Failed glow plug with an internal short to ground on bank 1; Chafed or grounded glow plug harness wiring near the cylinder head or exhaust manifold; Corroded or loose glow plug connector at one or more cylinders; Faulty glow plug control module or relay stuck in the on position; Moisture intrusion into the glow plug circuit connector causing a low-resistance path; PCM internal driver fault on the glow plug control circuit (rare)

DTC codes / Ford codes / P1391

P1391 · FordGlow Plug Circuit Low Input Bank 1

Q: What does code P1391 mean?

Glow Plug Circuit Low Input Bank 1. This code means the powertrain control module (PCM) has detected an abnormally low voltage on the glow plug circuit for bank 1, which is the set of glow plugs on one side of the engine. The PCM expects to see a specific voltage when the glow plug relay is commanded on, and a reading below the expected threshold indicates a short to ground, a failed glow plug, or a wiring fault.

This code means the powertrain control module (PCM) has detected an abnormally low voltage on the glow plug circuit for bank 1, which is the set of glow plugs on one side of the engine. The PCM expects to see a specific voltage when the glow plug relay is commanded on, and a reading below the expected threshold indicates a short to ground, a failed glow plug, or a wiring fault.

SystemPowertrain

Code typeFord specific

SeverityMedium

Fix difficultyIntermediate

AreaIgnition system and misfire

Symptoms you may notice

Illuminated check engine light or glow plug warning light
Hard cold starting or extended cranking before the engine fires
Rough idle and white smoke immediately after a cold start
Reduced engine performance and poor fuel economy during warm-up
Often no symptoms besides the warning light, especially in mild weather

Common causes, most likely first

Failed glow plug with an internal short to ground on bank 1
Chafed or grounded glow plug harness wiring near the cylinder head or exhaust manifold
Corroded or loose glow plug connector at one or more cylinders
Faulty glow plug control module or relay stuck in the on position
Moisture intrusion into the glow plug circuit connector causing a low-resistance path
PCM internal driver fault on the glow plug control circuit (rare)

How to diagnose and fix it

Scan all modules and record freeze-frame data to confirm the code is current and not history
Visually inspect the bank 1 glow plug harness for chafing, corrosion, or loose connectors
Measure resistance of each glow plug on bank 1 and compare to the manufacturer's specification
Disconnect the glow plug control module and check for a short to ground on the feed circuit
Test the glow plug relay or control module for proper operation and command from the PCM
Repair or replace any damaged wiring, connectors, or glow plugs found during testing
Clear the code and perform a cold-start verification to confirm the repair

What Ford techs check before glow plug replacement

A single shorted glow plug can drag the entire bank 1 circuit low, so testing each plug individually prevents replacing a full set unnecessarily.
The glow plug harness on Ford diesel engines often rub through on the intake manifold or EGR cooler bracket, creating a short that only appears when the engine is cold and vibrating.
This code does not always mean the glow plugs themselves are bad; a stuck glow plug relay can keep voltage low and set the same fault.
Aftermarket glow plugs with incorrect resistance values are a common cause of repeat P1391 codes on Ford vehicles, so use OEM-spec parts.
A weak battery or poor ground connection at the engine block can mimic a glow plug circuit fault by pulling system voltage down during the glow plug on-time.

Can I keep driving with P1391?

You can drive the vehicle, but cold starting will be difficult and prolonged cranking can damage the starter and battery. Avoid heavy loads and long idling until the fault is repaired, and do not rely on the vehicle for cold-weather operation.

Tools that help

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Guidance is general and written for Ford vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.