P1389

What Does Code P1389 Mean?

DTC P1389, which translates to “Glow Plug Circuit High Side Low Input,” indicates that the Powertrain Control Module (PCM), also referred to as the Engine Control Module (ECM), has detected an abnormal voltage condition within the glow plug control circuit. Specifically, the PCM monitors the voltage on the “high side” of the glow plug circuit, which typically refers to the main battery voltage supply (B+) to the glow plug relay or module, or the voltage supplied to the glow plugs themselves after passing through the control module. A “low input” signifies that the PCM is observing a voltage significantly lower than expected on this critical supply path when the glow plugs are commanded active, or when it expects to see full battery voltage. This condition prevents the glow plugs from receiving adequate power to heat properly, directly impacting the engine’s ability to start and run smoothly in cold conditions. The PCM identifies this fault by comparing the sensed voltage against a pre-programmed threshold. If the voltage remains below this threshold for a calibrated period during glow plug operation, P1389 is set.

Common Symptoms

• Difficulty Starting a Cold Diesel Engine: The most prominent symptom, as glow plugs are essential for igniting fuel in cold cylinders.
• Rough Idle After Cold Start: Engine may run erratically, misfire, or produce excessive smoke until operating temperature is reached.
• White Smoke from Exhaust During Cold Start: Unburnt fuel due to insufficient combustion heat can lead to visible white smoke.
• Increased Exhaust Emissions: Incomplete combustion due to cold cylinders contributes to higher levels of pollutants.
• Reduced Engine Performance During Warm-Up: Engine may lack power or respond poorly until fully warmed.
• Check Engine Light (CEL) or Malfunction Indicator Lamp (MIL) Illumination: The primary indication that a fault has been detected by the PCM.

What Causes the Code P1389?

• Faulty Glow Plug Relay or Control Module: This is the most prevalent cause. The relay or module may be unable to effectively switch or deliver full battery voltage to the glow plug circuit, or its internal monitoring circuit may be defective.
• Open Circuit or High Resistance in Wiring: Damage, corrosion, or a loose connection in the main power supply wire leading to the glow plug relay/module, or in the high-side output wiring from the module to the glow plugs.
• Blown Fuse or Fusible Link: A protective fuse or fusible link for the glow plug circuit’s high-side power supply may have failed, interrupting the voltage supply.
• Corroded or Damaged Connectors: Poor electrical contact at the glow plug relay/module connector, or the main power feed connector, can lead to voltage drop.
• Defective PCM (Powertrain Control Module): While rare, an internal fault within the PCM’s glow plug driver circuit or voltage monitoring system could cause an erroneous low input reading or failure to properly command the relay.
• Short to Ground in the High-Side Circuit: A short in the wiring between the relay/module and the glow plugs could pull the circuit voltage low, although this often manifests as a blown fuse first.

How to Diagnose and Troubleshoot

Diagnosing P1389 requires methodical electrical testing using a digital multimeter (DMM) and an OBD-II scanner with bi-directional control capabilities.

1. Verify DTC and Gather Freeze Frame Data: Connect an OBD-II scanner to confirm P1389. Record any accompanying codes. Analyze freeze frame data to understand engine conditions (temperature, RPM, load) when the fault occurred, which provides context. Clear the code and attempt to replicate the fault, preferably during a cold start cycle.
2. Perform a Thorough Visual Inspection:
   • Inspect all wiring harnesses and connectors associated with the glow plug system, including the main power feed, ground straps, and connections at the glow plug relay/module. Look for signs of chafing, corrosion, melted insulation, or loose terminals.
   • Locate and inspect all fuses and fusible links related to the glow plug circuit. Refer to the vehicle’s service manual for exact locations and ratings. Check for continuity using a DMM if visual inspection is inconclusive.
   • Examine the glow plug relay/module itself for external signs of damage, burning, or corrosion.
3. Test Main Power Supply to Glow Plug Relay/Module:
   • Identify the main B+ (battery positive) input wire to the glow plug relay or module. This is typically a large gauge wire.
   • With the ignition on (and glow plugs commanded active by temperature, or using a bi-directional scan tool), measure the voltage between this input terminal and chassis ground using a DMM. Expect near battery voltage (e.g., 12.0V-14.5V).
   • Perform a voltage drop test: Measure the voltage between the positive battery post and the B+ input terminal of the glow plug relay/module. A reading greater than 0.2 volts indicates excessive resistance in the supply wiring or fuse.
4. Test Glow Plug Relay/Module Operation and Output:
   • Using a bi-directional scan tool, command the glow plug relay/module ON. Listen for an audible click from the relay. If no click, verify the control signal (usually a ground or voltage signal) from the PCM to the relay, and ensure the relay has a proper power and ground supply.
   • While the relay/module is commanded ON (or during a cold pre-heat cycle), measure the voltage at the output terminal(s) of the glow plug relay/module that feed the individual glow plugs. Expect to see full battery voltage. A significantly lower voltage (e.g., 5-8V) or no voltage indicates a faulty relay/module or a problem with its internal power supply.
   • Verify the ground integrity of the glow plug relay/module by measuring resistance between its ground terminal and the battery negative post. Resistance should be less than 0.5 ohms.
5. Inspect Glow Plug Harness and Connections:
   • Disconnect the main harness leading to the glow plugs. Inspect the connector for corrosion or bent pins.
   • While P1389 primarily points to the supply side, excessive current draw due to a shorted glow plug could sometimes trigger this code if the module’s protection circuit limits voltage. Test the resistance of individual glow plugs. Disconnect each glow plug and measure its resistance between the terminal and engine ground. Consult service information for specifications (typically 0.5 to 2.0 ohms). An open circuit (OL) indicates a failed glow plug. A very low resistance (near 0 ohms) could indicate a short.
6. PCM Output Signal Verification (Advanced): If all previous tests indicate proper wiring, fuses, and relay/module functionality, but the relay is still not activating, it may point to a PCM issue. Use an oscilloscope to verify the control signal from the PCM to the glow plug relay/module. Check for correct voltage levels and pulse width modulation (if applicable) according to manufacturer specifications. This step requires advanced diagnostic tools and expertise.

Recommended Repairs and Solutions

Once the diagnostic steps have identified the root cause, the following repairs are typically recommended:

• Replace Blown Fuses or Fusible Links: If a fuse is found blown, replace it with one of the correct amperage. It is crucial to investigate why the fuse blew (e.g., a short circuit) to prevent immediate recurrence. Address any underlying short before fuse replacement.
• Repair or Replace Damaged Wiring and Connectors: If visual inspection or voltage drop tests reveal high resistance or an open circuit in the wiring, repair or replace the affected section. Ensure all connections are clean, tight, and protected from environmental factors. Use appropriate repair techniques, such as soldering and heat-shrink tubing for wiring, and dielectric grease for connectors.
• Replace the Glow Plug Relay or Control Module: This is the most common repair for P1389. If the relay/module fails to activate or consistently outputs low voltage when commanded ON, it is defective and requires replacement. Always use an OEM-quality or equivalent component to ensure proper function and compatibility with the PCM.
• Replace Faulty Glow Plugs: While less common as a direct cause for P1389 (which points to the supply side), if individual glow plugs are found to be shorted or causing excessive current draw on the circuit, they should be replaced. If one glow plug is faulty, it is often advisable to replace all glow plugs as a set, especially if they have similar age and mileage, to ensure even heating and prevent future issues.
• PCM Replacement or Reprogramming: This should only be considered as a last resort after all other components and wiring have been thoroughly tested and verified to be in good working order. A faulty PCM driver circuit can lead to P1389. If a PCM replacement is necessary, it often requires programming to the specific vehicle.

Mechanic’s Tip: After any repair, clear the DTCs with the OBD-II scanner and perform several cold starts to ensure the issue is resolved and the code does not return. Always consult the specific vehicle’s factory service manual for detailed wiring diagrams, component locations, and diagnostic procedures.