Error Code P0671 is defined as Cylinder 1 Glow Plug Circuit Fault. This means there’s a problem in the cylinder #1 glow plug circuit.

This error code is a generic trouble code, meaning it applies to all vehicles equipped with OBD-II or those made since 1996 up to present. Specifications on the definition, troubleshooting and repairs of course, vary from one make and/or model to another.

Definition

Unlike a gas engine, a diesel engine doesn’t depend on compressed fuel mixture, and electrical source of ignition to work. This is because diesel has a higher compression than gas engines.

Higher compression causes the air inside the cylinder to reach more than 600˚ C, enough to ignite diesel fuel. When the piston reaches top dead center of the cylinder, high-pressure fuel sprays into the cylinder. It quickly ignites when it encounters super-heated air to ignite the fuel, when the engine is cold a problem will arise. A cold start makes it difficult for the engine to super-heat the air, as heat is quickly lost due to the cold cylinder head.

This is where the job of the glow plug comes into the picture; installed in the cylinder, this pencil-shaped component heats up and glows up for about 10 seconds. This results in an increase of temperature of the surrounding wall, allowing heat compression to rise for the ignition.

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Error Code P0671 happens when there’s a problem in the glow plug circuit, causing the cylinder #1 glow plug to fail from heating up. This will require a complete and thorough inspection to locate the fault.

Common Symptoms

Since this code means one bad plug, the engine can normally start with minimal symptoms. For that, it’s most common symptom would be the illumination of the Check Engine light. For vehicles under freezing conditions, there could be more symptoms, such as:

  • Hard starting or no start condition when sitting long enough
  • Engine lacks power until it heats up completely
  • Engine hesitates when accelerating
  • Engine miss due to cylinder head temperature being colder than normal
  • No pre-heat period, Pre-heat light doesn’t go off

Possible Causes

There can be multiple of factors to contribute this error code, such as:

  • Faulty cylinder #1 glow plug
  • Damaged wiring connector
  • Open or shorted glow plug circuit
  • Faulty glow plug control module

How to Check

To diagnose this code, technicians start by using a DVOM (digital volt-ohmmeter). They disconnect the glow plug’s connector, set the DVOM on ohms and place the red lead on the terminal of the glow plug and at the back end of a good ground. The range should be between 0.5 to 2.0 ohms. They verify the measurements from the factory service manual. If the range is outside the specifications, then the glow plug must be replaced.

Nest, they check the resistance in glow plug wire to the glow plug relay buss bar on the valve cover. Notice how the relay (which looks like a starter relay) has large gauge wire that leads to a buss bar where all the wires of the glow plugs are attached.

Next, they test the wire to the #1 glow plug by replacing the red lead on the buss bar wire and the black lead to the glow plug inside. Once again, the range must be 0.5 to 2.0 ohms (with 2ohms  being the highest resistance). If the resistance is higher, then the wires from the buss bar to the glow plug must be replaced. They make sure to reconnect wires, as this may cause the buss bar to splurge.

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Then, technicians look for signs of cracks, looseness, or missing insulation.

Lastly, they connect the code scanner to the OBD port on the vehicle (usually located under the dash), turn the key at “on” with the engine off. Then they clear the codes.

How to Fix

Common repairs for this error code are of course, derived from the diagnosis. Examples of the repairs include:

  • Repair or replace any damaged, corroded, or loose wiring, connectors, and harnesses
  • Replacing blown fuses and repair shorts
  • Repair or replace damaged or bent pins in electrical connectors
  • Replacing the cylinder #1 glow plug
  • Replacing the glow plug relay

Note that, if P0670 trouble code is present along with this DTC, it is important to diagnose P0670 before diagnosing this code.