P1391 Glow Plug Circuit Low Input (Bank #1)
What does P1391 mean?
P1391 — Glow Plug Circuit Low Input (Bank #1). This is an OBD-II diagnostic trouble code a scanner retrieves when your vehicle's computer detects glow plug circuit low input (bank #1). The level of risk depends on how the vehicle is currently running and how long you've been driving with the light on.
Common causes of P1391
- Failing ignition coil, spark plug, or spark plug wire
- Faulty crankshaft or camshaft position sensor
- Wiring or connector damage in the ignition circuit
- Low compression or engine mechanical wear
- Incorrect ignition timing or a stretched timing chain
How to fix P1391
These are general steps for this system. The exact cause depends on your vehicle — confirm with live scan data before replacing parts.
- Read misfire counts and freeze frame data with an OBD-II scanner
- Test spark output and inspect spark plugs and coils
- Check crankshaft and camshaft position sensor signals while cranking
- Swap suspect ignition parts between cylinders to see if the fault moves
- Compression test if the issue persists on the same cylinder