P0670 — Glow Plug Module Control Circuit

P0670 — Glow Plug Module Control Circuit. A fault in the diesel glow plug system. The car is usually drivable while you plan the repair. Typical causes: burned-out glow plug, damaged glow plug wiring or connector, faulty glow plug relay or control module.

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 P0670 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 P0670 does not return.

Approximate repair cost

Repair cost depends on which component actually failed. Typical scenarios for P0670 (parts and labor, USD, independent shop rates): Replace the glow plug relay/module: $150–$450; Replace one glow plug: $80–$200; Replace a full set of glow plugs: $250–$600. Overall, fixing P0670 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

Will P0670 clear itself?

Sometimes P0670 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.

How much does it cost to fix P0670?

Replace the glow plug relay/module — $150–$450; Replace one glow plug — $80–$200; Replace a full set of glow plugs — $250–$600. 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 P0670?

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.

What is the most common cause of P0670?

The most common causes are: burned-out glow plug, damaged glow plug wiring or connector, faulty glow plug relay or control module.

What are the symptoms of P0670?

Drivers most often notice: white smoke after a cold start, hard starting in cold weather, Check Engine Light on, rough idle until warmed up.

Diesel engines rely on glow plugs to heat the combustion chambers during cold starts. Without that initial warmth, fuel injection doesn't ignite properly in low temperatures. A control module manages the entire glow plug circuit, activating the plugs for a brief period before engine start and monitoring their operation while the system is energized. When something interrupts this circuit—through a bad connection, failed component, or electrical fault—the engine's ECU detects the abnormality and stores a diagnostic trouble code.

Code P0670 specifically points to a problem with the glow plug module control circuit. The ECU doesn't just log this the first time it sees a strange reading; it runs multiple checks across successive drive cycles before confirming a genuine fault. Once verified, the check engine light illuminates and freeze frame data gets captured, preserving the engine's operating conditions at the exact moment the problem appeared. Those snapshot values—engine speed, load, temperature, vehicle speed—give you a precise starting point for troubleshooting P0670.

The glow plug system draws significant current when active, which makes it vulnerable to heat-related failures and connector degradation over time. Because the ECU continuously watches this circuit, it catches intermittent problems that might disappear once the engine warms up. That's why P0670 often returns if the underlying issue isn't actually fixed—you need to address the root cause, not just clear the code and hope for the best.

Symptoms

Check Engine Light or dedicated glow plug warning lamp stays illuminated
White smoke billowing from the exhaust pipe during cold starts
Engine cranks sluggishly or refuses to start in cold weather
Rough, uneven idle that smooths out once the engine reaches normal operating temperature
Delayed or incomplete glow plug activation cycle before startup

Causes

Corroded or moisture-damaged connectors at the glow plug harness
Failed glow plug relay or control module
One or more burned-out glow plugs with open circuits
Blown fuse protecting the glow plug circuit
Chafed, melted, or otherwise damaged wiring in the glow plug circuit
Poor ground connection at the glow plug control circuit

How to Diagnose P0670

Pull the codes with an OBD2 scanner and confirm P0670 is active. Save the associated freeze frame data—this snapshot of engine RPM, load percentage, coolant temperature, and vehicle speed tells you exactly what was happening when the fault triggered.
Check for related diagnostic trouble codes before touching anything mechanical. Codes like P0671 through P0674 indicate specific cylinder faults, and those often point straight to the bad component. Also look up any TSBs (technical service bulletins) for your specific make, model, and model year—manufacturers publish service updates for known P0xxx patterns, and sometimes a simple software flash resolves the issue.
Perform a thorough visual inspection of the entire glow plug wiring harness. Look for cracked insulation, wire chafing from engine movement, corrosion buildup on connectors, and any signs of heat damage or melting. Wiggle the connectors while watching the scanner's live data to spot intermittent opens.
Test each glow plug individually with a multimeter set to resistance (ohms). A functioning plug reads between 0.5 and 2 ohms depending on the design. Infinite resistance means an open element—you've found your culprit.
Verify voltage at the glow plug rail during the pre-glow sequence using a digital multimeter or test light. No voltage when the system should be active points to a relay, module, or fuse problem upstream.
Clear P0670 after repairs and complete a full drive cycle under the same conditions captured in the freeze frame. The code must stay cleared—if it returns, something else in the control circuit needs investigation.

Repair Options and Costs

Tackle the cheapest probable causes first before committing to expensive component replacement.

Glow plug relay or control module replacement: $150–$450
Single glow plug replacement: $80–$200
Complete glow plug set replacement (all cylinders): $250–$600

Individual glow plugs cost $15–$50 each, while a relay or control module typically runs $80–$250 for the part alone. Labor adds $75–$150 depending on engine configuration and accessibility.

How Much Does Fixing P0670 Cost?

The total bill hinges entirely on which part failed. For most P0670 cases, expect to pay between $80 and $600 when combining parts and labor at an independent shop. Replacing one bad plug represents the minimum repair scenario. Replacing a full set pushes toward the high end, especially on engines where accessing all cylinders requires significant disassembly. A stuck relay or failed control module falls somewhere in the middle.

Spending $50–$100 on a proper diagnosis before buying parts almost always saves money in the long run. Throwing a new relay at the problem when a single bad plug caused the code leaves the original fault unresolved—and you'll be back where you started.

Can You Drive with P0670?

Short-term, yes—the car remains driveable once the engine is running and warmed up. The glow plugs only matter during cold starts. However, if multiple plugs have failed, you risk being stranded on a freezing morning when the engine simply won't catch. The rough idle and white smoke also indicate incomplete combustion, which can dilute oil quality over time if the issue persists. Address P0670 before cold weather arrives.