P0673 Code Symptoms, Causes, Diagnostic Steps, Cost & Safety

P0673 Code – Symptoms, Causes, and How to Fix It

Drivers who encounter a P0673 code usually notice the vehicle struggling to start, especially in cold weather. Cranking time can increase dramatically, and the engine may fire unevenly once it finally turns over. A rough idle that wobbles at low RPMs, a noticeable loss of power on one cylinder, and an illuminated check‑engine light are common. In many cases the fault appears only when the engine is cold; after the engine warms up the symptoms may lessen, but the code remains stored until cleared.

Symptoms

• Hard starting or extended cranking – the engine requires several seconds longer than normal to fire.
• Rough idle – noticeable shaking or uneven RPMs at idle, often felt more on one cylinder.
• Cylinder 3 misfire – a slight hesitation or loss of power that is repeatable at low load.
• Check‑engine light (CEL) – the ECM stores P0673 and may illuminate additional related codes (e.g., P0672, P0674).
• Increased fuel consumption – the ECM may enrich the mixture to compensate for the weak cylinder, leading to a modest drop in MPG.

Why Cylinder 3 Glow‑Plug Circuit Problems Occur

Faulty Glow Plug

A glow plug that has burned out or developed an internal short will not draw the required voltage, causing the ECM to register an open circuit.

Wiring or Connector Damage

Corrosion, broken wires, or a loose connector in the circuit between the ECM and the glow plug can interrupt voltage flow. Vibration and exposure to engine heat accelerate such damage.

ECM Output Driver Failure

The ECM contains a driver transistor that supplies power to each glow‑plug circuit. If the driver for cylinder 3 fails, the ECM will read zero voltage even though the plug and wiring are intact.

Ground or Power Supply Issues

A compromised ground strap or a blown fuse that supplies the glow‑plug circuit can produce the same “open” condition.

*While a faulty glow plug or wiring could cause the symptoms, the issue may stem from the ECM not communicating correctly with the circuit. A diagnostic scan and module communication test will determine if module repair or replacement is needed.*

Diagnostic and Repair Procedures

1. Retrieve and clear codes – Connect a professional OBD‑II scanner, read all stored and pending codes, then clear them to verify if P0673 returns after a drive cycle.
2. Visual inspection – Locate the cylinder 3 glow‑plug harness. Check for cracked insulation, burnt pins, or loose connectors. Repair or reseat as needed.
3. Voltage test at the plug – With the ignition on (engine not running), measure voltage between the glow‑plug terminal and ground. Expected reading is typically 12 V ± 1 V. A reading of 0 V confirms an open circuit.
4. ECM output driver test – If voltage is absent at the plug but the wiring appears sound, bench‑test the ECM’s driver circuit using a scope or a dedicated ECM test board. Many aftermarket scan tools can command a “glow‑plug test” that forces the driver on; observe whether voltage appears.
5. Ground and fuse verification – Check the fuse for the glow‑plug circuit (often 10 A–15 A) and measure resistance to chassis ground at the ECM connector. Replace any blown fuse and repair grounding straps.
6. Component replacement –

– Glow plug – Replace the cylinder 3 plug if it fails the resistance test (typically > 5 Ω indicates a bad plug).

– Wiring/connector – Repair any damaged sections, replace corroded pins, and apply heat‑shrink tubing.

– ECM – If the driver test shows no output despite good wiring and plug, the ECM is the likely culprit.

1. Re‑programming – After any ECM replacement, the new unit must be programmed with the vehicle’s VIN and calibrated to the engine’s control parameters. Flagship One offers VIN‑matched modules pre‑programmed for immediate installation.
2. Final verification – Clear all codes, perform a cold start, and confirm that the engine starts promptly, idles smoothly, and no CEL reappears.

Typical labor for a complete diagnosis ranges from $120‑$180. Glow‑plug replacement alone is $15‑$30 for the part plus $50‑$80 labor. ECM driver repair is rarely feasible; replacement is the standard remedy.

When Replacement Makes Sense

If the ECM’s internal driver for cylinder 3 is defective, repairing the board is often a temporary fix because the failure usually indicates broader circuit degradation. Replacing the ECM eliminates the risk of recurring faults and restores full control over all glow‑plug circuits.

Modern control modules are complex and integrated with security and immobilizer systems. That’s why choosing a replacement isn’t only about the hardware—it’s about correct programming and compatibility. Flagship One specializes in VIN‑matched control modules, providing a plug‑and‑drive solution backed by warranty. Their units arrive pre‑programmed to your vehicle’s specifications, eliminating dealer‑only re‑coding steps and reducing installation time.

Preventive Maintenance

• Inspect glow‑plug wiring during routine under‑hood checks; look for exposed conductors or moisture intrusion.
• Replace glow plugs at the manufacturer’s interval (often every 60,000 mi for diesel engines) to avoid premature failure.
• Maintain proper grounding by tightening chassis straps and cleaning corrosion from grounding points.
• Use quality fuel and keep the fuel system clean; excessive soot can accelerate glow‑plug wear.
• Run periodic diagnostics with a scan tool that can command glow‑plug tests; early detection of voltage loss prevents hard‑start situations.