The P1391 code indicates that the engine control module has detected a voltage input below the normal threshold in the Bank 1 glow plug circuit. This typically means the circuit is not receiving the expected current to heat the combustion chambers for a cold start.
An open circuit or internal failure in one or more glow plugs prevents the circuit from maintaining the required voltage.
Corroded connectors, frayed wires, or broken circuits between the relay and the glow plugs can cause low voltage input.
A failing relay or control module may not be delivering the full battery voltage to the glow plug bus bar.
A blown fuse in the glow plug circuit will completely interrupt power, resulting in a low input signal.
An internal failure in the ECM's monitoring circuit may incorrectly report low voltage.
Yes, the vehicle will generally run, but it will be significantly harder to start when the engine is cold.
While only one may be failed, glow plugs often wear out at similar rates; however, the code only requires the failed component to be addressed.
No. While some may suggest replacing the ECM, P1391 is almost always caused by a failed glow plug, wiring issue, or relay failure.