P1392

What Does Code P1392 Mean?

DTC P1392 signifies a detected High Input condition within the Glow Plug Circuit specific to Bank #1. This code is primarily associated with diesel engines, where glow plugs are critical for initiating combustion during cold engine starts. The Engine Control Module (ECM) or Powertrain Control Module (PCM) actively monitors the electrical characteristics of the glow plug circuits, including voltage, resistance, and current draw, to ensure proper operation. A “High Input” reading indicates that the ECM/PCM is receiving a voltage signal from the glow plug circuit on Bank #1 that is higher than its calibrated expected range under certain operating conditions. This typically suggests an open circuit, a high resistance connection, or a short circuit to a voltage source within the glow plug wiring, the glow plug itself, or the glow plug control module that interfaces with Bank #1. The ECM/PCM flags this anomaly because the glow plug circuit is not functioning as commanded, which can severely impact cold start performance and exhaust emissions.

Common Symptoms

• Difficulty Starting Engine: Particularly noticeable in cold weather, as glow plugs are essential for preheating the combustion chambers.
• Rough Idling: Especially prominent immediately after a cold start, due to uneven combustion in cylinders with non-functional glow plugs.
• White Smoke from Exhaust: During cold starts, unburnt or partially burnt fuel can exit the exhaust, producing white smoke.
• Check Engine Light (CEL) Illumination: The Malfunction Indicator Lamp (MIL) will activate once the ECM/PCM detects the fault.
• Reduced Engine Performance: While less direct, compromised combustion at cold temperatures can indirectly affect overall engine response and fuel efficiency.
• Excessive Crank Time: The engine may crank for an extended period before starting, especially when cold.

What Causes the Code P1392?

• Faulty Glow Plug(s): An internal open circuit within one or more glow plugs on Bank #1, preventing current flow and leading to a high voltage reading on the monitoring circuit.
• Damaged Wiring Harness: An open circuit (break) or a short to battery voltage within the wiring connecting the glow plug(s) on Bank #1 to the glow plug control module or ECM.
• Corroded Electrical Connectors: High resistance or an intermittent open circuit at the glow plug terminals or harness connectors due to corrosion, moisture intrusion, or physical damage.
• Malfunctioning Glow Plug Control Module/Relay: The module or relay responsible for supplying power to Bank #1 glow plugs may be defective, causing an incorrect voltage signal to be sent back to the ECM/PCM, or failing to supply current, which can result in an open circuit condition from the ECM’s perspective.
• ECM/PCM Malfunction: While less common, an internal fault within the Engine Control Module itself could lead to misinterpretation of the glow plug circuit voltage, though other related codes would typically accompany this.

How to Diagnose and Troubleshoot

Diagnosis of P1392 requires a systematic approach utilizing specialized tools:

1. Verify Code and Freeze Frame Data: Connect an OBD-II scan tool to confirm P1392. Note any accompanying codes and review freeze frame data, which captures engine conditions at the moment the DTC was set. This information (e.g., engine temperature, RPM) can provide valuable context.
2. Visual Inspection: Perform a thorough visual inspection of the glow plugs, wiring harness, and connectors for Bank #1. Look for signs of physical damage, chafing, corrosion, loose connections, or rodent damage. Ensure all connectors are securely seated.
3. Glow Plug Resistance Test (Individual):
   • Disconnect the electrical connector from each glow plug on Bank #1.
   • Using a Digital Multimeter (DMM) set to ohms, measure the resistance between the glow plug terminal and a known good engine ground (or the glow plug body if accessible).
   • A healthy glow plug typically shows very low resistance, usually between 0.5 to 2.0 ohms, depending on the manufacturer and design.
   • An “OL” (Open Line) or infinite resistance reading indicates an open circuit within the glow plug, confirming it is faulty.
4. Glow Plug Circuit Voltage Test:
   • Reconnect glow plug connectors.
   • Access the glow plug control module or the main harness leading to Bank #1 glow plugs (refer to service manual for specific access points).
   • With the key ON, engine OFF (KOEO), and the glow plug cycle activated (either naturally during a cold start attempt or via bi-directional control with a scan tool), use a DMM set to DC volts to measure voltage at the glow plug side of the connector harness.
   • You should observe battery voltage (approximately 12V) being supplied for a short duration. A reading of 0V or a significantly lower voltage when commanded indicates a power supply issue from the glow plug control module or a wiring problem. A high input fault often implies the ECM is monitoring the voltage on a circuit that should be low (e.g., pulling current through a glow plug) but is instead seeing a high voltage, possibly due to an open circuit on the load side.
5. Glow Plug Control Module/Relay Test:
   • Check for proper power supply (battery voltage) and ground at the glow plug control module.
   • Verify that the ECM/PCM is sending the appropriate activation signal to the control module (often a pulse-width modulated signal or a simple ON/OFF signal).
   • If possible, perform a continuity check through the glow plug relay or test the control module’s output directly to Bank #1 during activation.
6. Wiring Harness Continuity and Short Test:
   • With the battery disconnected, disconnect the glow plug control module and glow plugs on Bank #1.
   • Using a DMM, check for continuity between the glow plug harness connector and the corresponding pin at the glow plug control module connector. An open circuit indicates a wiring break.
   • Check for shorts to ground and shorts to battery voltage by measuring resistance between the glow plug circuit wire and chassis ground, and between the glow plug circuit wire and other power wires (e.g., constant 12V supply).

Recommended Repairs and Solutions

Once the root cause of P1392 has been pinpointed through systematic diagnosis, the following repairs are typically recommended:

1. Replace Faulty Glow Plug(s): If the resistance test identified an open circuit, the defective glow plug(s) on Bank #1 should be replaced. It is often advisable to replace all glow plugs on a bank, or even all glow plugs in the engine, if they are of similar age and wear, to prevent future failures.
2. Repair or Replace Wiring Harness: If damaged or corroded wiring is found, repair the affected sections using appropriate automotive-grade connectors and wiring, or replace the entire sub-harness if damage is extensive. Ensure all repairs are weatherproofed.
3. Clean or Replace Corroded Connectors: If corrosion is the primary issue, carefully clean the electrical terminals using a specialized contact cleaner and a small brush. If corrosion is severe or physical damage is present, replace the affected connector pigtail.
4. Replace Glow Plug Control Module/Relay: If diagnostic tests confirm that the control module or relay is not supplying power correctly or is internally faulty, replacement is necessary. Always use an OEM equivalent part to ensure compatibility and reliability.
5. ECM/PCM Reprogramming or Replacement: This is a last resort and should only be considered after all other components and wiring have been meticulously tested and ruled out. If an ECM/PCM fault is suspected, consult a dealership or a specialist with programming capabilities.

Mechanic’s Tips:

• When replacing glow plugs, always apply a small amount of anti-seize compound to the threads to prevent future seizure, especially in aluminum cylinder heads.
• Use a torque wrench to tighten glow plugs to the manufacturer’s specified torque. Overtightening can damage the glow plug or cylinder head threads, while undertightening can lead to poor contact or combustion leakage.
• Exercise extreme caution when removing old glow plugs, as they can sometimes seize or break off in the cylinder head, requiring significant and costly repair. Allow the engine to be slightly warm (not hot) to aid removal.
• After completing repairs, clear the DTCs with a scan tool and perform several cold start cycles to confirm the repair and ensure the code does not return.