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…
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
- 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
- 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
- Glow-plug relay or Glow Plug Control Module (GPCM) fault affecting bank 1 — Federal-emissions trucks use a PCM-commanded glow plug relay; 1999.5+ California-emissions trucks use a glow plug control module (GPCM). A relay/GPCM fault can produce low input on the bank feedback circuit.12
- 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
- Verify code and freeze frame; confirm P1391 is present and note engine temperature at set.
- Inspect glow-plug relay and GPCM power/ground and control signals; check for battery voltage at relay output during glow cycle.
- Perform resistance/continuity check of bank-1 glow-plug circuit from GPCM/relay to each glow plug; compare bank 1 vs bank 2 values.
- 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.
- Measure individual glow-plug resistance to ground and check for opens/shorts; isolate a faulty plug if present.
- Check PCM feedback wiring for bank-1 glow-plug circuit for opens/shorts to ground or voltage; verify harness integrity outside valve cover.
- Clear codes, command glow cycle with scan tool, and monitor glow-plug feedback PID for bank 1 to confirm low input is resolved.
Specs and test values
| Check | Normal reading | When to measure | Source |
|---|---|---|---|
| Battery and charging voltage | 12.6–12.7 V at rest when fully charged; about 13.5 V running | Engine off, then running with loads on | DieselHub |
| Glow plug resistance | Low — usually no more than about 6 Ω — and similar on every plug; an open reading means a failed plug | Engine off, meter lead to engine ground | DENSO |
Common mistake to avoid
Replacing glow plugs or the glow module before checking the batteries. The glow system draws heavily, and low battery voltage causes circuit faults that no part swap will fix. Source: FleetRabbit — P0670 glow plug control module fault.
Parts and tools you may need
- Digital multimeter
- Scan tool with Ford enhanced PIDs
- Glow-plug relay test light
- Torque wrench for valve cover
- Replacement UVCH for bank 1
- Dielectric grease
- Safety glasses and gloves
Tools you’ll need
- digital multimeter — Measures resistance of each bank-1 glow plug and continuity through the UVCH to find opens, shorts, or high resistance causing low input.
- OBD-II scan tool — Reads P1391, freeze frame, and monitors glow-plug feedback PID while commanding a glow cycle to confirm bank-1 low input.
- test light — Quickly verifies battery voltage at the glow-plug relay output and GPCM supply during the glow cycle.
- inspection mirror — Lets you see UVCH connector seating, chafing, and oil in glow-plug wells under the valve cover without full disassembly.
Rough cost and difficulty
| Likely repair | Parts | Shop labor | Who usually does it |
|---|---|---|---|
| Wiring or connector repair | $10–$100 | 1–4 hrs | DIY with tools and a manual |
| Glow plugs (full set) | $80–$250 | 2–6 hrs | DIY with tools and a manual |
| Injector wiring harness / under-valve-cover harness | $50–$300 | 2–5 hrs | Shop |
| Glow plug relay or control module | $60–$400 | 0.5–2 hrs | Easy DIY |
Typical US independent-shop figures; labor runs about $120–$180 per hour (dealers charge more). Confirm the diagnosis before buying parts.
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.
Check for recalls
Recalls are tied to your truck’s VIN, so check yours directly: enter the VIN at NHTSA’s recall lookup. Open safety recalls are repaired free at a dealer.
Sources
Repair videos: P1391 on the 7.3 Power Stroke
Third-party videos, not ours — picked for this engine. Confirm anything against the factory service information for your truck.
Spotted an error, or something missing? Tell us
Run a 7.3 Power Stroke? This goes straight to the editor — it is not published.