Glow Plug Circuit Low Input Bank #1 (Right)
The P1391 code indicates that the engine control module (ECM) has detected a low voltage or resistance in the glow plug circuit for bank #1 (right side of the engine). This typically occurs in diesel engines where glow plugs are used to aid cold starts. The low input suggests a fault such as a failed glow plug, open or shorted wiring, or a bad relay. This can lead to hard starting, especially in cold weather, and increased emissions.
01. Most Common Causes
- Failed glow plug(s) in bank #1
- Open or shorted wiring in the glow plug circuit
- Faulty glow plug relay or fuse
- Corroded or loose connectors
02. Symptoms You Will Notice
- Hard starting, especially in cold weather
- Check Engine Light illuminated
- Rough idle or misfire on cold start
- Excessive smoke from exhaust during startup
03. Step-by-Step Diagnosis
Step 1: Scan for codes and record freeze frame data.
Step 2: Visually inspect glow plug wiring and connectors for damage or corrosion.
Step 3: Test glow plug resistance with a multimeter (typically 0.5-2 ohms).
Step 4: Check glow plug relay and fuse operation.
Step 5: If all else is fine, replace the glow plug(s) in bank #1.
04. Commonly Affected Makes
05. Frequently Asked Questions
Is it safe to drive with this code?▼
You can drive, but starting may be difficult, especially in cold weather. It's best to repair promptly to avoid further issues.
How much does it cost to fix?▼
Typically $50-$150 for parts and labor, depending on the number of glow plugs replaced and labor rates.