P1391 Code: Meaning, Symptoms, Causes & Fix

Overview

P1391 is triggered when monitoring logic identifies an issue with Glow Plug Circuit Low Input (Bank #1) affecting powertrain controls.

Symptoms

reduced performance
increased fuel use
check engine light

Common causes

Open circuit in wiring affecting glow plug circuit low input (bank #1)
Broken terminal inside harness
Connector pin damage/corrosion or harness stress near related components

How to diagnose P1391

Confirm P1391 and capture freeze-frame data with a professional scan tool.
Inspect wiring, connectors, and grounds related to glow plug circuit low input (bank #1) for obvious defects.
Use a continuity test from module pin to component to identify opens/high resistance.
Check for related DTCs to identify the root-cause chain before replacing parts.
After repair, clear codes and confirm readiness monitors complete without recurrence.

Severity of code P1391

Moderate: This code is moderate severity: the vehicle may still operate, but fault progression can increase repair cost over time.

Can you drive with P1391?

Most vehicles can be driven short-term with P1391, but postpone heavy load/highway driving until repaired.

If the warning lamp is flashing or drivability/safety is affected, avoid driving and diagnose immediately.

How to fix it (step-by-step)

Repair obvious wiring/connector faults first to prevent repeat parts replacement.
Address the root fault mode (open) in the glow plug circuit low input (bank #1) circuit/system.
Apply the required relearn/adaptation procedure if specified by service information.
Road-test and verify P1391 remains cleared under the conditions that originally set the code.

Fix options

Restore continuity and verify pin tension.
Clear corrosion and protect terminals.
Clear DTCs and verify with live data and completed drive cycle.

Typical repair cost

$200-$900

Common mistakes when diagnosing P1391

Replacing parts without verifying voltage, ground, and signal integrity first.
Ignoring related stored/pending codes that indicate upstream faults.
Skipping post-repair verification drive cycle and readiness checks.

P1391 in common vehicles

P1391 appears across makes such as Ford, Toyota, Honda, Chevrolet. Failure patterns differ by platform: wiring routes, software calibration, and component supplier revisions can change root cause.

Frequently asked questions

What causes P1391?

P1391 is commonly set by open faults affecting glow plug circuit low input (bank #1).

How much does it cost to fix P1391?

For P1391, repair costs are typically $200-$900 depending on exact root cause, labor rates, and part quality.

Can I clear P1391 without repair?

You can clear P1391 temporarily, but the code usually returns until the underlying fault is corrected.