Trailer Input Circuit Failure
Your vehicle's trailer connection system isn't communicating properly with the main computer, similar to a phone losing signal from its network. The ECU can't read the trailer's electrical status or control signals.
The ECU monitors the trailer input circuit voltage and signal continuity to detect trailer presence and brake requests. It expects a valid signal within a specific voltage range; an open circuit, short, or out-of-range voltage triggers the fault. The system typically uses a dedicated trailer control module or direct ECU inputs.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Trailer Circuit Voltage | 5V-12V steady state | 0V (open), >13.5V, or erratic fluctuation |
| Signal Continuity | Continuous valid signal detected | Intermittent or no signal for >2 seconds |
Code C1749 is classified as a serious fault. If your check engine light is flashing — not just steady — pull over safely and do not continue driving. A flashing CEL indicates an active misfire or critical failure that can cause catalytic converter damage within minutes or permanent engine harm within miles. Contact a certified mechanic immediately. Do not attempt roadside repairs on high-severity codes unless you are trained to do so.
Once the fault is repaired, C1749 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.