P0673 Code: Cylinder 3 Glow Plug Circuit Malfunction
The P0673 diagnostic trouble code is specific to diesel engines and signals that the engine control module (ECM) has detected a malfunction in the glow plug circuit for cylinder 3. Glow plugs are heating elements that warm the combustion chamber before engine startup in cold conditions, allowing diesel fuel to ignite more easily. When this code appears, it means the glow plug system for cylinder 3 isn’t functioning properly, which can make cold starts difficult and trigger a check engine light.
What Does P0673 Mean?
P0673 is a manufacturer-specific code that breaks down as follows:
- P = Powertrain code
- 0 = Generic OBD-II code
- 6 = Ignition system or glow plug circuit
- 73 = Cylinder 3 glow plug circuit malfunction
The code is triggered when the ECM detects an electrical fault in the glow plug circuit for cylinder 3. This could mean the glow plug itself has failed, the wiring is damaged or corroded, the relay is faulty, or the control module isn’t receiving the proper voltage signal. Unlike spark plugs in gasoline engines, glow plugs in diesel engines are critical for cold-start performance and engine efficiency.
Common Symptoms
- Check Engine Light: The most obvious indicator that P0673 has been stored
- Difficult Cold Starts: Engine cranks but takes longer to start, especially in cold weather
- Rough Idle After Cold Start: Engine may run rough or misfire until it warms up
- White Smoke on Startup: Unburned diesel fuel creates visible white smoke, particularly in cold conditions
- Reduced Fuel Economy: Engine compensates for poor combustion with less efficiency
- Hesitation During Acceleration: Cylinder 3 may not fire properly under load
- No Symptoms in Warm Weather: Code may not trigger or symptoms may be minimal when starting in warm conditions
Possible Causes (Ranked by Frequency)
- Failed Glow Plug (Most Common): The glow plug for cylinder 3 has burned out or lost its heating element. This is the most frequent cause and typically requires replacement.
- Corroded or Damaged Wiring: The electrical connector or wiring harness connected to the cylinder 3 glow plug may be corroded, loose, or broken, preventing proper voltage delivery.
- Faulty Glow Plug Relay: The relay that controls power to the glow plugs may have failed, cutting off voltage to cylinder 3’s glow plug.
- Engine Control Module (ECM) Fault: The ECM may have a software glitch or internal fault preventing it from properly controlling the glow plug circuit.
- Poor Ground Connection: A loose or corroded ground wire can prevent the glow plug circuit from completing properly.
- Battery Voltage Issues: Low battery voltage or a weak battery may prevent the glow plug from reaching proper operating temperature.
- Faulty Glow Plug Control Module: Some vehicles have a dedicated glow plug control module separate from the main ECM that may require replacement.
Diagnostic Steps
Step 1: Scan for Additional Codes
Use a diesel-compatible OBD-II scanner to retrieve all stored and pending codes. Other glow plug codes (P0670-P0676) may indicate a broader system issue rather than just cylinder 3.
Step 2: Visual Inspection
Inspect the cylinder 3 glow plug connector and wiring harness for:
- Corrosion or green/white oxidation on connectors
- Loose or disconnected wires
- Damaged insulation or burnt wiring
- Water intrusion or moisture in connectors
Step 3: Check Battery Voltage
Verify that the battery is in good condition and providing adequate voltage (typically 12.6V at rest, 13.5-14.5V while running). Low battery voltage can cause glow plug circuit faults.
Step 4: Test the Glow Plug Relay
Locate the glow plug relay (usually in the fuse box) and test it with a multimeter or swap it with an identical relay from another circuit to see if the code clears. A clicking sound when the relay is powered indicates it’s functioning.
Step 5: Measure Glow Plug Resistance
Disconnect the cylinder 3 glow plug and measure its resistance with a multimeter. A healthy glow plug typically reads 0.5-2 ohms. An open circuit (infinite resistance) indicates a failed glow plug.
Step 6: Test Voltage at the Glow Plug
With the ignition in the “on” position (before cranking), measure the voltage at the cylinder 3 glow plug connector. You should see approximately 12 volts during the glow plug preheat cycle (usually 3-10 seconds before starting). No voltage indicates a wiring, relay, or control module issue.
Step 7: Check Ground Connections
Verify that the glow plug has a solid ground connection. Test continuity between the glow plug body and the engine block using a multimeter.
Step 8: Professional Diagnostics
If the above steps don’t identify the issue, use a professional diesel diagnostic scanner to monitor glow plug control parameters in real-time and check for ECM or glow plug module faults.
Repair Cost Estimates
The cost to repair P0673 varies depending on the underlying cause:
- Glow Plug Replacement: $50-$200 per plug, plus $100-$300 labor. Most common repair.
- Wiring Repair or Connector Replacement: $75-$250 in parts and labor, depending on accessibility.
- Glow Plug Relay Replacement: $30-$100 for the relay, plus $50-$150 labor.
- Glow Plug Control Module Replacement: $200-$600 for the module, plus $100-$200 labor.
- ECM Reprogramming or Replacement: $300-$1,200+ if the main engine control module requires service.
Total Average Repair Cost: $150-$500 for most P0673 repairs, with glow plug replacement being the most common and least expensive fix.
Can I Still Drive?
Severity: Low to Moderate
You can typically continue driving with P0673 stored, but with some caveats:
- Cold Weather Driving: Avoid prolonged cold-weather operation. The vehicle will be difficult to start and may produce white smoke. Extended driving in freezing temperatures could damage the engine.
- Warm Weather Driving: In mild or warm conditions, the vehicle will likely start and run normally, though fuel economy may suffer slightly.
- Avoid Extended Idling: Don’t let the engine idle for long periods, as cylinder 3 may not be firing properly.
- Schedule Repair Soon: While not an emergency, repair this issue within a week or two to avoid potential engine damage and ensure reliable cold starts.
- Check for Additional Codes: If multiple glow plug codes appear (P0670-P0676), the issue is more serious and the vehicle should be serviced immediately.
Frequently Asked Questions
Q: Can a bad glow plug damage my diesel engine?
A: A single failed glow plug won’t immediately damage the engine, but it can cause incomplete combustion in that cylinder, leading to reduced power, increased emissions, and slightly higher fuel consumption. Over time, this can contribute to carbon buildup. It’s best to replace the faulty glow plug promptly to maintain engine health and performance.
Q: Why do glow plugs fail so often?
A: Glow plugs are heating elements that cycle on and off thousands of times during the life of a diesel engine. They’re exposed to extreme heat, vibration, and electrical stress. Most glow plugs last 80,000-100,000 miles before failing. Environmental factors like moisture, salt, and corrosion can accelerate failure, especially in harsh climates.
Q: Can I replace just the cylinder 3 glow plug, or should I replace all of them?
A: You can replace just the faulty glow plug if only P0673 is present. However, if your vehicle has high mileage (over 80,000 miles) or multiple glow plug codes appear, it’s often more cost-effective to replace all glow plugs at once. This prevents repeat failures and ensures consistent cold-start performance across all cylinders.
Q: Will clearing the code fix the problem?
A: No. Clearing the code with a scanner will temporarily turn off the check engine light, but the underlying problem remains. The code will return after a few cold-start cycles. The glow plug circuit must be physically repaired—either by replacing the glow plug, fixing wiring, or replacing the relay—for a permanent fix.