P1397

Glow Plug Circuit High Input (Bank #1)

Powertrain Ignition System Glow Plug Circuit 🟡 Moderate — Fix within a week ⚠️ Drive with Care
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What This Actually Means

In plain language — no jargon

The glow plug heating circuit is receiving too much electrical current, like an overloaded outlet trying to power something too demanding. This typically means a wiring short, failed relay, or shorted glow plug element in bank #1 of a diesel engine.

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Symptoms You May Notice

3 known symptoms for this code
Difficulty starting in cold weather
Engine misfire or rough idle after cold start
Glow plug warning light illuminated on dashboard
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How Your ECU Detects This

Technical sensor logic and voltage thresholds

The ECU monitors the voltage and current draw through the glow plug circuit. When the circuit draws excessive current (above threshold), the ECU detects a short or failed component and sets the fault. The sensor measures voltage feedback from the glow plug relay control signal.

Voltage & Parameter Thresholds

ParameterNormal RangeFault Condition
Glow Plug Circuit Current 0.5–3.5 amps during warm-up cycle >4.5 amps or sustained high draw
Control Signal Voltage 0–5V modulated signal Constant high voltage or shorted-low resistance path
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Diagnostic & DIY Fix Guide

Check these in order — from cheapest to most complex
1
Wiring harness and connectors
Inspect glow plug circuit wiring for damage, corrosion, or exposed conductors causing a short to ground; repair or replace damaged sections.
2
Glow plug relay
Test relay with a multimeter for continuity and replace if stuck closed or failed, as this is a common cause of high input voltage.
3
Glow plugs (Bank #1)
Remove and inspect glow plugs for internal shorts or cracks; replace any failed plugs found during testing.
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When to See a Professional Mechanic

Not all fault codes are safe to DIY

Code P1397 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.

Safety note: OBD-II codes identify the system or circuit where a fault was detected — they do not always identify the exact failed component. A professional mechanic using live sensor data will diagnose the root cause more accurately than replacing parts based on the code alone.
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How to Clear Code P1397

What happens after you fix the fault

Once the fault is repaired, P1397 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.

The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.

✅ Safe to Clear When
  • Fault has been diagnosed and repaired
  • You want to confirm the repair worked
  • Code appeared after a sensor was cleaned
⚠️ Do Not Clear When
  • Preparing for an emissions/PUC test
  • Root cause is still undiagnosed
  • Check engine light is flashing
Emissions test note: Clearing codes resets OBD readiness monitors. Most vehicles need 50–100 km of mixed driving before monitors complete. Do not clear codes immediately before an emissions or PUC inspection.