P1389

Ford

Glow Plug Circuit High Side Low Input

Definition source: Manufacturer reference catalog
Definition verification: Not independently verified for display.
Time: Varies by diagnosis and vehicle | Diagnostic difficulty: Medium
Medium Severity

The engine computer has detected an input that is too low in the glow-plug circuit’s high-side control path. Glow plugs help warm a diesel engine for starting and early operation. This code indicates a detected circuit condition, not proof that a glow plug or control module needs replacement. Cold starting may be affected, though some vehicles show no obvious symptoms. Exact applicability depends on model, year, and engine.

Report an error with this code

⚠️ Before You Begin (Safety First)

If the engine is difficult to start, avoid repeated prolonged cranking, which can drain the battery and strain the starter. Glow-plug circuits carry substantial current and can become hot; do not probe or disconnect energized components. Stop driving and arrange diagnosis if the engine stalls, runs severely poorly, or warning signs indicate another serious fault.

Common Symptoms

  • The check-engine light may be on because the computer has detected a glow-plug circuit input below the expected level.
  • Cold starting may take longer or become difficult if the glow plugs are not being controlled or powered correctly.
  • The engine may run unevenly just after a cold start, and exhaust smoke may occur while it warms up.
  • There may be no noticeable symptoms, particularly in mild conditions or if the circuit fault is intermittent.

Common Causes

  • A damaged, corroded, loose, or high-resistance connection in the glow-plug high-side circuit can reduce the input seen by the control system.
  • A wiring fault, including a break or an unintended connection to ground, may pull the circuit input low.
  • A power-supply or ground problem affecting the glow-plug control circuit may cause a low reading.
  • A failed glow plug or a fault within the glow-plug control circuitry may contribute, depending on the vehicle's design.
  • Low system voltage or an intermittent electrical fault may affect the circuit reading.

Diagnostic Steps

  1. Check the owner's manual for the correct starting procedure and note whether the problem occurs mainly when the engine is cold.
  2. With the engine off and cool, visually inspect accessible glow-plug wiring and connectors for looseness, corrosion, chafing, or heat damage. Do not disconnect or probe energized components.
  3. Check the battery condition and visible battery connections; weak system voltage or poor connections can complicate electrical diagnosis.
  4. Record the exact code and any related codes before clearing anything. If a scan tool is available, save the available freeze-frame information for a technician.
  5. Do not replace glow plugs or a control module based on this code alone. If wiring damage is not obvious, have the circuit tested using vehicle-specific service information.

When to Call a Professional

Confirm the code and review related codes and available freeze-frame data before replacing parts. Use the vehicle-specific wiring diagram to check the high-side circuit, connectors, power and ground paths, and control-module operation. Compare commanded operation with circuit readings, then test the plugs and wiring as the applicable service information directs. A low input can result from wiring, connections, supply, a plug, or control circuitry; identify the cause before replacing components.

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Commonly Asked Questions

Does P1389 prove that a glow plug is bad?

No. It identifies a low input in the glow-plug circuit's high-side control path. Wiring, connections, supply, a plug, or control circuitry may be involved; testing is needed to distinguish them.

Does “high side” mean the circuit voltage is too high?

No. “High side” describes the powered side of a control circuit. “Low input” describes the detected condition; it does not mean the circuit voltage is too high.

Can I keep driving with this code?

If the engine runs normally, the code alone does not establish that the vehicle is unsafe to drive. Cold starting may be unreliable. Stop and seek help if the engine stalls or runs severely poorly.

Should I replace the glow-plug control module first?

No. Check the circuit and its connections, power and ground paths, plugs, and control operation using the correct information for the vehicle before choosing a part.

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