P1391

Ford

Glow Plug Circuit Low Input (Bank #1)

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

On an applicable Ford diesel, the glow plug system helps warm the engine for starting and initial running when cold. P1391 indicates the control system detected a low input in the Bank #1 glow plug circuit. That points to an electrical condition, not proof that a glow plug itself has failed. Starting and cold running may be affected, though some vehicles show no obvious symptoms.

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⚠️ Before You Begin (Safety First)

A glow plug circuit can carry high current, and engine components may be hot. Do not probe energized wiring or attempt tests that require bypassing circuit protections. If the engine stalls, runs severely unevenly, or produces heavy smoke, stop driving and arrange diagnosis.

Common Symptoms

  • Longer or difficult cold starts if the glow plug system is not warming the engine as intended.
  • Rough running shortly after a cold start, because cold combustion may be less consistent.
  • Increased exhaust smoke during cold start or warm-up may occur, depending on the engine and fault.
  • The warning light may be on, with or without noticeable drivability symptoms.
  • No obvious symptoms are possible, particularly when the engine is warm or conditions do not require much glow-plug assistance.

Common Causes

  • A glow plug or other component in the Bank #1 circuit may have an electrical fault.
  • Damaged, corroded, loose or poorly connected wiring or terminals may lower the circuit input.
  • A fault in the circuit's power supply, ground, fuse or related connection may affect input.
  • A glow plug control component or its wiring may not be operating as intended.
  • An intermittent connection or harness fault may cause the condition to appear only at certain times.

Diagnostic Steps

  1. Check whether the engine is difficult to start or runs differently when cold; note when the symptoms occur.
  2. With the engine off and cool, visually inspect accessible Bank #1 glow-plug wiring and connectors for damage, looseness or corrosion. Do not pull on wiring or disconnect components unless the vehicle service information permits it.
  3. Check the owner's manual for applicable glow-plug circuit fuse information and inspect only as directed. Replace a fuse only with the specified type and rating; if it fails again, stop and have the circuit diagnosed.
  4. Record any other diagnostic codes and available scan information before clearing codes, since related faults can help narrow the cause.
  5. If visual checks do not identify an issue, have a qualified technician test the circuit and control components using vehicle-specific service information. Avoid probing energized, high-current wiring.

When to Call a Professional

Confirm the code and check for related codes and scan data before replacing parts. Use model-, year- and engine-specific service information to identify the Bank #1 circuit. Inspect its wiring, connectors, power and ground, then test the circuit and control components using the applicable procedure. A low input can result from a connection or supply problem as well as a failed component.

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Commonly Asked Questions

Does P1391 prove that a glow plug needs replacement?

No. It reports a low input in the Bank #1 glow plug circuit. Wiring, connections, power supply, ground or a control component can also cause that condition.

What does Bank #1 identify?

It identifies the engine bank specified by the code. The exact component layout and circuit access depend on the vehicle's year and engine; do not assume a location without service information.

Can the vehicle still start with this code?

It may start, especially when warm, but cold starting or initial running can be affected. The result depends on the engine and the extent of the circuit fault.

Can I test the glow plugs with a basic meter?

Some checks may be possible, but the correct method and interpretation are vehicle-specific. Glow plug circuits can carry high current, so avoid energized testing unless you have the right equipment and procedure.

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