OBD-II Diagnostic Trouble Code
P0671 — Cylinder 1 Glow Plug Circuit
P0671 means a fault was detected in the glow plug circuit for cylinder 1. Current or resistance on that circuit was out of range, usually an open plug or wire, or a short. It applies to diesels with individual glow plug monitoring.
Severity 2/5
Truck-relevant
Service soon
Severity
Service soon
Cylinder 1 Glow Plug Circuit
Code
P0671
Category
Powertrain
Subsystem
Cold Start
Truck-relevant
Yes
What this means for truckers
P0671 (Cylinder 1 Glow Plug Circuit) — specific cylinder's glow plug not drawing current. Replace that glow plug ($50-180 part).
Before you replace parts — try this first
P0671 on the Cold Start system is most often burned-out or open cylinder 1 glow plug — not a dead part. On commercial diesels especially, work these in order before spending money on parts:
- Confirm the symptoms. Match what your truck is doing to the Symptoms section below. If the code is set but the symptoms are not there, clear the code and re-test after a drive cycle — it may not return.
- Clean and dry the sensor connector. Water, road salt, or oil seeping into a connector is the #1 fake sensor failure. Disconnect, look for green corrosion or moisture, spray with electrical contact cleaner, dry, reseat. Many faults clear here for $0.
- Inspect the wiring harness. Trace the wire from the sensor toward the ECM — look for chafing against the engine block, melted insulation near the turbo or exhaust, or pinch points where a clamp has rubbed through.
- Test the sensor before swapping it. Most Common Causes (below) list the expected voltage or resistance range — a $20 multimeter saves a $300 sensor swap that does not fix the problem.
- Only then, replace the part. Use the OEM or a tier-1 aftermarket. Cheap sensors throw the same code in two weeks.
Symptoms
Slightly harder cold starts, rough idle or misfire, and white smoke for the first minute after a cold start, often worse in freezing weather. With only one plug out, warm starts and normal driving are usually fine.
Common causes
- CommonBurned-out or open cylinder 1 glow plug ($120–$450)
- CommonLoose, corroded or damaged glow plug connector or harness lead at cylinder 1 ($60–$300)
- OccasionalGlow plug shorted to ground (swollen tip) ($120–$450)
- OccasionalFailed glow plug control module output channel ($150–$600)
How to fix
Measure the cylinder 1 glow plug resistance at the plug (typically about 0.5 to 2 ohms; check the spec) and compare it to the other cylinders
When to use: First step
Inspect the cylinder 1 connector and harness lead for heat damage or corrosion
When to use: When the plug resistance is normal
Replace the cylinder 1 glow plug, using penetrating oil and the correct torque to avoid breaking it in the head
When to use: When the plug is open or shorted
Check the glow plug module output for cylinder 1
When to use: When the plug and harness test good
Affects most
No specific vehicle pattern found in our complaint database. This code can appear on any vehicle that supports OBD-II diagnostics.
Related
- → Browse all DTC trouble codes
- → Decode a VIN to check codes for your truck
- → Owner-operator maintenance guides
- → P0670 — Glow Plug Control Module Circuit
- → P0672 — Cylinder 2 Glow Plug Circuit
- → P0673 — Cylinder 3 Glow Plug Circuit
- → P0674 — Cylinder 4 Glow Plug Circuit
- → P0675 — Cylinder 5 Glow Plug Circuit
Frequently Asked Questions
Can I drive with code P0671?
P0671 is severity 2 — service this week. Not an immediate stop, but ignoring it usually compounds the problem. Most common cause for this code is a worn sensor or minor performance fault that won't strand you today but will get more expensive the longer you wait.
How much does it cost to fix P0671 on a commercial truck?
Total repair cost ranges $60–$600 depending on the root cause. The cheapest fix is usually the first one listed below — Burned-out or open cylinder 1 glow plug — which is also the most common. On commercial trucks the cost is usually higher than passenger vehicles due to part availability and shop labor rates ($120–185/hour at most truck shops, vs $90–130 at general auto shops). DIY-capable fixes (sensors, hoses, filters) are at the low end; major component replacements (turbo actuator, high-pressure fuel pump, transmission solenoid pack) are at the high end. Always diagnose with a scan tool BEFORE replacing parts — guessing wrong on a $1,500 turbo actuator when the actual issue was a $30 boost hose is a common expensive mistake.
Will P0671 fail a DOT inspection?
P0671 is generally not a direct DOT inspection fail item by itself. Inspectors focus on safety-critical systems (brakes, steering, tires, lights, hours-of-service). However, the check engine light alone can trigger additional inspection scrutiny. Best practice: fix the code and clear it before a planned inspection.
What's the difference between P0671 and related codes?
P0671 is closely related to: P0670 (Glow Plug Control Module Circuit), P0672 (Cylinder 2 Glow Plug Circuit), P0673 (Cylinder 3 Glow Plug Circuit), P0674 (Cylinder 4 Glow Plug Circuit). These codes share the same subsystem (cold-start) and often appear together. If you see this code, scan for the full code list — fixing the root cause usually clears multiple companion codes at once. If only P0671 is set without the companions, the issue is more isolated.
How do I prevent P0671 from coming back after a repair?
For P0671 (Cylinder 1 Glow Plug Circuit), prevention centers on the cold start system. Test glow plug system annually before winter. Use a battery in good condition — weak batteries make glow plugs work harder. Park indoors below -10°F if possible.