What Is Code P0671?
Code P0671 is a diagnostic trouble code (DTC) specific to diesel engines that indicates a malfunction in the glow plug circuit for cylinder 1. Glow plugs are heating elements that warm the combustion chamber before engine startup, allowing diesel fuel to ignite more easily in cold conditions. When the engine control module (ECM) detects an electrical problem with cylinder 1’s glow plug circuit—such as an open circuit, short circuit, or excessive resistance—it triggers code P0671.
What Does P0671 Mean?
The “P” in P0671 stands for “Powertrain,” indicating the code relates to the engine’s fuel and ignition systems. The “0671” specifically refers to the glow plug circuit for cylinder 1. The ECM monitors glow plug operation during cold starts and warm-up cycles. When it detects a voltage or resistance problem in the cylinder 1 glow plug circuit—either too high resistance, no voltage, or a short to ground—it logs this code and illuminates the check engine light.
This code is exclusive to diesel engines, as gasoline engines use spark plugs instead of glow plugs. If you see this code on a gasoline vehicle, it may indicate a scanner error or a vehicle with a diesel option that’s being misdiagnosed.
Common Symptoms
- Check engine light illuminated – The primary indicator of code P0671
- Hard starting in cold weather – Difficulty cranking the engine, especially in freezing temperatures
- White smoke on startup – Unburned fuel producing visible exhaust smoke during cold starts
- Rough idle after cold start – Engine runs unevenly until it fully warms up
- Loss of power during acceleration – Reduced engine performance, particularly when cold
- Increased fuel consumption – Engine compensates for poor combustion by burning more fuel
- No symptoms in warm weather – The problem may only appear during cold starts or in winter months
Possible Causes (Ranked by Frequency)
1. Faulty Glow Plug (Most Common)
The glow plug itself has failed and no longer heats properly. This is the most common cause of P0671, accounting for the majority of cases. Glow plugs have a limited lifespan and typically fail after 80,000–100,000 miles.
2. Broken or Corroded Glow Plug Connector
The electrical connector that attaches to the glow plug has corroded, loosened, or broken, preventing proper electrical contact. Corrosion is especially common in vehicles exposed to road salt or moisture.
3. Damaged Glow Plug Wiring or Harness
The wiring that supplies power to the glow plug circuit is damaged, frayed, or has a break. This can result from engine vibration, heat damage, or rodent damage.
4. Faulty Glow Plug Control Module or Relay
The module or relay that controls power delivery to the glow plugs has failed, preventing the circuit from receiving voltage. This is less common but still possible.
5. Engine Control Module (ECM) Malfunction
The ECM itself is faulty and incorrectly reading the glow plug circuit or failing to send the proper control signal. This is relatively rare but should be considered after other causes are ruled out.
6. Poor Ground Connection
A loose or corroded ground wire in the glow plug circuit prevents proper electrical flow, causing the ECM to detect a fault.
Diagnostic Steps
Step 1: Retrieve and Document the Code
Use an OBD-II scanner to confirm code P0671 is present. Note whether it’s a current fault or a stored fault, and check for any additional codes that might provide clues (such as P0672, P0673, etc., which indicate problems with other cylinders’ glow plugs).
Step 2: Visual Inspection
Inspect the glow plug connector and wiring for the following:
- Corrosion or white/green oxidation on the connector
- Loose or disconnected connectors
- Damaged, frayed, or burnt wiring
- Signs of water intrusion or moisture
- Loose or corroded ground connections
Step 3: Test Glow Plug Resistance
Disconnect the glow plug connector and use a multimeter to measure resistance across the glow plug terminals. A healthy glow plug typically reads between 0.5–2 ohms. If the reading is infinite (open circuit) or extremely high, the glow plug has failed and needs replacement.
Step 4: Check Voltage Supply
With the connector still disconnected, turn the ignition to the “On” position (without starting). Use a multimeter to measure voltage at the connector. You should see approximately 12 volts. If there’s no voltage, the problem is in the wiring, relay, or control module—not the glow plug itself.
Step 5: Inspect the Ground Circuit
Check that the glow plug has a solid ground connection. Trace the ground wire and ensure it’s clean, tight, and free of corrosion. A poor ground can prevent the circuit from functioning properly.
Step 6: Test the Glow Plug Control Module
If voltage is present but the glow plug still won’t heat, the control module may be faulty. This requires specialized diagnostic equipment and is best performed by a professional technician.
Step 7: Clear the Code and Test Drive
After repairs, clear the code using your scanner and perform a cold start test drive. If the code doesn’t return after several cold starts, the repair was successful.
Repair Cost Estimates
Glow Plug Replacement
Cost Range: $150–$300
Replacing a single faulty glow plug is the most common and affordable repair. Parts typically cost $30–$75, with labor ranging from $120–$225 depending on engine accessibility and your location.
Glow Plug Connector or Wiring Repair
Cost Range: $100–$250
Cleaning, repairing, or replacing a corroded connector or damaged wiring is usually straightforward. Parts are inexpensive, and labor is minimal.
Glow Plug Control Module Replacement
Cost Range: $300–$600
If the control module is faulty, replacement costs are higher due to the module’s price ($150–$350) and potential programming requirements.
Multiple Glow Plugs (All Cylinders)
Cost Range: $400–$800
If multiple glow plugs are failing simultaneously, replacing all of them at once is often recommended to prevent future codes. This is more economical than replacing them one at a time.
Professional Diagnosis
Cost: $75–$150
If you’re unsure of the cause, a professional diagnostic fee may apply before repair work begins.
Can I Still Drive?
Severity: Low to Moderate
Code P0671 is generally not a critical safety issue, and your vehicle will likely still be drivable. However, there are important considerations:
Cold Weather Driving
If you live in a cold climate or it’s winter, the faulty glow plug will make cold starts extremely difficult or impossible. You may not be able to start your engine reliably in freezing temperatures.
Warm Weather Driving
In warm climates or during summer months, you may experience minimal symptoms. The engine will start normally once it’s warm, though you might notice slightly reduced performance.
Fuel Economy
Expect reduced fuel economy as the engine compensates for poor combustion in cylinder 1 by running richer.
Recommendation
While you can technically drive with code P0671, it’s best to repair it promptly—especially if cold weather is approaching. The repair is relatively inexpensive and will restore normal cold-start performance and fuel economy.
Frequently Asked Questions
Q: Is code P0671 the same as a bad glow plug?
A: Not necessarily. While a faulty glow plug is the most common cause of P0671, the code indicates a problem with the entire glow plug circuit, which includes the plug itself, wiring, connectors, relay, and control module. Proper diagnosis is needed to pinpoint the exact cause.
Q: Can I drive with a bad glow plug?
A: Yes, you can drive in warm weather, but cold starts will be very difficult or impossible in freezing temperatures. It’s recommended to repair the issue promptly, especially before winter.
Q: How much does it cost to fix code P0671?
A: Most commonly, $150–$300 for a single glow plug replacement. If the problem is a corroded connector or wiring, it may be $100–$250. A faulty control module is more expensive at $300–$600.
Q: Can a bad glow plug damage my engine?
A: A faulty glow plug won’t directly damage your engine, but it can cause rough running, increased fuel consumption, and difficulty starting. Over time, poor combustion in one cylinder may slightly increase engine wear, but this is not a major concern.
Q: Do all cylinders’ glow plugs fail at the same time?
A: Not always. While glow plugs wear out over time and may eventually all need replacement, they don’t typically fail simultaneously. If you see codes for multiple cylinders (P0671, P0672, P0673, P0674), it’s worth considering a control module or relay issue rather than multiple failed plugs.