Glow Plugs: P0671-P0678, P0670 and the Removal That Snaps Them Off in the Head

Read This Before You Touch a Glow Plug

This is the one piece of advice on the page that can save you thousands, so it goes first. Glow plugs thread into a hot cylinder head and stay there for years through constant heat cycling. They seize. When someone puts a wrench on a seized plug and keeps pulling, the plug snaps, leaving the tip buried in the head -- and depending on the engine, retrieving it can mean removing the cylinder head. Ford's 6.0L Power Stroke is the notorious example, but it happens on plenty of engines. The technique that avoids it is patience: work on a warm-but-not-hot engine, soak the threads with penetrating oil and give it real time, then loosen and re-tighten in small increments rather than forcing steadily in one direction. If a plug will not move, stop and let a shop with an extraction kit handle it. A seized plug left in place costs you a cold start. A snapped one can cost you the top end of the engine.

What These Codes Actually Tell You

Glow plugs are a cold-start aid on diesel engines, heating the combustion chamber so the fuel ignites reliably before the engine is warm. That has a diagnostic consequence worth knowing: a dead glow plug often produces NO symptom at all in summer, and the fault only announces itself as hard starting, white smoke and rough running for the first minute on the first cold morning. P0671 through P0678 name a specific cylinder's glow plug circuit -- 1 through 8 in order. P0380 and P0382 are the general glow plug heater circuits. P0670 is the control module's circuit and P0683 is the communication link between that module and the PCM. The useful distinction: a code on ONE cylinder points at that plug or its wiring, while several cylinders reporting at once points at the control module, the relay, or the harness feeding them all rather than at multiple plugs failing simultaneously.

Codes P0671-P0678, P0380, P0382: Glow Plugs -- Buy the Set

Even when a single cylinder reports, replacing one plug is usually a false economy. They are the same age, they have heat-cycled the same number of times, and the labour to reach them is largely shared -- so replacing one now means doing the same job again shortly for the next one. Buy the set. Brand genuinely matters here more than in most categories: cheap glow plugs are known for swelling in the head, which makes the next removal dramatically harder, and for failing early. Motorcraft, ACDelco, Bosch, Beru and NGK are the established names, and several supply the original equipment. Check your engine's plug type before ordering, because ceramic and metal-sheath plugs are not interchangeable and ceramic units are considerably more fragile in the hand as well as more expensive.

Codes P0670, P0683: Glow Plug Control Module

The part that gets blamed on the plugs. The control module switches power to the glow plugs and monitors them, and when it fails you get symptoms indistinguishable from dead plugs -- hard cold starting, white smoke, and often several cylinder codes at once. P0670 is its control circuit; P0683 is the communication link between the module and the PCM, which points at wiring or the module rather than at anything in the cylinder head. This is genuinely worth checking before pulling plugs, because the module is accessible and the plugs are not. On the 6.0L Power Stroke the module lives where it collects water and corrodes, which is a well-documented failure. Inspect its connector for corrosion before condemning anything: a green-tinged terminal produces exactly these codes with a healthy module.

The Cheap Failure Between Them: Glow Plug Harness

The wiring from the control module to each plug sits directly on top of a hot engine, in some designs strapped along the head where it bakes continuously. Insulation hardens and cracks, terminals corrode, and the little connectors that push onto each plug lose their grip. The result is per-cylinder codes on plugs that are perfectly healthy -- which is why a set of new plugs sometimes changes nothing. Before ordering plugs, pull the connector off the cylinder that is reporting and look at it, then compare with a cylinder that is not. Brittle insulation, a terminal that has backed out, or corrosion is your answer. Harnesses are inexpensive relative to the labour of removing plugs, and on several engines they are a known wear item replaced as routine alongside the plugs.

Buy This At The Same Time: Penetrating Oil and Anti-Seize

Not glamorous, and the single highest-value thing you can add to this order. Penetrating oil applied to the threads and given genuine soaking time -- hours, not minutes, and ideally through a heat cycle or two -- is what turns a seized plug into one that comes out. And a thin film of anti-seize on the threads of the NEW plugs is what makes the next change a normal job rather than a gamble. Keep the compound strictly on the threads and off the tip. This is the same advice as any exhaust fastener, and the consequence of skipping it is larger here, because the failure mode is a broken plug in a cylinder head rather than a broken stud you can drill out on the bench.

If You Have a Cummins, You Probably Do Not Have Glow Plugs

Worth stating plainly because it sends people looking for parts their engine does not have. Dodge and Ram trucks with the Cummins inline-six use an intake air grid heater rather than glow plugs -- a heating element in the intake that warms incoming air for the whole engine at once, instead of individual plugs per cylinder. So there is no set of six plugs to buy, and the codes and failures are different: grid heater element failure, and the heavy relays that switch it, which draw enormous current and are a known wear point. If your cold-start complaint is on a Cummins, you are looking for a grid heater and its relays. Some other engines use an intake air heater alongside or instead of glow plugs too, so confirm what your engine actually has before ordering anything.

Quick Reference: Which Part for Which Code

Glow plug set: P0671 (cyl 1) through P0678 (cyl 8), and the general P0380 and P0382 heater circuits — bought as a set even when one cylinder reports. Glow plug control module: P0670 for its control circuit, P0683 for its communication link to the PCM, and any case where several cylinders report at once. Glow plug harness: per-cylinder codes where the connector at that cylinder is corroded or brittle, and a sensible companion purchase on a high-mileage engine. Penetrating oil and anti-seize: every time, without exception. Grid heater and relays instead of all of the above: Cummins and other intake-air-heater engines. P0381 is the dashboard glow plug indicator circuit rather than the plugs themselves — a lamp or wiring fault, not a heating fault. As always, check your specific code page first, since glow plug system design varies more between manufacturers than most systems on this site.

Frequently Asked Questions

Related Guides

• Winterizing Your Diesel: Cold Weather Prep Checklist Diesel trucks need specific winter prep: anti-gel additive, block heater check, fuel filter water drain, battery load test, and glow plug inspection. Miss any of these and sub-zero mornings turn into tow calls. Covers codes P0380 and P0671-P0678.
• Car Hard to Start in Cold Weather: Causes & Fixes Your car cranks slowly, takes forever to fire up, or won't start at all when it's cold outside? Learn the most common cold-weather starting problems, which OBD-II codes to check, and what to fix first.
• Fuel Pump, Sensor or Module? What P0087, P0190-P0193, P0230 and P2635 Actually Need Modern direct-injection engines have TWO fuel pumps, and a $40 sensor can impersonate a $900 one. Match your fuel code to the right part -- rail pressure sensor, high-pressure pump, in-tank module, driver module or level sender -- before you spend.
• Sensor Cleaners & Shop Chemicals: Which Can for Which Code (and Which One Kills Sensors) MAF cleaner, throttle body cleaner, contact cleaner and brake cleaner look identical and are not interchangeable -- the wrong can destroys a mass airflow sensor. Match the chemical to your code, from P0101 and P0171 to the U-code connector faults.