P0670 Code: Complete Guide to Glow Plug Control Circuit Issues
Comprehensive diagnostic guide, symptoms, repair costs, and professional solutions for diesel engine problems
Understanding P0670 Code
The P0670 diagnostic trouble code (DTC) indicates a malfunction in the glow plug control circuit for cylinder #1. This code is specific to diesel engines and relates to the system that preheats the engine’s cylinders for easier starting in cold conditions.
Technical Details
The P0670 code is a generic powertrain code related to the glow plug control circuit. In diesel engines, glow plugs are essential for cold starts as they heat the combustion chamber to ensure proper fuel ignition when the engine is cold.
How Glow Plug Systems Work
Modern diesel engines use a sophisticated glow plug system that includes:
- Glow Plugs: Heating elements that warm the combustion chamber
- Glow Plug Control Module: Electronic controller that manages glow plug operation
- Glow Plug Relay: Provides high current to the glow plugs
- Temperature Sensors: Monitor engine temperature to determine glow plug activation time
- PCM Communication: The Powertrain Control Module communicates with the glow plug system
Quick Tip
Glow plugs are essential for diesel engines in cold weather. A malfunction can lead to hard starting, especially in temperatures below 40°F (4°C). Modern diesel engines may have post-glow cycles that continue heating after startup to reduce emissions.
Complete Symptoms List
When the P0670 code appears, you may experience one or more of the following symptoms:
Primary Symptoms
- Check Engine Light illuminated – The most common and immediate indicator
- Hard starting in cold weather – Extended cranking time before engine starts
- Rough idle after starting – Engine runs unevenly for the first few minutes
- Excessive white smoke from exhaust – Unburned fuel exiting the exhaust system
Secondary Symptoms
- Reduced engine performance – Noticeable power loss, especially when cold
- Possible misfire in cylinder #1 – Engine runs unevenly due to incomplete combustion
- Decreased fuel economy – Up to 10-15% reduction in fuel efficiency
- Longer warm-up period – Engine takes more time to reach optimal operating temperature
- Failed emissions test – Increased hydrocarbon emissions during testing
Symptom Severity: Medium Severity
While not immediately dangerous, prolonged driving with a P0670 code can lead to additional issues including increased wear on the starter motor, battery drain, and potential damage to the catalytic converter from unburned fuel.
Comprehensive Diagnostic Process
Diagnosing a P0670 code requires a systematic approach to identify the root cause. Follow these detailed steps:
Step 1: Code Verification and Preliminary Checks
Begin by confirming the P0670 code with a professional OBD-II scanner. Check for any additional codes that might provide context. Perform a visual inspection of the glow plug wiring harness for obvious damage, corrosion, or loose connections.
Tools Needed
OBD-II scanner, flashlight, basic hand tools
Step 2: Glow Plug Resistance Test
Use a digital multimeter to measure the resistance of the glow plug for cylinder #1. Disconnect the electrical connector from the glow plug and set your multimeter to the ohms (Ω) setting.
- Normal resistance: 0.1 to 6 ohms (varies by vehicle)
- Infinite resistance: indicates an open circuit (failed glow plug)
- Zero or very low resistance: indicates a short circuit
Compare the reading with manufacturer specifications for your specific vehicle.
Step 3: Power Supply Verification
Check that the glow plug control module is receiving proper voltage. With the ignition on and engine cold, test for battery voltage (typically 12V) at the power supply wire to the glow plug control module.
Safety Note
Always disconnect the battery before working on electrical components. Glow plugs can become extremely hot – allow the engine to cool completely before testing.
Step 4: Control Module Activation Test
Using a diagnostic scanner with bidirectional controls, activate the glow plug control module. Listen for an audible click from the glow plug relay and check for voltage at the glow plug connector during activation.
Step 5: Wiring Continuity Check
Test the continuity of wires between the control module and glow plug, and between the control module and PCM. Look for:
- Open circuits (broken wires)
- Short circuits (wires touching ground or each other)
- High resistance connections (corrosion or damage)
Step 6: PCM Signal Verification
As a last resort, verify the PCM is sending the correct signals to the glow plug control module. This may require an advanced scan tool or oscilloscope to monitor communication signals.
Diagnostic Flowchart
For complex cases, follow this decision process: Start with visual inspection → Test glow plug resistance → Check power to control module → Verify control module operation → Test wiring continuity → Check PCM signals.
Comprehensive Causes Analysis
| Cause | Likelihood | Description | Detection Method |
|---|---|---|---|
| Faulty Glow Plug | High | The glow plug itself has failed due to age, overheating, or manufacturing defect | Resistance test with multimeter |
| Wiring Harness Issues | High | Damaged, corroded, or loose wiring in the glow plug circuit | Visual inspection, continuity test |
| Glow Plug Control Module Failure | Medium | The electronic module that controls the glow plugs has malfunctioned | Activation test, voltage output check |
| Blown Fuse | Low | A fuse in the glow plug circuit has blown due to electrical overload | Visual inspection, voltage test at fuse |
| Faulty Glow Plug Relay | Medium | The relay that provides high current to the glow plugs has failed | Audible click test, voltage input/output check |
| PCM Software Issue | Low | Rare cases where the PCM needs reprogramming or replacement | Professional diagnostic scan, software update check |
| Poor Electrical Connections | Medium | Loose, corroded, or high-resistance connections in the circuit | Visual inspection, voltage drop tests |
Diagnostic Priority
Always start with the most common and easily accessible components (glow plugs and wiring) before moving to more complex and expensive components (control module, PCM).
Detailed Repair Costs Analysis
| Repair | Parts Cost | Labor Cost | Total Estimate | Complexity |
|---|---|---|---|---|
| Single Glow Plug Replacement | $20 – $60 | $50 – $100 | $70 – $160 | Low |
| Complete Glow Plug Set Replacement | $100 – $250 | $150 – $300 | $250 – $550 | Medium |
| Glow Plug Control Module | $100 – $300 | $80 – $150 | $180 – $450 | Medium |
| Wiring Harness Repair | $20 – $50 | $60 – $120 | $80 – $170 | Medium |
| Glow Plug Relay Replacement | $30 – $80 | $40 – $80 | $70 – $160 | Low |
| PCM Reprogramming/Replacement | $200 – $800 | $100 – $200 | $300 – $1000 | High |
Cost Factors
Costs vary significantly by vehicle make and model. Luxury and European vehicles typically have higher parts and labor costs. Additional factors include:
- Vehicle Accessibility: Some engines require removal of components to access glow plugs
- Glow Plug Type: Ceramic glow plugs are more expensive than metal ones
- Location: Labor rates vary by geographic region
- Dealer vs. Independent Shop: Dealerships typically charge 20-40% more
Professional Repair Procedures
Step 1: Preliminary Preparation
Gather all necessary tools and parts. Ensure you have the correct replacement glow plug for your specific vehicle. Allow the engine to cool completely before beginning work.
Critical Safety Step
Always disconnect the negative battery terminal before starting any electrical work to prevent short circuits or accidental activation of the glow plug system.
Step 2: Access the Glow Plugs
Remove any components necessary to access the glow plugs. This may include the intake manifold, engine cover, or other components depending on your vehicle’s design.
Step 3: Remove the Faulty Glow Plug
Carefully remove the electrical connector from the glow plug. Use a proper glow plug socket and extension to remove the glow plug from cylinder #1. Apply steady pressure and avoid excessive force to prevent breakage.
Pro Tip
If a glow plug is difficult to remove, apply penetrating oil and allow it to soak for 15-30 minutes before attempting removal again.
Step 4: Inspection and Verification
Check the removed glow plug visually for signs of damage, overheating, or carbon buildup. Test with a multimeter to confirm it’s faulty before proceeding with replacement.
Step 5: Install New Components
Install the new glow plug, applying appropriate anti-seize compound if recommended by the manufacturer. Tighten to the specified torque using a torque wrench to avoid damage.
Step 6: Reassembly and Testing
Reconnect all electrical connectors and reassemble any components that were removed. Reconnect the battery, clear codes with your scanner, and test the glow plug system operation.
Step 7: Verification and Final Check
Start the engine and confirm the Check Engine Light remains off after several drive cycles. Test the vehicle in cold conditions to ensure proper starting.
Additional Considerations
- Consider replacing all glow plugs if one has failed, as others may be near failure
- Inspect and clean glow plug connectors during replacement
- Check glow plug operation timing with a scan tool if available
- Verify proper glow plug system operation in various temperature conditions
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