Code P0670 is a diagnostic trouble code that signals a malfunction in the glow plug module control circuit. This code is specific to diesel engines, which rely on glow plugs to preheat the combustion chambers before starting in cold conditions. When your vehicle’s onboard diagnostic system detects a problem with the electrical circuit that controls the glow plug module, it triggers the P0670 code and illuminates the check engine light.
What Does P0670 Mean?
The P0670 code breaks down as follows:
- P = Powertrain system code
- 0 = Generic OBD-II code (standardized across all manufacturers)
- 6 = Transmission, ignition, or auxiliary emissions control system
- 70 = Glow plug module control circuit malfunction
The glow plug module is an electronic control unit responsible for managing the glow plugs in diesel engines. These plugs generate intense heat to warm the combustion chambers, making cold starts possible. The module controls the timing and duration of glow plug operation based on engine temperature, ambient conditions, and other factors. When the engine control module (ECM) detects a fault in the communication or operation of this circuit—such as an open circuit, short circuit, or module failure—it sets the P0670 code.
Common Symptoms
- Check Engine Light – The most obvious indicator that the P0670 code has been triggered
- Difficult Cold Starts – The engine struggles to start in cold weather or cold conditions
- Rough Idle After Cold Start – The engine runs rough or misfires immediately after starting
- Reduced Fuel Economy – Inefficient combustion due to inadequate preheating
- Excessive White Smoke – Unburned fuel produces white smoke from the exhaust, especially on startup
- Engine Hesitation – Poor acceleration or hesitation during initial driving after a cold start
- Glow Plug Indicator Light – Some vehicles display a dedicated glow plug warning light on the dashboard
- Limp Mode – In severe cases, the vehicle may enter reduced power mode to protect the engine
Possible Causes
The following issues are ranked from most to least common causes of the P0670 code:
1. Faulty Glow Plug Module (Most Common)
The glow plug module itself may fail due to age, electrical stress, or manufacturing defects. This is the most frequent cause of P0670 codes and typically requires module replacement.
2. Damaged or Corroded Wiring
The electrical harness connecting the ECM to the glow plug module can become damaged, corroded, or loose. Water intrusion, road salt, or physical damage can compromise these connections.
3. Faulty Connectors
Connectors at the glow plug module or ECM may be loose, corroded, or damaged, breaking the electrical circuit. Oxidation is a common culprit in older vehicles.
4. ECM/PCM Malfunction
Although less common, the engine control module itself may have a fault that prevents proper communication with the glow plug module. This typically requires professional diagnostics.
5. Defective Glow Plugs
While individual glow plug failure is more common than module failure, multiple failed glow plugs can trigger the P0670 code if the module detects excessive current draw or circuit resistance.
6. Blown Fuse or Relay
A blown fuse or failed relay in the glow plug circuit can prevent power from reaching the module, triggering the code.
7. Battery or Charging System Issues
Weak battery voltage or alternator problems can cause intermittent circuit faults that set the P0670 code.
Diagnostic Steps
Follow these steps to diagnose the P0670 code:
Step 1: Scan for Additional Codes
Use an OBD-II scanner to read all stored and pending codes. Related codes like P0671-P0679 (individual glow plug faults) or P0380 (glow plug system malfunction) will help narrow down the issue.
Step 2: Inspect the Battery
Check the battery voltage with a multimeter. It should read 12.6V or higher when the engine is off. Low battery voltage can cause false P0670 codes. Test the alternator output while the engine is running (should be 13.5-14.5V).
Step 3: Visually Inspect Wiring and Connectors
Locate the glow plug module (typically mounted on or near the engine block in diesel vehicles). Inspect the wiring harness for:
- Corrosion or oxidation on connectors
- Damaged, pinched, or melted insulation
- Loose or partially disconnected connectors
- Water intrusion or moisture inside connectors
Clean any corroded connectors with electrical contact cleaner and reseat them firmly.
Step 4: Test the Glow Plug Module Connectors
With the engine off and the key in the “On” position (not running), use a multimeter to check for voltage at the glow plug module connector. You should see battery voltage on the power pin. If there’s no voltage, check the fuse and relay in the glow plug circuit.
Step 5: Check the Glow Plug Fuse and Relay
Locate the fuse box (consult your vehicle’s service manual for the exact location). Inspect the glow plug fuse for signs of burning or damage. Test the glow plug relay with a multimeter or by swapping it with an identical relay to see if the code clears.
Step 6: Test Individual Glow Plugs (Optional)
If you have electrical testing experience, you can remove individual glow plugs and test them with a multimeter. A good glow plug should show minimal resistance (typically 0.5-2 ohms). However, this is time-consuming and may not be necessary if the module is the problem.
Step 7: Professional Diagnostics
If the above steps don’t identify the issue, use a professional-grade diesel diagnostic scanner that can communicate directly with the glow plug module. This can help determine if the module is responding to commands from the ECM and if there are internal faults.
Repair Cost Estimates
The cost to repair a P0670 code varies depending on the root cause and your vehicle:
- Glow Plug Module Replacement: $300-$800 (parts and labor combined). OEM modules are typically more expensive than aftermarket alternatives.
- Wiring Harness Repair: $150-$500 depending on the extent of damage and labor time required.
- Connector Replacement: $50-$200 for parts and labor.
- Fuse or Relay Replacement: $20-$100, one of the least expensive fixes.
- Glow Plug Replacement (set of 4-8): $200-$600 if multiple plugs are faulty.
- ECM Reprogramming or Replacement: $500-$2,000+ if the engine control module is at fault (rare).
Note: Many shops will charge $100-$150 for a comprehensive diagnostic to pinpoint the exact cause before providing a repair estimate.
Can I Still Drive?
Severity: Moderate to High
Whether you can safely drive with a P0670 code depends on the severity of the problem and your driving conditions:
- Cold Weather: If you’re in cold climates, do not drive the vehicle. Without functioning glow plugs, the diesel engine may not start at all, leaving you stranded.
- Warm Weather: In warm conditions, the vehicle may start and run, but cold starts will be difficult and rough. You can drive to a repair shop, but avoid prolonged driving.
- Limp Mode: If the vehicle enters limp mode due to the P0670 code, power and performance will be severely reduced. Have the vehicle towed to a repair facility.
- Emissions: Driving with a P0670 code may cause the vehicle to fail emissions testing and could result in fines in some jurisdictions.
Recommendation: Have the code diagnosed and repaired as soon as possible. While you may be able to drive short distances in warm weather, the underlying electrical fault could worsen and cause additional damage to the glow plug system or engine.
Frequently Asked Questions
Q: Can a P0670 code clear itself?
A: In rare cases, a P0670 code may clear itself if the fault is intermittent (such as a loose connector that reseats itself). However, if the code returns, the underlying problem must be diagnosed and repaired. Do not ignore a code that clears on its own, as it indicates a developing issue.
Q: Is the glow plug module the same as the glow plug relay?
A: No, they are different components. The glow plug relay is a simple switch that supplies power to the glow plug module based on signals from the ECM. The module is a more complex electronic control unit that manages individual glow plugs. A faulty relay can cause a P0670 code, but the module is the more common culprit.
Q: Will replacing the glow plugs fix a P0670 code?
A: Not necessarily. If the code is caused by a faulty module, wiring, or connector, replacing the glow plugs alone will not resolve it. However, if multiple glow plugs have failed and are causing excessive current draw, replacing them may help. A proper diagnosis is essential before replacing parts.
Q: How long do glow plug modules typically last?
A: Glow plug modules can last the lifetime of the vehicle (150,000+ miles) if well-maintained, but many fail between 80,000 and 120,000 miles due to electrical stress and heat exposure. Extreme temperatures, frequent cold starts, and electrical system problems can shorten their lifespan.
Q: Can I drive with the glow plug warning light on?
A: If only the glow plug indicator light is on (not the check engine light), the vehicle should still be drivable in warm weather, though cold starts may be difficult. However, if the check engine light is also illuminated with a P0670 code, you should have the vehicle serviced promptly.