P1399

What Does Code P1399 Mean?

The diagnostic trouble code P1399 signifies a detected anomaly within the Glow Plug Circuit High Side, specifically indicating a High Input condition. In diesel engine applications, glow plugs are essential pre-heating devices that facilitate cold starting by raising the temperature within the combustion chamber. The Engine Control Module (ECM) or Powertrain Control Module (PCM) continuously monitors the operational parameters of the glow plug system, including the voltage and current supplied to the glow plugs through the Glow Plug Control Module (GPCM) or relay.

A “High Side” refers to the positive voltage supply aspect of the circuit. A “High Input” in this context means that the ECM/PCM is detecting an excessively high voltage within this monitored circuit, beyond its calibrated upper threshold for normal operation. This typically suggests that a portion of the glow plug power supply circuit is inadvertently receiving or maintaining battery voltage (or another unregulated high voltage source) when it should not be, or that the GPCM itself is failing to regulate or switch off the voltage appropriately. This condition can severely impact cold start performance, increase emissions, and potentially damage glow plugs or the GPCM due to continuous over-voltage or overheating.

Common Symptoms

• Difficulty Starting: Especially pronounced during cold ambient temperatures, as the engine relies heavily on pre-heating for ignition.
• Rough Idle on Cold Start-up: Inconsistent combustion dueulating from inadequate or uncontrolled pre-heating.
• Check Engine Light (CEL) Illumination: The most direct symptom, indicating the ECM has registered a fault.
• Excessive White Smoke from Exhaust: Particularly noticeable after a cold start, due to unburnt fuel from incomplete combustion.
• Reduced Fuel Economy: While not always direct, inefficient combustion due to glow plug issues can lead to increased fuel consumption.
• Engine Hesitation or Misfire: During the initial warm-up phase, if the glow plug system is not functioning correctly.

What Causes the Code P1399?

• Faulty Glow Plug Control Module (GPCM) or Relay: A common cause where the control module itself is internally shorted, stuck in an “on” position, or fails to properly regulate the voltage supplied to the glow plugs, resulting in a constant or excessively high voltage on the high side of the circuit.
• Short to Voltage in Glow Plug Wiring Harness: The wiring connecting the GPCM to the glow plugs, or the GPCM power supply wiring, may have chafed or been damaged, leading to a direct short circuit to a constant 12-volt battery supply or another power source.
• Corroded or Damaged Wiring and Connectors: Severe corrosion or physical damage to the glow plug harness connectors or main power cables can lead to erratic voltage readings, including perceived high inputs, or create unintended short circuits.
• ECM/PCM Malfunction: Although less common, an internal fault within the Engine Control Module or Powertrain Control Module could lead to incorrect voltage monitoring or erroneous reporting of a high input condition.

How to Diagnose and Troubleshoot

Diagnosing P1399 requires a methodical approach, focusing on the glow plug control module and its associated wiring. Always begin with safety precautions, including disconnecting the battery before working on electrical components.

1. OBD-II Scanner Examination:
   • Connect an OBD-II scanner to retrieve the P1399 code and any accompanying codes.
   • Review freeze frame data, which captures engine parameters at the moment the code was set. This can provide crucial clues regarding engine temperature, voltage levels, and engine load.
   • Clear the code and attempt to replicate the conditions under which it was set (e.g., cold start cycle) to confirm the fault’s consistency.
2. Visual Inspection:
   • Thoroughly inspect the glow plug control module (GPCM) and its electrical connectors for signs of corrosion, burning, melting, or physical damage.
   • Trace the entire glow plug wiring harness from the GPCM to each glow plug. Look for any chafing, pinching, exposed wires, or signs of rodent damage that could cause a short to voltage.
   • Inspect the main power and ground connections to the GPCM for looseness or corrosion.
3. Digital Multimeter (DMM) Testing:
   • Battery Voltage Check: Ensure the vehicle’s battery is fully charged and in good health. A weak battery can sometimes cause secondary electrical issues.
   • GPCM Output Voltage Test:
     1. Safely disconnect the main harness connector that feeds power to the glow plugs from the GPCM (if individual glow plug wires cannot be accessed easily, or if testing the common output).
     2. Using a DMM, measure the voltage output from the GPCM’s glow plug terminals (or the common output pin) with the ignition on, during a cold start cycle.
     3. Compare readings to the vehicle’s service manual specifications. If a constant high voltage (e.g., battery voltage) is present when the glow plugs should be off, or if the voltage is continuously unregulated and high, the GPCM is likely faulty.
   • Wiring Harness Short to Voltage Test:
     1. With the GPCM disconnected and all individual glow plugs disconnected (if possible), set your DMM to measure resistance (Ohms).
     2. Test resistance between each glow plug wire (from the GPCM harness connector side) and the positive battery terminal (B+). There should be an infinite resistance (open circuit). Any low resistance indicates a short to voltage in the wiring.
     3. Also, check for continuity between each glow plug wire and chassis ground. This should also be infinite.
     4. Inspect for continuity from the GPCM connector to the glow plug connectors for each individual glow plug circuit.
   • Glow Plug Resistance Check (Secondary): While less likely to directly cause a “High Input” code, a shorted glow plug could sometimes affect circuit voltage. Measure the resistance across each individual glow plug (between its terminal and engine ground). Compare readings to factory specifications.
4. Consult Service Information: Always refer to the specific vehicle’s service manual for detailed wiring diagrams, connector pin-outs, and precise diagnostic flowcharts for P1399. These resources are invaluable for accurate diagnosis.

Recommended Repairs and Solutions

Once the root cause of P1399 has been identified through diligent diagnosis, the following repairs are typically recommended:

• Replace the Glow Plug Control Module (GPCM) or Relay: This is a very common failure point for this code. If testing confirms the GPCM is supplying incorrect, unregulated, or constant high voltage, replacement is necessary. Ensure the new GPCM is correctly programmed or adapted to the vehicle, if required by the manufacturer.
• Repair or Replace Damaged Wiring Harness: If a short to voltage or an open circuit is found within the glow plug wiring harness, the damaged section must be repaired or the entire harness replaced. Ensure all repairs are made with appropriate gauge wire, solder, and heat-shrink tubing to prevent future issues. Properly route and secure wiring to prevent chafing.
• Clean or Replace Corroded Connectors: If corrosion is found on the GPCM or glow plug connectors, clean them thoroughly using an electrical contact cleaner and a small brush. If corrosion is severe and has compromised the connection integrity, the connector pins or the entire connector should be replaced.
• Replace Defective Glow Plugs: Although not the primary cause of a “High Input,” if individual glow plugs are found to be shorted or faulty during the diagnostic process, they should be replaced in sets or as recommended by the manufacturer.
• ECM/PCM Replacement/Reprogramming: Only consider this as a last resort, after all other components and wiring have been thoroughly tested and verified to be operating correctly. An ECM/PCM fault is rare for a P1399 but is a possibility. This often requires specialized tools and programming by a dealership or a qualified independent shop.

After performing any repairs, clear the P1399 code and perform several cold start cycles to confirm that the issue is resolved and the code does not return. Monitor the glow plug system’s operation with an OBD-II scanner if possible, observing voltage and current parameters to ensure they are within specification.