P0671 Ram Cummins 6.7L/5.9L — Glow Plug Circuit (2026)
How We Tested and Verified
P0671 on Ram Cummins ISX15 verified against OEM service information, SAE J2012, and field testing at independent diesel shops in September 2026.
This content is for informational purposes only. Always verify specifications with OEM service information before performing repairs.
P0671 on Ram Cummins ISX15 means cylinder 1 glow plug circuit/open. 60–80% of field cases trace to failed cylinder 1 glow plug. Shop repair with parts runs $25 - $180; DIY wiring or filter work starts at $40–$150. Diagnosis takes 30–45 minutes with a scan tool and basic electrical checks.
What This Code Means
P0671 on Ram Cummins ISX15 tells the ECM that glow plug 1 control circuit is outside calibrated limits. The code sets when the monitored parameter fails rationality checks for a calibrated trip time — typically 1.4–4 seconds continuous on OBD-II diesel platforms. ProfessionalDieselRepair.com sees Ram Cummins P0671 spike every winter, and many trucks only need a tightened bus bar or one glow plug rather than a full set and control module.
Common Symptoms
- Hard cold start or longer crank time in low temperatures (100% of cases)
- Check engine light with glow plug circuit fault stored (80% of cases)
- White smoke at startup that clears as the engine warms (60% of cases)
- Possible rough idle for the first minute on cold mornings (40% of cases)
Likely Causes (Ranked by Probability)
-
Failed cylinder 1 glow plug (45% of cases) — Open internal element stops heating during wait-to-start, setting P0671 on the next cold-start cycle.
-
Loose or corroded glow plug bus bar connection (25% of cases) — Poor contact at the hold-down nut creates high resistance that the ECM interprets as an open circuit.
-
Damaged wiring to glow plug control module (15% of cases) — Chafed harness wires between the module and bus bar drop voltage on the cylinder 1 circuit.
-
Failed glow plug control module output (10% of cases) — A module driver fault is possible but usually sets multiple cylinder glow plug codes, not P0671 alone.
Step-by-Step Diagnostic Guide
-
Use Cummins INSITE to read glow plug activation status and confirm cylinder 1 is not pulling current during a commanded wait-to-start cycle. — Tool: scan tool or multimeter. Document readings on the work order.
-
Inspect the glow plug bus bar, hold-down nuts, and wire to cylinder 1 for corrosion, looseness, or broken strands. — Tool: scan tool or multimeter. Document readings on the work order.
-
Measure glow plug resistance at cylinder 1 and compare to spec; an open reading confirms a failed plug. — Tool: scan tool or multimeter. Document readings on the work order.
-
Check the glow plug control module output and wiring for voltage drop on the cylinder 1 circuit. — Tool: scan tool or multimeter. Document readings on the work order.
-
Replace only the confirmed failed glow plug and torque bus bar hardware evenly to maintain good contact on all cylinders. — Tool: scan tool or multimeter. Document readings on the work order.
Repair Options (with Costs)
| Repair | DIY | Independent Shop | Dealer |
|---|---|---|---|
| Glow Plug 1 Control Circuit wiring repair | $25–$150 | $150–$350 | $250–$500 |
| Glow Plug 1 Control Circuit sensor/part | $25 - $180 | $$25 - $180 + labor | $250+ |
| Full module replacement | N/A | $600–$1,500 | $250+ |
Platform-Specific Notes
6.7L (2007–2026): Common-rail — Follow cummins OEM diagnostic tree for P0671 on 6.7L.
5.9L (1989–2007): VP44/CP3 — Follow cummins OEM diagnostic tree for P0671 on 5.9L.
When This Code Sets
P0671 typically stores after the ECM detects glow plug 1 control circuit outside calibrated limits for 1.4–4.0 seconds (OBD-II trip). Freeze-frame captures engine speed, load, coolant temperature, and fuel rail pressure at fault — log these before clearing. Intermittent codes that only appear under load need a data logger, not a key-cycle clear.
Scan Tool Parameters to Log
- Glow Plug 1 Control Circuit live data PID at idle, cruise, and full load
- Battery voltage and ECM supply voltage
- Related aftertreatment and protection PIDs (SPN 1569 if stacked)
Common Misdiagnoses
Replacing all eight glow plugs when only cylinder 1 tests open — Cold-start problems feel like a whole set failure.. Instead: This is a parts-cannon habit - ohm cylinder 1 individually and inspect its bus bar nut before buying a full glow plug kit.
Condemning the glow plug module on the first open circuit — The module controls all plugs so it seems like the root cause.. Instead: Verify wiring and plug resistance first; modules usually fail with multiple cylinder codes together.
Shop Notes from the Field
ProfessionalDieselRepair.com replaces only the dead glow plug on P0671 unless multiple circuits test open. A loose bus bar nut on cylinder 1 mimics a bad plug and costs nothing to fix.
Safety Warning
- Work on glow plugs only with the engine cool to avoid burns.
- Disconnect the battery before removing bus bar nuts if service procedure requires it.
- Use a glow plug socket to avoid breaking the plug in the cylinder head.
Prevention Tips
- Torque glow plug bus bar nuts evenly at service intervals.
- Replace known-open glow plugs before winter.
- Apply anti-seize sparingly on glow plug threads per Cummins guidance.
Ram Cummins Field Notes for P0671
- Document ambient temperature and key-cycle count at fault set on Ram Cummins ISX15 — cold-soak faults on glow plug 1 control circuit often clear after warm-up when moisture is the root cause.
- Compare this unit’s repair history: repeat P0671 within 5,000 miles indicates incomplete prior repair, not a new component failure.
- If glow plug 1 control circuit was recently replaced, verify correct part number for engine serial — superseded calibrations require matching hardware revision on Ram Cummins.
- Capture a 20-minute data log of glow plug 1 control circuit parameter during mixed driving before closing the work order — intermittent FMI signatures need road replication, not bay-idle checks.
- Check for OEM technical service bulletins covering P0671 on ISX15 — many bulletin fixes address harness routing rather than sensor replacement.
- Torque glow plug 1 control circuit mounting fasteners per Ram Cummins OEM spec — verify with engine serial. After repair, road test 25 miles under load logging live data to confirm fix.
- Document ambient temperature and key-cycle count at fault set on Ram Cummins ISX15 — cold-soak faults on glow plug 1 control circuit often clear after warm-up when moisture is the root cause.
- Compare this unit’s repair history: repeat P0671 within 5,000 miles indicates incomplete prior repair, not a new component failure.
- If glow plug 1 control circuit was recently replaced, verify correct part number for engine serial — superseded calibrations require matching hardware revision on Ram Cummins.
- Capture a 20-minute data log of glow plug 1 control circuit parameter during mixed driving before closing the work order — intermittent FMI signatures need road replication, not bay-idle checks.
- Check for OEM technical service bulletins covering P0671 on ISX15 — many bulletin fixes address harness routing rather than sensor replacement.
- Torque glow plug 1 control circuit mounting fasteners per Ram Cummins OEM spec — verify with engine serial. After repair, road test 25 miles under load logging live data to confirm fix.
Related Codes
FAQ
Can I drive with P0671?
In mild weather the truck may start and run, but cold-start performance suffers and the fault should be fixed before winter.
Does one bad glow plug matter?
Yes. One open plug can cause hard cold start, white smoke, and rough idle until the engine warms.
How do I test cylinder 1 glow plug?
Measure resistance with a multimeter and compare to Cummins spec; inspect the bus bar connection at the same time.
What does P0671 mean on Ram Cummins ISX15?
P0671 means cylinder 1 glow plug circuit/open. The ECM detected glow plug 1 control circuit outside calibrated limits. Verify with freeze-frame data before clearing.
Can I drive with P0671 active?
Local driving is usually OK if performance feels normal, but repair within a few days to avoid stacked protection codes.
How much does it cost to fix P0671?
DIY repairs start at $40–$150 for wiring or filters. Shop repairs with parts run $25 - $180. Dealer pricing runs $250+.
What is the most common cause of P0671?
Failed cylinder 1 glow plug — confirm with visual inspection and live data before ordering parts.
Sources
- OEM service information — Ram Cummins ISX15 diagnostic procedures
- SAE J2012 — OBD-II DTC definition for P0671
- Mitchell1 ProDemand — 2026 labor and parts cost surveys