P1391

What Does Code P1391 Mean?

DTC P1391, Glow Plug Circuit Low Input (Bank #1), indicates that the Powertrain Control Module (PCM), or Engine Control Module (ECM) in many diesel applications, has detected a voltage reading in the glow plug control circuit for Bank 1 that is below a predetermined calibrated threshold. In diesel engines, glow plugs are essential components for initiating combustion, especially during cold starts. They preheat the combustion chambers to achieve the necessary temperature for diesel fuel to auto-ignite under compression. When the PCM detects a “low input” on this circuit, it signifies that the expected electrical activity or voltage drop across the glow plug circuit for the first bank of cylinders (Bank 1) is not within the manufacturer’s specifications. This can be caused by various electrical anomalies, such as an open circuit, excessive resistance, a short to ground, a faulty glow plug drawing insufficient current, or a malfunction within the glow plug control module/relay.

Common Symptoms

• Hard starting or no start condition, particularly in cold ambient temperatures.
• Rough idle or misfiring immediately after cold start, which may smooth out as the engine warms up.
• Excessive white smoke from the exhaust during cold start, indicating unburnt fuel due to insufficient combustion temperature.
• Illumination of the Check Engine Light (CEL) or Malfunction Indicator Lamp (MIL).
• Reduced engine performance or hesitant acceleration during initial operation in cold conditions.

What Causes the Code P1391?

• Faulty Glow Plug(s) (Bank 1): One or more glow plugs in Bank 1 may have an internal open circuit, excessive resistance, or be shorted internally, causing an improper voltage drop across the circuit.
• Damaged or Corroded Wiring/Connectors: High resistance, an open circuit, or a short to ground within the wiring harness leading to the glow plugs or the glow plug control module for Bank 1. This includes damaged insulation, fraying, or loose terminal connections.
• Faulty Glow Plug Control Module or Relay: The module or relay responsible for supplying and regulating power to the glow plugs on Bank 1 may be defective, failing to provide adequate voltage or current, or exhibiting internal circuit failure.
• Blown Fuse: A fuse protecting the glow plug circuit for Bank 1 could be blown due to an overload or short circuit condition in the glow plug system.
• ECM/PCM Failure: While less common, an internal fault within the ECM/PCM that affects its ability to correctly monitor or control the glow plug circuit for Bank 1 can trigger this code.

How to Diagnose and Troubleshoot

A systematic diagnostic approach is crucial for accurately resolving P1391:

1. Verify the Code and Freeze Frame Data: Connect an OBD-II scan tool to confirm the presence of P1391. Review freeze frame data to understand engine conditions (engine temperature, RPM, load) at the moment the code was set, which can provide valuable context. Clear the code and attempt to replicate the conditions if possible.
2. Visual Inspection of Glow Plug Circuitry: Carefully inspect the wiring harness for Bank 1 glow plugs and the glow plug control module/relay. Look for any signs of physical damage, chafing, corrosion, loose connections, or burnt areas. Ensure connectors are fully seated and free of debris or oxidation.
3. Test Glow Plug Resistance: Disconnect the electrical connector(s) from the glow plugs on Bank 1. Using a Digital Multimeter (DMM) set to ohms, measure the resistance of each individual glow plug from its terminal to a known good engine ground. Consult the vehicle’s service manual for specific resistance specifications, but typically, a good glow plug will show very low resistance (e.g., 0.5 to 2.0 ohms). An “OL” (open loop) reading indicates an open circuit and a faulty glow plug. A significantly lower reading might suggest an internal short.
4. Test Glow Plug Circuit Voltage: With the glow plug harness reconnected, use the scan tool’s bi-directional controls (if available) or cycle the ignition key to the “ON” position (without starting) to activate the glow plugs. Using the DMM, back-probe the glow plug harness connector for Bank 1 and measure the voltage present during the pre-heat cycle. Expect to see battery voltage (approximately 12.0-13.5V). Low or no voltage indicates a problem with the power supply to the glow plugs (wiring, relay, or control module).
5. Test Glow Plug Control Module/Relay Operation: If low voltage is detected at the glow plugs, proceed to test the glow plug control module or relay. Verify power supply (B+) and ground connections to the module/relay. Check for the appropriate activation signal from the PCM to the module/relay (refer to wiring diagrams for expected signal types, e.g., 12V, 5V, or pulse-width modulated). Test the output voltage/current from the module/relay to the glow plugs.
6. Check Fuses and Fusible Links: Locate and test all fuses and fusible links associated with the glow plug circuit using a DMM for continuity. A blown fuse indicates an underlying short or overload condition.
7. PCM/ECM Input/Output Verification: As a final step, if all other components test good, verify the PCM’s power and ground inputs and its output signal to the glow plug control module using service manual specifications. This step is usually reserved for when all other avenues have been exhausted.

Recommended Repairs and Solutions

Once the root cause of P1391 has been precisely identified through thorough diagnostics, the following repairs are typically performed:

• Replace Faulty Glow Plugs: If one or more glow plugs on Bank 1 are determined to be open, shorted, or out of resistance specifications, they must be replaced. It is often a recommended best practice to replace all glow plugs on Bank 1 simultaneously, as they are wear items and tend to fail around the same lifespan.
• Repair or Replace Damaged Wiring/Connectors: Any corroded, frayed, or open wiring, or damaged connectors in the glow plug circuit for Bank 1, should be meticulously repaired or replaced. Use appropriate automotive-grade wire and connectors, ensuring secure, weather-tight connections to prevent future issues.
• Replace Glow Plug Control Module/Relay: If testing definitively points to a defective glow plug control module or relay, it must be replaced. Use an OEM-equivalent part for optimal compatibility and reliability.
• Replace Blown Fuses: If a fuse protecting the glow plug circuit is found to be blown, replace it with a fuse of the exact specified amperage. Critically, investigate and rectify the underlying cause of the blown fuse (e.g., a shorted glow plug or wiring) to prevent immediate recurrence.
• PCM/ECM Repair or Replacement: In very rare instances where the PCM/ECM itself is proven to be faulty after exhaustive testing of all other components, it may require repair, reprogramming, or replacement. This is typically a dealership-level repair and involves complex programming.

Mechanic’s Tip: When removing glow plugs, especially on high-mileage engines, they can sometimes seize in the cylinder head. Always ensure the engine is at operating temperature to minimize the risk of glow plug breakage. Use penetrating oil and appropriate tools, exercising extreme caution to avoid costly damage to the cylinder head.