The P0671 code indicates a fault in the glow plug circuit of cylinder 1. Glow plugs are essential for starting diesel engines in cold conditions, as they heat the combustion chamber to enable fuel ignition. When this code is triggered, it suggests a problem with the electrical circuit connected to the glow plug, including the plug itself, wiring, or control module, which can lead to starting issues or reduced engine performance in cold weather.
P0671 quick overview
| Meaning | P0671: Cylinder 1 Glow Plug Circuit |
| Is it serious? | Yes, especially in cold weather as it can affect the engine’s ability to start. |
| Possible causes | – Faulty or incorrect glow plug – Issues with the glow plug harness or wiring – Malfunctioning glow plug control module – Incorrect glow plug type (ceramic vs. steel) |
| How to diagnose? | Check glow plug operation for cylinder 1 through amp draw or resistance measurement Inspect the wiring and harness for damage or corrosion Verify glow plug type and ensure compatibility with the engine Consider control module functionality and software updates Swap glow plugs between cylinders to see if the issue follows the plug |
P0671 Meaning
The P0671 code is a sign that there’s an issue with the first cylinder’s glow plug circuit in a diesel engine. Glow plugs are crucial for getting the engine to start smoothly by warming up the air-fuel mixture in the combustion chamber. If this system isn’t working correctly, you might find your vehicle struggling to start on cold mornings or exhibiting uneven performance.
The problem could stem from a variety of sources, including the glow plug itself, any connecting wires, or the control module that manages the plug’s operation. Addressing this issue promptly ensures your engine starts reliably, especially in colder climates.
Car Diagnostics Training
- Car Electrical basics
- OBD-II scanner basics
- How to use an OBD reader
- Introduction to a Bi-directional Scan tool
- OBD scanner buyer’s guide + discounts
How to fix P0671 – learn from mechanics
I looked into P0671 service cases at iatn.com website where in the private forums the mechanics seek advice from other mechanics. I looked up a bunch of cases to help you decide how to fix your issue and see what would a real mechanic do to solve it.
Case #1: Volkswagen
Vehicle information
2006 Volkswagen Jetta TDI 1.9L – MIL Lamp on
Issue + repair history
- Replaced glow plugs & harness
Other OBD2 Codes + Test Results
- PO671: #1 Glow plug or harness
- P2185 / 08581: Related to coolant temp sensor in Radiator outlet
- 05658: Same as PO671, indicating #1 glow plug issue
Mechanic suggestions
- Glow plugs may be numbered opposite the cylinder numbers, suggesting #1 glow plug could be on cylinder #4
- Suggestion to amp draw the plugs individually and compare
- Recommendation to use OEM glow plugs and replace all four
- A recall on the glow plugs from ceramic to steel for specific engine models, and the VW dealer may need to reflash the ECU when a glow plug update is made
Final fix
- P2185/08581 issue was due to the coolant temp sensor in the Radiator outlet
- 05658 and PO671 issue resolved by addressing the #1 glow plug, which had been replaced earlier
Case #2: Volkswagen
Vehicle Information 2002 Volkswagen Jetta TDI 1.9L Customer uses Bio-Diesel
Issue + Repair History
- Check engine light comes on after start every time.
- Code P0671 found on generic scan mode, snap on mt2500.
- All glow plugs start out drawing 18 amps and decrease to 9 amps after being on for about 20 secs.
- All glow plugs draw the same amount of current.
- Connections look good at each plug.
- Also, as each glow plug is disconnected, it generates the appropriate related code.
Other OBD2 Codes + Test Results
- P0671 – Glow Plug #1 Cyl. – Open/Short to Ground
- Current to glow plugs: all start at 18 and go to 9 amps
Mechanic Suggestions
- Replace the glow plug wire harness as it’s a common problem.
- Tie the two wires running out to the glow plugs together.
- Check plugs resistance; it must be absolutely the same on all plugs otherwise the code is set.
- Swap the #1 and #2 glow plugs and see if the fault comes back the same or goes to the #2 glow plug.
- Replace glow plug control unit.
- Note that glow plug #1 might actually refer to cylinder #4.
- Try replacing one glow plug then recheck.
- Some late TDIs might have different numbering for glow plugs and cylinders.
- Consider replacing the glow plug harness, glow plugs, and clear the codes.
Final Fix
- Inspected the glow plug harness; appeared ok.
- Swapped what was thought to be #1 and #2 glow plugs for further diagnosis.
- The light has not returned in two weeks after the swap, indicating a potential fix. Time will tell.
Case #3: Ford
Vehicle Information 2000 Ford Excursion XLT 7.3L
Issue + Repair History
- Complaint of blowing black smoke and needing an estimate.
- Check engine light on with codes P0671 (cyl 1 glow plug circuit) and P0340 (camshaft pos sensor ckt malf).
- Camshaft position sensor (CMP) has been replaced.
- Ran KOEO injector test, checked out ok.
- Ran glow plug test, same code comes up.
- Ran a cylinder contribution test, got a code for cylinder 3 balance.
- Previous mechanic did extensive work including replacing two injectors (5 and 7), high-pressure pump, CMP, and resealing injectors, but no cylinders were replaced.
- Glow plug on cylinder 1 found broken off, replaced glow plugs on passenger side bank, replaced one injector in cylinder 3, and a new harness.
Other OBD2 Codes + Test Results
- P0671 – Cyl 1 glow plug circuit
Mechanic Suggestions
- Check under valve cover harnesses and connections; UVC connectors may partially unplug inside the valve covers causing glow plug problems and injector firing issues.
- Glow plugs should have less than 1 ohm resistance.
- Check compression on all cylinders through the glow plug holes; compression should be 450 psi or better.
- Injector seals may fail around this mileage, causing fuel in oil; consider resealing injectors.
- Consider replacing injector o-rings as they may be bad.
- Test glow plugs, if no internal harness shorting is found, proceed with compression and leak down tests.
- Consider bench testing or swapping injectors.
- Use an ICP sensor to compare injection pressure between cylinders.
- Perform a cylinder pressure test before pulling the engine.
Final Fix
- Found number three piston damaged, no rings, all in the oil pan.
- Sold the customer a new engine, which resolved the issue.
How to diagnose P0671
I made you an overview of solving this DTC, based on all the advice that was suggested by certified mechanics to address this issue in previously mentioned service cases.
| Diagnostic Step | Brief Explanation |
|---|---|
| Inspect and replace faulty glow plugs | Essential for ensuring proper starting and functioning in cold conditions |
| Check and repair the glow plug harness | Fixes common wiring issues that can lead to circuit faults |
| Test glow plug operation and compare amp draw | Identifies underperforming or faulty glow plugs |
| Swap glow plugs between cylinders | Helps diagnose if the issue is with the plug or cylinder position |
| Update or reflash ECU if necessary | Required when changing glow plug types or to fix software issues |
| Verify correct glow plug type (ceramic vs. steel) | Ensures compatibility and proper operation of glow plugs |
| Inspect under valve cover harnesses and connections | Addresses potential internal wiring and connection issues |
Drawing from these service cases, it’s clear that diagnosing a P0671 code can involve a mix of straightforward and complex steps, from simply replacing a glow plug to more in-depth electrical troubleshooting. The cases also highlight the importance of using OEM parts and ensuring that all components are correctly numbered and installed, reflecting the need for careful attention to detail in resolving this issue.
Car Diagnostics Training
- Car Electrical basics
- OBD-II scanner basics
- How to use an OBD reader
- Introduction to a Bi-directional Scan tool
- OBD scanner buyer’s guide + discounts
I test OBD-II scanners and make DIY Engine diagnostics guides to help you solve your car problems without having to depend on the mechanic. A lot of them will try to scam you or are just no help at all. About Juraj Lukacko