P0674 OBD-II Trouble Code: Cylinder 4 Glow Plug Circuit

P0674 code definition

When a malfunction occurs in the glow plug circuit for cylinder 4 in a diesel engine, the Powertrain Control Module (PCM) stores a P0674 trouble code. When the PCM stores a P0674 trouble code, other related glow plug circuit codes are also stored, including P0670, P0671, P0672, P0673, P0675, P0676, P0677, P0678, P0679, P0680, P0681, P0682, P0683, and P0684.

What the P0674 code means

A P0674 trouble code is stored on the PCM when the reference voltage from the glow plug circuit in cylinder 4 has a variance of 10 percent over manufacturer specifications. In addition to storing a P0674 trouble code, the Check Engine Lights also illuminates.

What causes the P0674 code?

In addition to a faulty glow plug, glow plug relay, or glow plug module, some other causes of a P0674 trouble code include shorted, corroded, or loose wiring and electrical connectors in the glow plug circuit. Another common cause of a P0674 trouble code is a blown fuse.

What are the symptoms of the P0674 code?

Symptoms common to a P0674 trouble code include difficulty in starting the vehicle, especially in cold weather, an increase in white smoke from the exhaust, increased engine noise when starting, engine misfires, and the glow plug indicator lamp staying on too long or remaining illuminated.

How does a mechanic diagnose the P0674 code?

Diagnosing a P0674 trouble code requires the mechanic to use a scanner, or code reader, and a digital volt/ohmmeter. In addition to the proper diagnostic equipment, the mechanic should perform the following:

• Inspect all wiring, connectors, and components associated with the glow plug system. They should look for any loose, damaged, or corroded wiring, components, or connectors, replacing them if needed.

• Download the freeze frame data and any stored trouble codes on the PCM using the scanner.

• Clear any codes and test drive the vehicle to see if the codes return.

• If the code returns, check the individual glow plug connector to see if a reference voltage or ground signal are present.

• If the code returns after replacing a faulty glow plug, test the resistance using the volt/ohmmeter, comparing the readings gathered to the manufacturer's specifications.

• If the mechanic detects no voltage at the glow plug connector, they should test the fuses of the glow plug system. This step requires that the mechanic test any fuses with the glow plug heater activated. This helps ensure that the fuses are not breaking down while under load.

• Next, the mechanic should test the glow plug module, relay, and timer, making sure to compare their findings to the manufacturer's specifications.

• If the glow plug system seems to be in proper working order, the mechanic should only disconnect and test the PCM as a last resort.

• Once all of the repairs have been made, the mechanic should clear the trouble code and retest the system to see if the code returns.

Common mistakes when diagnosing the P0674 code

Mechanics often mistakenly replace glow plug timers and relays when diagnosing a P0674 trouble code. The mechanic should test the glow plug system wiring, connectors, and the glow plugs before doing so to avoid a misdiagnosis.

How serious is the P0674 code?

Diesel engines utilize glow plugs to heat the compression chambers prior to help ignite the fuel. When the glow plug fails to work properly, due to faulty wiring, the glow plug circuit, or even the glow plug itself, the diesel engine could fail to start. This can present a problem in cold weather, as the ability to generate enough heat for proper combustion in such temperatures is even more important.

What repairs can fix the P0674 code?

After repairing all pending codes stored on the PCM prior to the storage of the P0674 trouble code, the mechanic can proceed with repairs. These repairs include:

• Replacing any damaged wiring, connectors, or components.

• Replacing the glow plug connector or the glow plug if either tests as faulty.

• Replacing any fuses that have blown. The mechanic should also determine what is causing the overload with the fuses and fix that problem before proceeding further.

• Repairing any faults within the glow plug heater system circuitry.

• If the glow plug relay, timer, or module is bad, replacing those components as well.

• Replacing the PCM if it is tested and found at fault.

Additional comments for consideration regarding the P0674 code

When repairing a P0674 trouble code, make sure to repair any codes stored on the PCM in the order that they are stored. This requires the mechanic to download any stored codes or freeze frame data using an advanced code reader.

Need help with a P0674 code?

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