Easy one-page printout for the garage or repair shop
This summary uses the existing diagnostic information shown in full below.
This means that the part of the car that helps start the engine when it's cold isn't working well. It's like when a light bulb doesn't turn on because there's a problem with the switch or the wires.
The P1391 code indicates that the Engine Control Module (ECM) has detected a low input signal from the glow plug circuit on bank 1. This can affect the starting and running of a diesel engine, especially in cold conditions.
The glow plug circuit is crucial for pre-heating the combustion chamber in diesel engines to ensure smooth starting. A low input signal suggests that the voltage or current in the circuit is below the expected range, which could be due to a faulty glow plug, wiring issues, or a problem with the control module.
• Difficulty starting the engine in cold weather
• Engine misfires
• Illuminated check engine light
• Faulty glow plug
• Damaged wiring or connectors
• Faulty glow plug control module
Glow Plug: A device used to preheat the air in a diesel engine's cylinder to help start the engine.
ECM: Engine Control Module, the computer that controls engine functions.
• Check for any other related codes and clear them.
• Inspect the glow plug and related wiring for damage.
• Test the glow plug control module for proper operation.
• Replace faulty glow plugs.
• Repair or replace damaged wiring or connectors.
• Replace the glow plug control module if necessary.
• Digital multimeter
• OBD-II scanner
• Socket set
$150 - $400
Moderate
• Ford F-Series Diesel
• Chevrolet Silverado Diesel
• Dodge Ram Diesel
• Regularly inspect glow plugs and wiring.
• Ensure proper maintenance of the glow plug control module.
• Use high-quality diesel fuel.
Sensor Issue
• Have your vehicle inspected by a professional mechanic.
• Avoid driving in extremely cold conditions until the issue is resolved.
Check with your vehicle manufacturer for specific warranty coverage on engine components.
This code has been commonly reported in diesel vehicles, especially in colder climates where glow plugs are critical for engine starting.
Moderate – It is important to address this issue to ensure reliable engine starting and performance.
Driving with a malfunctioning glow plug circuit can lead to increased emissions and potential engine damage.
• Cold engine start
• Ambient temperature below freezing
The glow plugs are located in the cylinder head of the engine, typically accessible from the top of the engine bay.
Q: Can I drive with a P1391 code?
A: It's not recommended, especially in cold weather, as it can lead to starting issues and increased emissions.
Q: How do I know if my glow plugs are bad?
A: Symptoms include difficulty starting the engine, especially in cold weather, and a rough idle.
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Universal · Digital multimeter
A solid entry-level multimeter for voltage, continuity, resistance, and basic electrical troubleshooting.
Helpful for checking voltage drops, grounds, and basic circuit health when modules are not communicating.
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Universal · Wire connectors and repair kit
Useful when diagnosis points toward damaged wiring, corroded terminals, or connector repairs.
Useful if diagnosis points toward damaged connectors, wiring, or terminal repairs.
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Fitment Required · Vehicle-fitment part
Commonly relevant for misfire, ignition-efficiency, and some drivability-related repairs.
Misfire and ignition-related faults commonly involve spark plugs as part of the repair path.
Check the correct plug type, gap, and engine application for your vehicle.
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Fitment Required · Vehicle-fitment part
Relevant when the fault points toward ignition-coil performance or cylinder-specific ignition issues.
Some ignition faults involve coil performance or cylinder-specific coil problems.
Match the coil to your engine and confirm the failed component before ordering.
View options on AmazonThese parts may be related to this fault category, but a code by itself does not confirm exact fitment or guarantee that replacing a part will solve the problem.