P1393

What Does Code P1393 Mean?

DTC P1393 indicates that the Engine Control Module (ECM), also commonly referred to as the Powertrain Control Module (PCM), has detected an abnormally low voltage or current signal within the glow plug circuit for Bank 2. In diesel engine applications, glow plugs are critical heating elements designed to elevate the temperature within the combustion chambers, facilitating cold starts, particularly in low ambient temperatures. The ECM monitors the electrical integrity and operational status of each glow plug circuit, typically by observing voltage drop, current flow, or resistance within the circuit. When the ECM perceives a voltage or current input from Bank 2’s glow plug circuit that falls below a calibrated threshold, indicating either excessive resistance, an open circuit, or insufficient power delivery, it logs P1393. Bank 2 generally refers to the cylinder bank that does not contain cylinder number one, commonly cylinders 4-6 in a V6 engine or 5-8 in a V8 engine, or the second half of an inline engine’s cylinders if the glow plug system is segmented. This issue directly affects the pre-heating subsystem responsible for ensuring proper engine combustion at startup.

Common Symptoms

• Difficulty Starting Engine: Especially pronounced in cold weather, requiring prolonged cranking.
• Rough Idling or Misfires: Particularly noticeable immediately after a cold start until the engine warms up.
• White Smoke from Exhaust: Unburnt fuel due to insufficient combustion chamber temperature during startup.
• Reduced Engine Performance: Though less common, prolonged issues can impact overall engine efficiency.
• Check Engine Light (CEL) Illumination: The Malfunction Indicator Lamp (MIL) will be illuminated.

What Causes the Code P1393?

• Faulty Glow Plugs: One or more glow plugs in Bank 2 may be open-circuited, have excessively high resistance, or be completely inoperative.
• Damaged Wiring or Connectors: Corrosion, chafing, or breaks in the wiring harness connecting the Glow Plug Control Module (GPCM) or ECM to the glow plugs in Bank 2. Loose or corroded electrical connectors can also cause low input.
• Faulty Glow Plug Control Module (GPCM): The GPCM, responsible for supplying voltage to the glow plugs, may be malfunctioning, failing to deliver adequate voltage or current to Bank 2.
• Blown Fuse or Relay: A fuse protecting the glow plug circuit or a faulty glow plug relay (if applicable) can cut off power to Bank 2.
• ECM/PCM Malfunction: While less common, a rare internal fault within the ECM/PCM could incorrectly report a low input condition.

How to Diagnose and Troubleshoot

Diagnosis of P1393 requires a systematic approach utilizing a digital multimeter (DMM) and an OBD-II scan tool.

1. Retrieve and Document Freeze Frame Data: Connect an OBD-II scanner to extract all stored DTCs and any associated freeze frame data. This provides valuable insights into engine operating conditions at the time the code was set. Clear the code and attempt to replicate the conditions if possible.
2. Visual Inspection: Begin by thoroughly inspecting the wiring harness, connectors, and glow plugs on Bank 2. Look for signs of chafing, corrosion, disconnections, or physical damage to the glow plugs themselves. Pay close attention to the multi-pin connector at the GPCM and individual connectors at each glow plug.
3. Glow Plug Resistance Test: Disconnect the electrical connector from each glow plug on Bank 2. Using a DMM set to ohms, measure the resistance between the glow plug’s terminal and a known good engine ground. Most glow plugs should exhibit very low resistance, typically between 0.5 to 2.0 ohms. Consult service information for exact specifications. A reading of infinity (open circuit) or significantly high resistance indicates a faulty glow plug. Replace any glow plug that falls outside specifications.
4. Voltage Supply Test at Glow Plugs: Reconnect the glow plug harness. Access the glow plug connector or the main supply wire for Bank 2 at the GPCM output. With the ignition key in the “ON” position (or during the pre-glow cycle if the engine is cold and the GPCM is active), use a DMM to measure voltage at the glow plug terminals or the output to Bank 2. You should observe battery voltage (typically 10.5V to 12.6V) for a short duration during the pre-glow period. No voltage, or significantly reduced voltage, points towards a wiring issue, GPCM failure, or fuse/relay problem.
5. Glow Plug Control Module (GPCM) Input/Output Test: If no voltage is present at the glow plugs, test the GPCM.
   • Power and Ground: Verify battery voltage and a solid ground connection at the GPCM’s main power input terminals using a DMM.
   • Signal Input: Check the control signal input to the GPCM from the ECM/PCM (if applicable, consult wiring diagrams). This is typically a pulse-width modulated (PWM) signal or a simple ON/OFF signal.
   • Output Test: If the GPCM has proper power, ground, and control signals, but no voltage is output to Bank 2 glow plugs, the GPCM is likely faulty.
6. Continuity Test of Wiring Harness: If individual glow plugs are good and the GPCM output is present, disconnect the harness from the GPCM and at the glow plugs. Perform a continuity test using the DMM between the corresponding terminals to check for open circuits or shorts to ground within the wiring harness for Bank 2.

Recommended Repairs and Solutions

Addressing DTC P1393 typically involves replacing the faulty component identified during diagnostics:

• Replace Faulty Glow Plugs: The most common resolution is replacing any glow plug on Bank 2 that failed the resistance test. It is often recommended to replace all glow plugs on the affected bank simultaneously, as they typically have similar lifespans and one failure often presages others. Always use OEM-equivalent or higher-quality glow plugs.
• Repair or Replace Damaged Wiring/Connectors: If visual inspection or continuity tests reveal damaged wiring or corroded connectors, repair them using appropriate automotive-grade connectors and wiring. Ensure all connections are clean, tight, and sealed to prevent future corrosion.
• Replace the Glow Plug Control Module (GPCM): If the GPCM is determined to be faulty (e.g., no output voltage despite proper input power and control signals), it will need replacement. Ensure the new module is correctly programmed or adapted to the vehicle’s ECM if required by the manufacturer.
• Check Fuses and Relays: Verify the integrity of any fuses or relays associated with the glow plug circuit. Replace any blown fuses or faulty relays.
• ECM/PCM Replacement (Rare): In very rare cases, if all other components and wiring are confirmed good, and the ECM/PCM is not sending the correct control signal to the GPCM, the ECM/PCM might require re-flashing or replacement. This should only be considered after exhaustive testing of all other potential causes.

Mechanics’ Tips:

• When removing glow plugs, ensure the engine is warm (but not hot) to minimize the risk of stripping threads or breaking the glow plug off in the cylinder head. Always apply anti-seize compound to the new glow plug threads before installation.
• Use a torque wrench for glow plug installation to prevent overtightening, which can damage the cylinder head or the glow plug itself.
• After any repairs, clear the DTCs with a scan tool and perform several cold starts to ensure the issue is resolved and the code does not return.
• Always consult the vehicle’s specific service manual for precise wiring diagrams, resistance values, and diagnostic procedures, as these can vary significantly between manufacturers and models.