P1391 — Glow Plug Circuit Low, Cylinder 1 (Diesel)
What P1391 means
The diesel engine control unit monitors current and voltage in the cylinder 1 glow plug circuit. Glow plugs warm the combustion chamber to help a cold engine start. P1391 sets when the signal in that circuit stays below the expected range, which can come from an open circuit, a short to ground, a failed plug or a glow plug module fault. The exact definition varies by manufacturer.
Symptoms
- Hard starting when the engine is cold
- White smoke on cold start
- Check Engine light on
- Rough running right after startup
- Higher fuel use until the engine warms up
Possible causes
- Failed or burned-out glow plug on cylinder 1
- Short to ground or damaged wiring in the glow plug circuit
- Corroded or loose connector terminals
- Faulty glow plug control module or relay
- Weak battery causing low supply voltage
- ECU fault (rare)
How to diagnose
- Scan all codes and freeze frame data and look for other glow plug related codes
- Inspect the cylinder 1 connector and harness for chafing, corrosion or heat damage
- Check battery voltage and terminal condition
- Measure glow plug resistance with a multimeter and compare it with the other cylinders
- Test the circuit for a short to ground with the connector unplugged
- Verify power and operation of the glow plug control module using the factory wiring diagram
- Run a scan tool glow plug test if available and compare current by cylinder
Common fixes
- Replace the cylinder 1 glow plug (ideally the full set)
- Repair damaged wiring or replace the connector
- Replace the glow plug control module
- Charge or replace the battery
- Clear the code and recheck after a cold start
P1391 on different brands
- Ford · manuals
- On Ford, this code means low input in the glow plug circuit (bank 1): the module sees voltage below the expected range.
- Honda · manuals
- Honda lists this code as a G-sensor performance fault, meaning an implausible signal rather than a glow plug problem.
- Suzuki · manuals
- Suzuki uses it for a G sensor rough road rationality check, not for glow plugs.
- VAG
- For Audi/VAG the notes describe a camshaft position sensor 2 (bank 2) short to ground, so confirm the definition for your specific model.
Questions and answers
What does P1391 mean on a diesel?
Most often it is a low signal in the cylinder 1 glow plug circuit, caused by the plug, wiring or glow plug module. Some brands define the code differently.
Can I drive with P1391?
Yes, for a short time. In summer you may barely notice it, but in cold weather the engine will start poorly and smoke. Don't put the check off for long.
Is replacing one glow plug enough?
If that plug is bad, replacing the whole set is wise since they wear at a similar rate. First make sure the wiring and control module are healthy.
This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.