demand Command Signal Circuit Malfunction
The engine computer can't properly communicate with the fuel injection demand circuit, like a broken messenger between the brain and the fuel system. This electrical signal problem prevents the ECU from commanding the fuel injectors to deliver the right amount of fuel.
The ECU monitors the fuel demand command signal voltage and circuit continuity to the fuel control actuator or injector driver module. It detects open circuits, shorts to ground/voltage, or signal dropout conditions that fall outside normal operating parameters. The ECM expects a clean, responsive signal that varies with load and engine speed demands.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Signal Voltage | 0.5–4.8V varying with demand | Out of range, stuck high/low, or no change |
| Circuit Resistance | <5 ohms wire harness | >10 ohms or open circuit detected |
Code P1681 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, P1681 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.