Ram Cummins · Glow Plug 1 Control CircuitMedium

Ram Cummins P0671: Cylinder 1 Glow Plug Circuit

P-Code P0671 (FMI N/A) — Cylinder 1 Glow Plug Circuit/Open

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Quick Answer

P0671 means the ECM detected an open or high-resistance fault on the cylinder 1 glow plug circuit. Check bus bar connections and glow plug resistance before replacing parts.

Overview: Ram Cummins P-Code P0671

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.

Repair Snapshot

Time

45 Minutes - 2 Hours

Difficulty

Beginner

Parts Cost

$25 - $180

Dealer Labor

$250+

Tools Required

  • Cummins INSITE
  • Noregon JPRO
  • Multimeter
  • Glow Plug Socket

What P-Code P0671 Means

P0671 on Ram Cummins trucks indicates an open or high-resistance fault on the cylinder 1 glow plug circuit. Cold-start complaints, long crank in winter, and white smoke on startup are common when one or more glow plugs are not heating properly.

Symptoms Drivers Notice

  • Hard cold start or longer crank time in low temperatures
  • Check engine light with glow plug circuit fault stored
  • White smoke at startup that clears as the engine warms
  • Possible rough idle for the first minute on cold mornings

Most Likely Root Causes

Failed cylinder 1 glow plug

Common

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

Common

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

Occasional

Chafed harness wires between the module and bus bar drop voltage on the cylinder 1 circuit.

Failed glow plug control module output

Rare

A module driver fault is possible but usually sets multiple cylinder glow plug codes, not P0671 alone.

Common Misdiagnoses (Don't Waste Parts)

Replacing all eight glow plugs when only cylinder 1 tests open

Why it happens: Cold-start problems feel like a whole set failure.

Do this 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

Why it happens: The module controls all plugs so it seems like the root cause.

Do this instead: Verify wiring and plug resistance first; modules usually fail with multiple cylinder codes together.

Master Tech Commentary

Mechanic's Notes

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.

Step-by-Step Diagnostic Procedure

  1. Step 1: Use Cummins INSITE to read glow plug activation status and confirm cylinder 1 is not pulling current during a commanded wait-to-start cycle.
  2. Step 2: Inspect the glow plug bus bar, hold-down nuts, and wire to cylinder 1 for corrosion, looseness, or broken strands.
  3. Step 3: Measure glow plug resistance at cylinder 1 and compare to spec; an open reading confirms a failed plug.
  4. Step 4: Check the glow plug control module output and wiring for voltage drop on the cylinder 1 circuit.
  5. Step 5: Replace only the confirmed failed glow plug and torque bus bar hardware evenly to maintain good contact on all cylinders.

Live Data & Freeze-Frame Tips

During a commanded glow plug activation in INSITE, watch module output and compare cylinder 1 current draw to neighbors. A cylinder with zero draw while others activate confirms an isolated open circuit.

Safety Warnings

  • 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.

When to Limp In vs Tow

P0671 alone does not require towing, but avoid long trips in extreme cold if the truck becomes difficult to start reliably.

Prevention Tips for Fleets & Owner-Operators

  • 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.

Related Fault Codes

P0672

Glow plug 2 circuit open, the same fault pattern on the next cylinder.

P0380

Glow plug/heater circuit A fault that can appear with broader control issues.

Frequently Asked Questions

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.

Will P0671 clear after replacing the plug?

After repair and a successful wait-to-start cycle, the code usually clears following a few drive cycles.

How much does repair cost?

A single glow plug and bus bar service is inexpensive compared with module replacement.

Can fuel problems cause P0671?

No. P0671 is an electrical open on the glow plug circuit, not a fuel delivery fault.

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