P1394

What Does Code P1394 Mean?

DTC P1394 signifies a detected “Glow Plug Circuit High Input (Bank #2)” condition. This indicates that the Powertrain Control Module (PCM), often referred to as the Engine Control Module (ECM) in diesel applications, has identified an electrical anomaly within the glow plug circuit specifically associated with Bank 2. On V-configuration engines, Bank 2 typically refers to the cylinder bank that does not contain cylinder #1. For inline engines, P1394 would generally point to a specific grouping of glow plugs or a secondary control circuit designated as “Bank 2” by the manufacturer. The ECM monitors the electrical characteristics of the glow plug circuits, primarily current draw or voltage drop, to ensure proper operation. A “high input” condition for a glow plug circuit generally implies an open circuit, excessively high resistance, or a short to a voltage source that exceeds the expected operational parameters. This fault prevents the glow plugs on Bank 2 from heating correctly, which is critical for initiating combustion in diesel engines, especially during cold-start conditions.

Common Symptoms

• Hard Start or No Start Condition: Particularly prevalent in cold ambient temperatures, as the engine relies heavily on glow plug preheating.
• Excessive White Smoke from Exhaust: Observed during initial startup and possibly for a brief period thereafter, indicating unburnt or poorly combusted fuel.
• Illumination of the Check Engine Light (CEL): The standard indicator for an emission-related or powertrain fault.
• Rough Idling or Misfires: Especially noticeable immediately after starting, until the engine reaches operating temperature.
• Reduced Engine Performance: A noticeable lack of power or sluggish acceleration in some cases, particularly if the ECM attempts to compensate for misfires.

What Causes the Code P1394?

• Faulty Glow Plug(s) on Bank 2: An open circuit (burnt-out filament) or significantly increased internal resistance within one or more glow plugs on Bank 2.
• Wiring Harness Damage: An open circuit, high resistance due to corrosion, or a short to voltage within the wiring harness connecting the glow plug control module to the Bank 2 glow plugs.
• Defective Glow Plug Control Module/Relay: An internal failure within the glow plug control module or a specific relay affecting the output circuit to Bank 2.
• Corroded or Loose Electrical Connections: High resistance at the connectors for the glow plugs, the glow plug control module, or the ECM itself.
• Faulty Engine Control Module (ECM/PCM): Although less common, an internal ECM fault preventing proper glow plug circuit monitoring or control for Bank 2 could be a cause.

How to Diagnose and Troubleshoot

Diagnosis of P1394 requires a methodical approach using appropriate diagnostic tools:

1. Visual Inspection: Begin by thoroughly inspecting the wiring harness leading to the glow plugs on Bank 2. Look for signs of chafing, corrosion, cuts, or damaged insulation. Examine all associated electrical connectors for corrosion, bent pins, or loose connections.
2. Retrieve Freeze Frame Data: Use an OBD-II scanner to retrieve freeze frame data along with the P1394 code. This data provides system parameters at the time the fault was detected, which can offer valuable clues regarding operating conditions.
3. Individual Glow Plug Resistance Test: Disconnect the electrical connector from each glow plug on Bank 2. Using a Digital Multimeter (DMM) set to ohms, measure the resistance between the glow plug’s terminal and a known good engine ground. A healthy glow plug typically exhibits very low resistance (e.g., 0.5 to 2.0 ohms, consult manufacturer specifications). An “OL” (open circuit) or significantly higher than specified resistance indicates a faulty glow plug.
4. Glow Plug Circuit Voltage Test: With the engine cold and the ignition switched to the “ON” position (or during a brief cranking attempt to activate the pre-heat cycle), use a DMM to measure the voltage at the harness connector for each glow plug on Bank 2. Compare this reading to battery voltage. A significantly lower voltage or no voltage indicates a problem upstream (wiring, control module).
5. Wiring Harness Continuity and Short Test: Disconnect the glow plug control module connector and the glow plug connectors on Bank 2. Using a DMM, check for continuity in the glow plug control circuit wiring between the module connector and each glow plug connector. Test for open circuits (OL) or excessive resistance. Also, check for short circuits to ground or to battery voltage in the harness.
6. Glow Plug Control Module Testing: Verify the glow plug control module is receiving proper power and ground signals. If available, use a manufacturer-specific diagnostic tool to command the glow plugs ON and monitor the output voltage from the module to Bank 2. An oscilloscope can be used to observe the duty cycle or pulse-width modulation (PWM) signals if the module employs such control.
7. ECM Cross-Reference: If all other components test within specifications, and a known good ECM is available, temporarily swap it to rule out an internal ECM fault. This should be a last resort due to cost and complexity.

Recommended Repairs and Solutions

Based on the diagnostic findings, the following repairs are typically recommended:

• Replace Faulty Glow Plugs: If individual glow plug resistance tests reveal an open circuit or resistance significantly outside manufacturer specifications, replace the defective glow plug(s) on Bank 2. It is often advisable to replace all glow plugs on a bank, or even all glow plugs on the engine, if they are of similar age and mileage, to prevent future failures and ensure balanced operation.
• Repair or Replace Wiring Harness: Address any identified damage to the glow plug wiring harness. This may involve repairing cut or chafed wires, replacing corroded terminals, or replacing entire sections of the harness if damage is extensive. Ensure all connections are clean, secure, and properly seated.
• Replace Glow Plug Control Module/Relay: If diagnostics confirm an internal failure of the glow plug control module or a specific relay responsible for Bank 2, replace the unit with a new, OEM-specification part.
• Clean Electrical Connectors: Use a specialized electrical contact cleaner to remove corrosion and ensure proper electrical conductivity at all glow plug, module, and ECM connections. Apply a dielectric grease to protect against future corrosion.
• ECM/PCM Replacement or Reprogramming: Only consider ECM replacement after all other diagnostic avenues have been exhausted and conclusively ruled out. This typically requires specialized tools for programming and adaptation to the vehicle.