P0674 — Cylinder 4 Glow Plug Circuit

powertrain (engine/transmission)

Is it safe to drive?

Short-term driving is OK Safe to drive once the engine is started and warmed up. The main risk is cold-weather starting: with several dead plugs the engine may not start on a cold morning. Fix before winter.

How to diagnose with an OBD scanner

Read the code and freeze frame Connect an OBD2 scanner, confirm P0674 is stored, and save the freeze frame data — it shows engine conditions when the fault was detected.
Check for companion codes and TSBs Related codes often reveal the real root cause. Also check manufacturer technical service bulletins (TSB) for your make and model — many P0xxx issues have factory fixes or software updates. Search by VIN on the manufacturer site or nhtsa.gov.
Inspect wiring and connectors Visually inspect the wiring harness and connectors related to the diesel glow plug system for damage, corrosion, chafing or poor contact.
Subsystem check Measure resistance of each glow plug — an open plug reads infinite resistance.
Subsystem check Check voltage at the glow plug rail during the pre-glow phase.
Verify the repair Clear the code, complete a full drive cycle including the conditions from the freeze frame, and confirm P0674 does not return.

Approximate repair cost

Repair cost depends on which component actually failed. Typical scenarios for P0674 (parts and labor, USD, independent shop rates): Replace a full set of glow plugs: $250–$600; Replace one glow plug: $80–$200; Replace the glow plug relay/module: $150–$450. Overall, fixing P0674 usually costs between $80 and $600. An accurate diagnosis before buying parts saves money — the most expensive scenario is rarely the first one to check.

Common questions

How much does it cost to fix P0674?

Replace a full set of glow plugs — $250–$600; Replace one glow plug — $80–$200; Replace the glow plug relay/module — $150–$450. Exact cost depends on the root cause, your vehicle and local labor rates — a proper diagnosis is the cheapest first step.

Is it safe to drive with P0674?

Safe to drive once the engine is started and warmed up. The main risk is cold-weather starting: with several dead plugs the engine may not start on a cold morning. Fix before winter.

Will P0674 clear itself?

Sometimes P0674 clears after a few drive cycles if the fault was intermittent — for example a loose connector or a tank of bad fuel. The underlying cause usually remains, so the code tends to return. Clear it once; if it comes back, diagnose rather than erase it repeatedly.

What is the most common cause of P0674?

The most common causes are: burned-out glow plug, blown fuse in the glow plug circuit, faulty glow plug relay or control module.

What are the symptoms of P0674?

Drivers most often notice: Check Engine Light on, rough idle until warmed up, glow plug warning light staying on, white smoke after a cold start.

When your scanner displays P0674, the engine computer is reporting that cylinder 4's glow plug system is behaving outside its designed parameters. P0674 translates to "Cylinder 4 Glow Plug Circuit" and falls under powertrain diagnostics, meaning it affects the engine or transmission control systems. The glow plug system matters most during cold starts with diesel engines—these plugs heat the combustion chamber so fuel ignites properly when temperatures drop. When the control module detects abnormal voltage, resistance, or current flow from the cylinder 4 plug for longer than expected during a defined number of drive cycles, P0674 gets stored and the check engine light activates.

Cylinder numbering follows a predictable pattern but varies by engine configuration. On inline engines, cylinder 4 sits fourth from the front, counting from the belt end of the block. V-type engines number differently depending on the manufacturer, so consulting your specific firing order diagram is essential before any repair work. This matters practically because reaching cylinder 4 may require removing intake components or other accessories on some layouts, directly affecting labor time and your repair bill.

The engine control unit saves freeze frame data alongside P0674, capturing exact conditions when the fault occurred—engine speed, load percentage, coolant temperature, and vehicle speed. This snapshot is your best diagnostic starting point, telling you whether the problem appeared at idle, under load, or during a cold start. Using this information strategically prevents guessing and unnecessary part replacements.

Common Symptoms

Check engine light illuminated
Engine idles roughly until reaching normal operating temperature
Glow plug warning lamp remains lit after startup
White exhaust smoke visible following a cold engine start
Difficulty starting the engine, especially in cold weather conditions

Common Causes

Corroded or loose glow plug electrical connectors
Failed glow plug relay or control module
Damaged wiring harness or connector in the glow plug circuit
Burned-out or open-circuit glow plug
Blown fuse protecting the glow plug circuit

How to Diagnose P0674

Connect an OBD2 scanner, confirm P0674 is present, and record all freeze frame data—this shows engine conditions at the exact moment the fault was detected.
Check for additional diagnostic trouble codes. Related codes often point to the root cause rather than P0674 itself. Also search manufacturer technical service bulletins for your make and model—many powertrain codes have documented software updates or factory fixes published online.
Perform a thorough visual inspection of the wiring harness and connectors feeding the glow plug system. Look for damaged insulation, corrosion buildup, loose terminals, or evidence of heat damage.
Measure the resistance of each glow plug individually with a multimeter. An open or infinitely resistant plug indicates failure and requires replacement.
Verify voltage availability at the glow plug rail connector while the pre-glow cycle is active using a multimeter or test light.
Clear P0674 after inspections, complete a full drive cycle replicating the freeze frame conditions, and verify the code does not return before considering the repair complete.

Repair Options and Costs

The fix depends entirely on which component triggered P0674. Work through cheaper possibilities before replacing expensive control modules.

Replace a complete set of glow plugs (all cylinders): $250–$600
Replace a single glow plug: $80–$200
Replace the glow plug relay or control module: $150–$450

Labor rates at independent shops typically add $75–$150 per hour depending on region. Replacing all glow plugs on a four-cylinder engine usually takes 1–2 hours of labor, while a single-plug replacement might finish in under an hour on accessible engines. Relay or module replacement generally requires 1–2 hours as well.

Total Cost to Fix P0674

Most P0674 repairs fall between $80 and $600 total including parts and labor. Single glow plug replacement represents the lower end of that range; full set replacement or control module replacement pushes toward the upper limit. An accurate diagnosis before purchasing parts prevents wasting money—replacing the most expensive component first without testing rarely saves time or money.

Can You Drive with P0674?

Yes, once the engine reaches operating temperature. Diesel engines run on glow plugs primarily during cold starts, so a warmed engine typically operates normally despite P0674. The practical concern emerges on cold mornings: engines with multiple failed plugs may not start reliably when temperatures drop. Addressing P0674 before cold weather arrives prevents being stranded. The fault does not represent an immediate safety hazard, but unresolved starting problems create inconvenient situations.