P0673 Code: Complete Guide to Cylinder 3 Glow Plug Circuit
Everything you need to know about diagnosis, repair costs, and prevention
Understanding P0673 Code
P0673 is an OBD-II diagnostic trouble code that indicates a malfunction in the glow plug circuit for cylinder 3. This code specifically relates to diesel engines where glow plugs are essential for cold starting by preheating the combustion chamber.
Technical Definition: The Engine Control Module (ECM) has detected an electrical issue with the glow plug or its circuit in cylinder 3. This could be an open circuit, short circuit, or resistance value outside the expected range.
How Glow Plugs Work
Glow plugs are heating devices used in diesel engines to aid starting in cold conditions. Unlike spark plugs in gasoline engines that create a spark to ignite the fuel-air mixture, glow plugs heat up to help ignite the diesel fuel through compression ignition.
| Component | Function | Common Issues |
|---|---|---|
| Glow Plug | Heats combustion chamber for cold starts | Burnout, carbon buildup, cracking |
| Glow Plug Control Module | Controls power to glow plugs | Internal failure, connection issues |
| Wiring Harness | Connects glow plugs to power source | Corrosion, damage, loose connections |
| Engine Control Module (ECM) | Monitors glow plug operation | Software issues, sensor faults |
Symptoms of P0673 Code
When your vehicle has a P0673 code, you may experience one or more of the following symptoms:
- Illuminated Check Engine Light – The most common and obvious symptom
- Hard starting in cold weather – Extended cranking time, especially in temperatures below 40°F (4°C)
- Rough idle – Engine may shake or vibrate excessively at idle due to incomplete combustion in cylinder 3
- Excessive white smoke from exhaust – Particularly during cold starts, indicating unburned fuel
- Reduced engine power – Vehicle may feel sluggish or lack acceleration, especially under load
- Decreased fuel economy – Up to 10-15% reduction in fuel efficiency
- Engine misfire – Especially noticeable in cylinder 3, may trigger additional codes
- Glow plug indicator light flashing – On some vehicles, the glow plug light may flash indicating a fault
Important: Continuing to drive with a P0673 code can lead to more severe engine problems over time, including potential damage to the catalytic converter, increased emissions, and in extreme cases, engine damage from incomplete combustion.
Common Causes of P0673 Code
The P0673 code can be triggered by several issues related to the glow plug system. Here are the most common causes:
- Faulty glow plug in cylinder 3 – The most common cause, typically due to normal wear or carbon buildup
- Damaged wiring or connectors – Frayed wires, corrosion, or loose connections in the glow plug circuit
- Failed glow plug control module – The module that controls power to the glow plugs may be malfunctioning
- Blown fuse – A fuse specifically for the glow plug system may be blown
- Poor electrical ground – Corroded or loose ground connections affecting the circuit
- Engine Control Module (ECM) issues – Rare, but possible software or hardware problems with the ECM
- Glow plug relay failure – The relay that provides power to the glow plugs may be faulty
Safety Warning: When working with glow plugs, be aware that they can become extremely hot. Always allow the engine to cool completely before attempting any repairs, and disconnect the battery to prevent accidental electrical shorts.
Comprehensive Diagnosis Guide
Diagnosing a P0673 code requires a systematic approach. Follow these detailed steps to identify the root cause:
Required Tools
Initial Inspection & Code Verification
Begin by verifying the code with an OBD-II scanner. Clear the code and see if it returns. Perform a visual inspection of the glow plug wiring harness and connectors for cylinder 3.
- Check for damaged, frayed, or burnt wires
- Look for corroded or loose connectors
- Inspect the glow plug itself for physical damage
- Check related fuses in the fuse box
Glow Plug Resistance Test
Using a digital multimeter, measure the resistance of the glow plug for cylinder 3:
- Disconnect the electrical connector from the glow plug
- Set your multimeter to ohms (Ω) setting
- Place one probe on the glow plug terminal and the other on the engine ground
- A properly functioning glow plug typically shows 0.5-2.0 ohms resistance
- Compare with other cylinders – significant differences indicate a faulty glow plug
- Infinite resistance indicates an open circuit (failed glow plug)
- Very low resistance (close to 0) indicates a short circuit
Voltage Supply Test
Check for proper voltage supply to the glow plug:
- Reconnect the electrical connector to the glow plug
- Use a back-pin probe or carefully pierce the wire with a pin to access the circuit
- With the ignition on (but engine not running), check for battery voltage at the glow plug connector
- No voltage indicates a wiring issue or problem with the glow plug control module
- Low voltage suggests high resistance in the circuit
Glow Plug Control Module Test
If the glow plug tests fine, the issue may be with the control module:
- Consult your vehicle’s service manual for specific testing procedures
- Check for power and ground at the control module
- Test the control signal from the ECM
- Use an oscilloscope to check the waveform if available
- Swap the control module with a known good one if possible to confirm the diagnosis
ECM Communication Check
In rare cases, the issue may be with the Engine Control Module itself:
- Use a professional scan tool to check for communication errors with the ECM
- Look for other related codes that might indicate an ECM problem
- Check for software updates for your vehicle’s ECM
- Consult with a professional technician for ECM diagnosis
| Diagnostic Step | Tools Needed | Expected Outcome | Possible Results |
|---|---|---|---|
| Visual Inspection | Flashlight, Basic hand tools | Identify obvious wiring issues | Damaged wires, loose connections |
| Resistance Test | Multimeter | 0.5-2.0 ohms resistance | Open circuit, short circuit, normal |
| Voltage Test | Multimeter, Back-pin probes | Battery voltage at connector | No voltage, low voltage, normal |
| Control Module Test | Multimeter, Service manual | Proper power and signal | Failed module, wiring issue |
| ECM Communication | Professional scan tool | No communication errors | ECM fault, network issue |
Repair Procedures & Cost Analysis
Once you’ve identified the cause of the P0673 code, follow these repair procedures:
Repairing a Faulty Glow Plug
- Disconnect the negative battery terminal for safety
- Remove any components obstructing access to the glow plug (intake manifold, etc.)
- Disconnect the electrical connector from the glow plug
- Use a deep socket and ratchet to remove the faulty glow plug
- Inspect the glow plug hole for carbon buildup and clean if necessary
- Apply anti-seize compound to the threads of the new glow plug (unless manufacturer specifies otherwise)
- Install the new glow plug and torque to manufacturer specifications (typically 10-20 Nm)
- Reconnect the electrical connector
- Reinstall any removed components
- Reconnect the battery and clear the trouble codes
- Test the repair by starting the engine and verifying no codes return
Repairing Wiring Issues
- Identify the damaged section of wiring
- Cut out the damaged portion
- Strip the insulation from both ends of the wire
- Use butt connectors or solder to join a new section of wire
- Protect the connection with heat shrink tubing
- Secure the wiring away from hot or moving components
- Test the repair for proper voltage and resistance
Cost Breakdown
Single Glow Plug Replacement
- Parts: $20 – $80
- Labor: $75 – $150
- Time: 0.5 – 1.5 hours
Complete Glow Plug Set
- Parts: $100 – $300
- Labor: $150 – $300
- Time: 1.5 – 3 hours
Control Module Replacement
- Parts: $100 – $400
- Labor: $100 – $200
- Time: 1 – 2 hours
Wiring Repair
- Parts: $10 – $30
- Labor: $50 – $150
- Time: 0.5 – 1 hour
Pro Tip: When replacing glow plugs, it’s often recommended to replace all of them at once rather than just the faulty one. This prevents future issues and ensures even heating across all cylinders. Additionally, consider replacing the glow plug controller if multiple plugs have failed.
Prevention & Maintenance
To prevent P0673 and similar glow plug issues, follow these maintenance guidelines:
- Follow your vehicle’s recommended maintenance schedule – Regular inspections can catch issues early
- Use high-quality glow plugs from reputable manufacturers – OEM parts typically offer the best performance and longevity
- Allow the glow plug system to complete its cycle before starting in cold weather – Wait for the glow plug indicator light to turn off
- Address any fuel system issues promptly – Poor fuel quality or injector problems can affect glow plug life
- Maintain your vehicle’s electrical system – Ensure the battery, alternator, and charging system are in good condition
- Use the correct engine oil viscosity – Thick oil in cold weather increases strain on the starting system
- Consider using a block heater in extreme cold – Reduces the demand on glow plugs during starting
- Regularly inspect glow plug connections – Look for corrosion or loose connections during oil changes
Maintenance Schedule: Most manufacturers recommend inspecting glow plugs every 30,000 miles and replacing them every 60,000-100,000 miles as preventive maintenance. However, consult your vehicle’s specific maintenance schedule for accurate intervals.
