Cylinder 4 Glow Plug Circuit Malfunction
The P0674 code indicates a fault in the glow plug circuit for cylinder 4 in a diesel engine. The glow plug is used to heat the combustion chamber for cold starts. The Powertrain Control Module (PCM) monitors the resistance of the glow plug circuit; if it detects an open or short circuit, or resistance outside the expected range, it sets this code. This can be caused by a faulty glow plug, damaged wiring, or a bad glow plug control module.
1. Most Common Causes
- Open or internally shorted glow plug in cylinder 4
- Damaged wiring or connector between the ECM and the glow plug (short to ground, short to voltage, or open circuit)
- Faulty glow plug relay (if individual relay per cylinder design)
- Failed glow plug control module (in systems with electronic glow plug control)
- Poor connection or corrosion at the glow plug connector
- Internal ECM failure (less common)
2. 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
3. Step-by-Step Diagnosis
Step 1: Use a scan tool to confirm P0674 and check for other related codes.
Step 2: Visually inspect the glow plug wiring and connector for cylinder 4 for damage, corrosion, or loose connections.
Step 3: Measure resistance of the cylinder 4 glow plug using a multimeter (typically 0.5-2 ohms). Compare with other cylinders.
Step 4: If glow plug is faulty, replace it. If wiring is damaged, repair or replace the harness. If all glow plugs and wiring are good, test the glow plug control module/relay.
4. Frequently Asked Questions
Is it safe to drive with this code?▼
You can drive, but starting may be difficult in cold weather. Prolonged driving with a faulty glow plug can cause incomplete combustion, leading to carbon buildup and potential damage to the engine. It's best to repair promptly.
How much does it cost to fix?▼
Replacing a single glow plug typically costs $50-$150 including parts and labor. If the control module or wiring needs repair, costs can be higher.
5. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.