P1399 Code: Meaning, Symptoms, Causes & Fix

Overview

Symptoms

• check engine light
• rough operation in certain conditions
• reduced performance

Common causes

• Open circuit in wiring affecting glow plug circuit high side, high input
• Disconnected or loose connector
• Connector pin damage/corrosion or harness stress near related components

How to diagnose P1399

1. Confirm P1399 and capture freeze-frame data with a professional scan tool.
2. Inspect wiring, connectors, and grounds related to glow plug circuit high side, high input for obvious defects.
3. Use a continuity test from module pin to component to identify opens/high resistance.
4. Check for related DTCs to identify the root-cause chain before replacing parts.
5. After repair, clear codes and confirm readiness monitors complete without recurrence.

Severity of code P1399

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

Can you drive with P1399?

Most vehicles can be driven short-term with P1399, 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)

1. Repair obvious wiring/connector faults first to prevent repeat parts replacement.
2. Address the root fault mode (open) in the glow plug circuit high side, high input circuit/system.
3. Apply the required relearn/adaptation procedure if specified by service information.
4. Road-test and verify P1399 remains cleared under the conditions that originally set the code.

Fix options

• Repair open circuit and secure connector lock.
• 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 P1399

• 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.

P1399 in common vehicles

P1399 appears across makes such as Mazda, Subaru, Lexus, Jeep. Failure patterns differ by platform: wiring routes, software calibration, and component supplier revisions can change root cause.

Frequently asked questions

What causes P1399?

P1399 is commonly set by open faults affecting glow plug circuit high side, high input.

How much does it cost to fix P1399?

For P1399, repair costs are typically $200-$900 depending on exact root cause, labor rates, and part quality.

Can I clear P1399 without repair?

You can clear P1399 temporarily, but the code usually returns until the underlying fault is corrected.

Helpful tools

• Estimate repair cost
• Decode your VIN
• Calculate fuel cost