Diesel engines use glow plugs to heat the combustion chambers so the engine can start in cold weather, and the engine computer runs them through a glow plug module or relay. A P0671 code means the engine computer found a problem with the cylinder 1 glow plug circuit.
This guide explains what the code means, what usually causes it and how to test the system before you buy parts.
P0671 Code Breakdown
Every OBD-II code follows the same five-character pattern set by the SAE J2012 standard. Each character narrows down which system has the fault. This table shows what each part of the code tells you.
| Character | Meaning | What It Tells You |
|---|---|---|
| P | Powertrain | The fault is in the engine or transmission systems. |
| 0 | Generic (SAE) code | The definition is the same on every OBD-II vehicle. |
| 6 | Computer and output circuits | The fault involves the engine computer, its memory, its network links or circuits it controls. |
| 71 | Specific fault | Cylinder 1 Glow Plug Circuit |
What the P0671 Code Means
The SAE J2012 standard defines the P0671 code as Cylinder 1 Glow Plug Circuit. Each glow plug is a small heater that glows red hot in a few seconds. Before cranking, and for a short time after the engine starts, the engine computer turns on a glow plug control module or relay, which sends battery power to the plugs. Many modern modules check the current to each plug and report faults.
If the module, a plug or the wiring fails, the computer sets a glow plug code. The engine may still start in warm weather, but cold starts get hard, with white smoke and a rough idle until it warms up.
This is a general circuit code for the cylinder 1 glow plug circuit. The PCM watches the circuit while it switches the part on and off. It set this code because the voltage or current on that circuit did not match what the PCM commanded, such as an open, a short or a part that does not respond.
Symptoms of a Glow Plug Problem
Symptoms show up mostly in cold weather. Common signs of a P0671 code include:
- Check engine light or glow plug light on
- Hard starting when cold
- Long crank before the engine fires
- White smoke at startup
- Rough idle or misfire until warm
- Glow plug light flashing
Common Causes of the P0671 Code
A general circuit fault can sit anywhere between the power feed, the part and the PCM. These are the usual causes:
- Loose or corroded connector. Heat, vibration and moisture attack the pins.
- Damaged wiring. Wires can chafe through, break or melt near hot or moving parts.
- Failed cylinder 1 glow plug circuit. The coil or motor inside opens or shorts.
- Blown fuse or lost power feed. The part cannot work without its supply.
- PCM driver fault. Rare, and often caused by a shorted part that overloaded the driver.
Tools You Will Need
You can do most of this testing at home with basic tools:
- OBD-II scan tool
- Digital multimeter
- Test light
- Glow plug socket
- Wiring diagram for your vehicle
How to Diagnose the P0671 Code
Check the glow plug fuse or fusible link and the module connector first.
Check Live Data and Freeze Frame
Check glow plug command and, if your scan tool shows it, each plug’s status. Many glow plug modules report which plug has failed.
Inspect the Connector and Wiring
Find the cylinder 1 glow plug circuit and check that the connector is fully seated and locked. Look for corrosion, bent pins and wiring that rubs on brackets or hot parts.
Test Power, Ground and Signal
Back-probe the connector with the key on. Check for the correct supply voltage, a good ground and a signal or command that matches the service manual. Any wire that fails tells you where to look next.
Test Each Glow Plug
Disconnect the glow plug wires and measure each plug’s resistance to ground. Most read about 0.5 to 2 ohms. An open reading means the plug has burned out. Some shops also measure the current each plug draws when powered.
Test the Module or Relay
With a scan tool, command the glow plugs on, or turn the key on in cold conditions. Check for battery voltage at the plug feed wire. If the module gets its command but sends no power, the module or its power feed is at fault.
How to Fix the P0671 Code
Match the repair to what your tests found. Then clear the code and drive under the same conditions that set it.
- Replace burned-out glow plugs with the correct type.
- Repair the glow plug wiring or connector
- Replace the glow plug module or relay if it fails its tests.
- Replace a blown fusible link after finding the cause.
Glow plugs can seize in the cylinder head. Remove them with the engine warm, using a glow plug socket and steady pressure, and stop if one does not turn freely. A broken glow plug can be expensive to remove.
Common Mistakes to Avoid
These mistakes waste money or leave the real fault in place:
- Replacing the cylinder 1 glow plug circuit before checking power, ground and the connector
- Replacing a blown fuse without finding out what shorted
- Forcing a seized glow plug, which can break off in the head
- Replacing only one plug when several are old
Repair Cost Breakdown
Repair costs depend on the cause. This table shows typical ranges for parts and labor.
| Repair | DIY Cost | Typical Shop Cost |
|---|---|---|
| Glow plug (each) | $15 to $60 | $60 to $150 |
| Full set of glow plugs | $60 to $300 | $300 to $900 |
| Glow plug module or relay | $50 to $300 | $200 to $600 |
| Wiring repair | $10 to $50 | $100 to $300 |
Is It Safe to Drive with a P0671 Code?
A P0671 code usually does not affect how the engine runs once it is warm. In cold weather, you may not be able to start the engine, so fix it before winter.
Related Codes
These codes often show up with this one or point to the same system:
| Code | Meaning |
|---|---|
| P0670 | Glow Plug Control Module Circuit |
| P0382 | Glow Plug/Heater Circuit B Malfunction |
| P0380 | Glow Plug/Heater Circuit A Malfunction |
Getting Easy Cold Starts Back
A P0671 code points to the diesel glow plug system. Burned-out plugs, a failed module or relay and wiring faults cause most cases.
- Check the fuse or fusible link.
- Test each glow plug.
- Test the module or relay.
- Remove plugs carefully.
Frequently Asked Questions
Is the P0671 Code Always a Bad Cylinder 1 Glow Plug Circuit?
No. Wiring and connector faults cause many circuit codes. Test the power feed, the control wire and the resistance of the cylinder 1 glow plug circuit first. Replace the part only when it measures out of range or fails a direct test.
Can I Drive with a Bad Glow Plug?
Yes, once the engine is running. A failed glow plug mainly makes cold starts harder and adds white smoke at startup. In very cold weather, several bad plugs can leave you unable to start, so replace them soon.
Is This Code the Same on Every Vehicle?
Yes. It is a generic SAE code, so the definition is the same on every OBD-II vehicle. The parts that cause it and the test steps vary by make and engine, so check the service manual for your vehicle before you buy parts.
Do I Need a Dealer to Fix This Code?
Not always. Most of the checks in this guide need only a scan tool, a multimeter and basic hand tools. A dealer or a well-equipped shop helps when the repair needs module programming, factory software or special tools.
Should I Check for Technical Service Bulletins?
Yes. Carmakers publish service bulletins for known problems, and some cover this code with updated parts or engine computer software. A dealer or a repair information service can check bulletins for your VIN before you spend money on parts.
Can a Basic Code Reader Diagnose This Code?
A basic reader can show the code number, but it often cannot read live data or run the tests that pin down the cause. A scan tool with live data, graphing and output tests saves a lot of guesswork.
Will This Code Clear Itself After the Repair?
If the fault is fixed, the check engine light usually turns off after about three drive cycles in a row with no fault. The stored code stays in memory for a while longer and then erases itself. You can clear it right away with a scan tool, but that also resets the readiness monitors, so the car will need some driving before an emissions test.