Diesel Trouble Codes

P1393 — Glow Plug Circuit Low Input Bank 2 — 7.3 Power Stroke

P1393 on a 7.3L Power Stroke means the PCM sees low current on the bank-2 glow-plug circuit. On this engine bank 2 is the driver side. The code points to an…

Updated October 15, 2026

What it means

Ford enhanced code: the bank 2 glow plug circuit current is lower than expected (an open plug or circuit). On the 7.3L this is usually a burned-out glow plug or a failed under-valve-cover harness (UVCH) connection on bank 2; bank 1 is P1391. Expect hard cold starts and white smoke.1

Most likely causes

  1. Open or burned-out glow plug(s) on bank 2 (driver side). — A failed plug reduces total bank current, triggering low-input detection.1
  2. Oil-soaked or chafed under-valve-cover harness (UVCH) on bank 2. — The UVCH carries glow-plug power under the valve cover; oil intrusion or connector fretting raises resistance or opens the circuit.1
  3. Glow-plug relay or glow-plug control module (GPCM on California-emissions trucks) supply/ground fault affecting bank 2. — Loss of power or ground to the bank-2 side of the glow-plug circuit mimics low current.1
  4. Corroded or loose external glow-plug wiring/connectors on bank 2. — High resistance at the valve-cover pass-through or at the relay/module feed reduces measured current.1

How to diagnose

  1. Verify freeze-frame conditions: cold start, ambient temperature, and whether hard start/white smoke is present.
  2. Inspect glow-plug relay and, on Super Duty, the glow-plug control module for proper power, ground, and activation during the wait-to-start cycle.
  3. Check external glow-plug wiring and connectors on bank 2 for corrosion, oil contamination, or loose terminals.
  4. Remove the valve cover on bank 2 and inspect the UVCH for oil saturation, chafing, backed-out pins, and proper seating at the valve-cover gasket and injector/glow-plug connectors.
  5. Measure glow-plug resistance to ground at the UVCH connector (typical good plug ~0.6–2.0 Ω; open = failed). Compare bank 2 plugs to bank 1.
  6. If a plug tests open, replace the affected glow plug(s) and inspect/replace the UVCH if oil-soaked or damaged; clear codes and retest cold-start behavior.

Specs and test values

CheckNormal readingWhen to measureSource
Battery and charging voltage12.6–12.7 V at rest when fully charged; about 13.5 V runningEngine off, then running with loads onDieselHub
Glow plug resistanceLow — usually no more than about 6 Ω — and similar on every plug; an open reading means a failed plugEngine off, meter lead to engine groundDENSO

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

Tools you’ll need

Rough cost and difficulty

Likely repairPartsShop laborWho usually does it
Wiring or connector repair$10–$1001–4 hrsDIY with tools and a manual
Glow plugs (full set)$80–$2502–6 hrsDIY with tools and a manual
Glow plug relay or control module$60–$4000.5–2 hrsEasy DIY
Engine wiring harness section$100–$5002–5 hrsShop

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 multiple glow plugs are open, the UVCH is brittle/oil-soaked, or the GPCM/relay tests faulty, have a diesel technician confirm harness integrity and perform a controlled current draw test to avoid repeated cold-start damage. Stop diagnosis if high-pressure oil system work is required. High-pressure oil work can cause injury. Do not attempt high-pressure oil system service without proper training and safety equipment.

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

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