P1391 Code: Glow Plug Circuit Low Input (Bank 1)
🟠 Moderate severity · drivable
Drivable, but cold starts will be rough or difficult because the glow plugs may not be heating.
What P1391 means
The glow plug circuit on bank 1 reports voltage that is too low. Glow plugs heat the combustion chambers so a diesel engine can start in cold weather, so a low-voltage fault means one or more plugs may not be getting power, making cold starts harder.
Symptoms
- Check engine light is on
- Hard starting in cold weather
- White smoke from the exhaust on startup
- Rough idle until the engine warms up
- Glow plug light behavior is abnormal
Common causes
- most common: Faulty glow plug
- common: Damaged wiring or connector at the glow plugs
- common: Failed glow plug relay or controller
- rare: Faulty powertrain control module
How to fix it
| Fix | DIY? | Typical cost |
|---|---|---|
| Test the glow plugs with a multimeter | Yes | $0–20 |
| Replace the faulty glow plugs | Yes | $60–150 |
| Replace the glow plug relay | Yes | $40–100 |
Frequently asked questions
Can I drive with a P1391 code?
Yes, but cold starts will be harder, especially in winter. The engine should still start and run once warm.
What do glow plugs do?
They heat the air inside the cylinders so a diesel engine can start in cold weather. Without them, the engine cranks a long time before firing.
How much does it cost to fix P1391?
Glow plugs cost about $60–$150 for the set if you install them yourself. A new relay is $40–$100.
Updated 2026-08. This page is informational and not a substitute for professional diagnosis.