Diesel Trouble Codes

P1391 — Glow Plug Circuit Low Input - Bank 1 (7.3 Power Stroke)

Diagnostic reference for 7.3L Power Stroke 1999-2003 Super Duty/Excursion. Code indicates PCM sees low voltage/current on bank-1 glow-plug feedback circuit…

Updated September 2, 2026

What it means

On the 7.3L Power Stroke, P1391 is Glow Plug Circuit Low Input, Bank 1 — the PCM sees low voltage/current on the bank-1 glow-plug feedback circuit (cylinders 1-4 on the passenger side). It is NOT a cylinder-misfire or catalyst-damage code, and the 7.3L has no catalytic converter.1

Most likely causes

  1. Under-Valve-Cover Harness (UVCH) failure on bank 1 — oil-soaked, chafed or connector/pigtail damage causing low input to glow plugs on that bank — UVCH connectors and pigtails feed injectors and glow plugs under each valve cover and are a known failure point. A failed UVCH commonly sets injector and glow-plug circuit codes such as P1391/P1393/P1394 and a rough miss on one bank.1
  2. Open or shorted glow plug(s) on bank 1 (cylinders 1-4) — The PCM detects low electrical current/voltage in the glow-plug heater circuit for the bank. Glow plugs with relay/module are used for cold start; P0380/P1391/P0381 relate to glow-plug circuit faults.12
  3. Glow-plug relay or Glow Plug Control Module (GPCM) fault affecting bank 1 — On Super Duty the glow-plug control module and relay supply the glow plugs. A relay/GPCM fault can produce low input on the bank feedback circuit.12
  4. Wiring/connector corrosion or high resistance in bank-1 glow-plug supply/return — Oil contamination and heat under the valve cover degrade the UVCH and external glow-plug wiring, leading to low input readings by the PCM.1

How to diagnose

  1. Verify code and freeze frame; confirm P1391 is present and note engine temperature at set.
  2. Inspect glow-plug relay and GPCM power/ground and control signals; check for battery voltage at relay output during glow cycle.
  3. Perform resistance/continuity check of bank-1 glow-plug circuit from GPCM/relay to each glow plug; compare bank 1 vs bank 2 values.
  4. Remove valve cover on bank 1 and inspect UVCH for oil saturation, chafing, melted/retracted pins, and connector seating; look for oil in glow-plug wells.
  5. Measure individual glow-plug resistance to ground and check for opens/shorts; isolate a faulty plug if present.
  6. Check PCM feedback wiring for bank-1 glow-plug circuit for opens/shorts to ground or voltage; verify harness integrity outside valve cover.
  7. Clear codes, command glow cycle with scan tool, and monitor glow-plug feedback PID for bank 1 to confirm low input is resolved.

Parts and tools you may need

Rough cost and difficulty

Parts cost varies by region and part quality; labor is higher if valve cover removal and UVCH replacement is required.

When to see a pro

If diagnosis requires removal of high-pressure oil components, internal engine access, or PCM reprogramming, have a qualified diesel technician perform the work.

Sources

  1. Curated DTC seed — 7.3 Power Stroke (curated) ↩︎ ↩︎ ↩︎ ↩︎ ↩︎

  2. Curated DTC seed — 7.3 Power Stroke (curated) ↩︎ ↩︎