OBD Code P0672: Cylinder 2 Glow Plug Circuit Malfunction

If your vehicle’s check engine light has illuminated with the P0672 diagnostic trouble code, you’re dealing with an issue specific to your diesel engine’s glow plug system. Glow plugs are essential heating elements in diesel engines that warm the combustion chamber before starting, especially in cold conditions. When the system detects a malfunction in cylinder 2’s glow plug circuit, it triggers this code. Understanding what this means and how to address it can save you time and money on repairs.

What Does P0672 Mean?

P0672 is a manufacturer-specific diagnostic trouble code that indicates the engine control module (ECM) has detected a problem with the glow plug circuit for cylinder 2. The code breaks down as follows:

• P = Powertrain code
• 0 = Generic OBD-II code
• 6 = Ignition system (which includes glow plugs in diesel engines)
• 7 = Glow plug circuit
• 2 = Cylinder 2

In diesel engines, glow plugs provide the initial heat needed to ignite fuel during cold starts. Unlike gasoline engines that use spark plugs, diesel engines rely on compression ignition combined with glow plug assistance. When the ECM detects a fault in the cylinder 2 glow plug circuit—whether it’s an open circuit, short circuit, or resistance issue—it stores this code and illuminates the check engine light.

Common Symptoms

Drivers experiencing P0672 may notice one or more of the following symptoms:

• Check engine light (CEL) illuminated on the dashboard
• Difficulty starting the engine, especially in cold weather
• Rough idle or stumbling during initial startup
• Increased white smoke from the exhaust during cold starts
• Extended cranking time before the engine fires
• Engine misfire or hesitation under load
• Reduced fuel economy due to incomplete combustion
• No symptoms at all if the issue is intermittent

In warmer climates or during warm-weather operation, you may not notice any symptoms at all, as glow plugs are less critical once the engine is warmed up. However, the code will still be stored in the ECM’s memory.

Possible Causes

P0672 can result from several different issues. Listed from most to least common:

1. Faulty Glow Plug (Most Common)

A burned-out or weakened glow plug is the most frequent cause of this code. Glow plugs have a limited lifespan and typically fail after 100,000 miles or more of use. When a glow plug fails, it creates an open circuit that the ECM detects immediately.

2. Corroded or Loose Glow Plug Connector

Corrosion, moisture, or vibration can cause the electrical connector on the glow plug to loosen or corrode. This creates intermittent contact issues or complete circuit breaks, triggering the code.

3. Broken or Damaged Wiring Harness

The wiring between the glow plug relay and cylinder 2’s glow plug can become damaged due to heat, vibration, or rodent damage. Broken or frayed wires prevent proper electrical flow.

4. Faulty Glow Plug Relay or Control Module

The relay that controls power delivery to the glow plugs may fail, or the ECM itself may have a software glitch. This prevents the proper voltage from reaching the glow plug circuit.

5. Poor Ground Connection

A corroded or loose ground wire in the glow plug circuit can prevent proper electrical flow, even if the glow plug itself is functional.

6. Low Battery Voltage

Insufficient battery voltage can cause the ECM to flag the glow plug circuit as faulty. A weak or dying battery may trigger this code.

7. ECM Software Issue or Programming Error

In rare cases, a software glitch in the engine control module or an incorrect calibration can cause false P0672 codes.

Diagnostic Steps

Follow these steps to diagnose the P0672 code:

Step 1: Retrieve and Document the Code

Use an OBD-II scanner to read the code and check for any additional codes. Document whether P0672 is the only code present or if there are related codes like P0671, P0673, or P0674 (which indicate issues with other cylinders’ glow plugs).

Step 2: Perform a Visual Inspection

Locate the glow plugs on your diesel engine (typically on top of the cylinder head). Inspect the connector on cylinder 2’s glow plug for:

• Corrosion or discoloration
• Loose or damaged connectors
• Burned or melted plastic
• Visible damage to the wiring harness

Step 3: Check Battery Voltage

Use a multimeter to measure battery voltage. It should read 12.6 volts or higher when the engine is off. If voltage is low, charge or replace the battery and retest.

Step 4: Test the Glow Plug Connector

With the ignition on (engine off), use a multimeter to check for voltage at the cylinder 2 glow plug connector. You should see approximately 12 volts during the glow plug preheat cycle (typically 5-10 seconds after turning on the ignition). If no voltage is present, the issue is upstream in the relay or wiring.

Step 5: Resistance Test the Glow Plug

Disconnect the glow plug connector and use a multimeter set to ohms to measure the resistance of the glow plug itself. A functioning glow plug typically reads between 0.5 and 2 ohms. A reading above 5 ohms or infinite resistance indicates a failed glow plug.

Step 6: Inspect the Wiring Harness

Trace the wiring from the glow plug relay to cylinder 2’s glow plug. Look for:

• Burned or melted insulation
• Pinched or crushed wires
• Rodent damage
• Loose connections at the relay or ECM

Step 7: Test the Glow Plug Relay

Locate the glow plug relay (consult your vehicle’s service manual for location). You can test it by listening for a clicking sound when the ignition is turned on, or use a multimeter to verify it’s switching properly.

Step 8: Clear the Code and Retest

After making repairs, clear the code using your OBD-II scanner and perform a test drive. If the code returns, there may be an intermittent issue or a secondary problem.

Repair Cost Estimates

The cost to repair a P0672 code varies depending on the root cause:

• Glow Plug Replacement: $150–$400 per plug (parts and labor). Most diesel engines have 4–6 glow plugs, so if multiple plugs are failing, costs can exceed $1,000.
• Connector or Wiring Repair: $100–$300 for cleaning, reseating, or replacing connectors and wiring.
• Glow Plug Relay Replacement: $200–$500 including parts and labor.
• Battery Replacement: $100–$300 if low voltage is the cause.
• ECM Reprogramming or Replacement: $500–$2,000+ if the engine control module is faulty.

Most commonly, a single glow plug replacement will resolve the P0672 code at a cost of $150–$400.

Can I Still Drive?

Severity: Low to Moderate

In most cases, you can continue driving with a P0672 code, but with some caveats:

• Warm Weather: If you’re driving in warm conditions, the faulty glow plug has minimal impact. Diesel engines can start and run without glow plugs once warmed up.
• Cold Weather: In freezing temperatures, a faulty glow plug can make starting difficult or impossible. You may experience extended cranking times or engine stalling.
• Performance: You may notice slightly reduced fuel economy and rougher idle during cold starts.
• Emissions: Increased white smoke during startup indicates incomplete combustion, which is not ideal for emissions.

Recommendation: While P0672 is not a critical safety issue, it’s best to repair it promptly, especially if you live in a cold climate or frequently start your engine in cold conditions. Ignoring the code could lead to more severe glow plug failures or damage to the engine’s combustion chamber.

Frequently Asked Questions