P1399 Code: Glow Plug Circuit High Side, High Input
🟠 Moderate severity · drivable
Drivable, but cold starting may be affected and the glow plug system should be repaired soon.
What P1399 means
The high side of the glow plug circuit is reporting voltage that is higher than expected. That can mean a shorted circuit, a faulty relay, or a failing glow plug. The glow plug system may not work correctly, which makes cold starts harder on a diesel engine.
Symptoms
- Check engine light is on
- Hard starting in cold weather
- White smoke at startup
- Glow plug light stays on or flashes
- Rough idle when cold
Common causes
- most common: Faulty glow plug relay or controller
- common: Shorted wiring on the high side of the circuit
- common: Faulty glow plug
- rare: Faulty powertrain control module
How to fix it
| Fix | DIY? | Typical cost |
|---|---|---|
| Test and replace the glow plug relay | Yes | $40–100 |
| Inspect and repair the glow plug wiring | Yes | $50–150 |
| Replace the faulty glow plugs | Yes | $60–150 |
Frequently asked questions
Can I drive with a P1399 code?
Yes, but cold starts may be harder and the glow plug light may act up. Fix it before winter weather arrives.
What does "high side, high input" mean?
The power side of the glow plug circuit is seeing voltage above normal, usually from a faulty relay or a shorted wire.
How much does it cost to fix P1399?
A glow plug relay is $40–$100, and glow plugs are $60–$150 for a set if you do the work yourself.
Related codes
Updated 2026-08. This page is informational and not a substitute for professional diagnosis.