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Vehicle engine and/or automatic transmission

Code: P0671 - The glow plug circuit for cylinder 1 (used to help start the engine in cold weather) is open or not working properly.

When you see the P0671 code pop up, your vehicle’s computer-what we call the PCM-has spotted a problem with the glow plug circuit in cylinder #1. If you’ve spent any time working on diesels, you know each cylinder usually has its own glow plug. Think of these as tiny heaters that make cold starts possible. When you turn that key on a chilly morning, the PCM or glow plug control module sends power to those plugs, giving your engine a fighting chance to fire up smoothly. But if something’s off-maybe the plug itself has given up, the wiring’s shot, or the circuit’s open-the PCM logs this code. Bottom line: glow plugs are essential for helping your diesel get going in cold weather, and if one’s not pulling its weight, you’ll feel it when you try to start up on a frosty day.

Causes and obd code P0671

From my time under the hood, I can tell you the most common reason for a p0671 code is a glow plug that’s packed it in. But don’t be fooled-sometimes it’s the control module acting up, or maybe the wiring has seen better days. I’ve found broken wires, corroded connectors, and even loose terminals more times than I can count. Every now and then, the PCM itself will be the culprit, but honestly, that’s rare. Here’s what I always check first:

Glow plug for cylinder #1 has failed
Glow plug control module malfunction
Damaged or corroded wiring harness or connectors
PCM failure (least common)

Symptoms and P0671 trouble code

If this code is hanging around, you’ll probably notice your diesel isn’t eager to start-especially when the temperature drops. A classic symptom is a rough, knocking sound right after startup, or maybe some white smoke billowing from the exhaust until the engine warms up. And, of course, the check engine light will be staring at you from the dash. If you’re fighting cold starts or getting that telltale smoke, there’s a good chance p0671 is in play. I’ve seen this show up on Fords, Renaults, and plenty of other brands.

Diagnosis and P0671 code

When I get a dtc p0671 in the shop, here’s my go-to routine. Always start simple-don’t overlook the basics!

First, check the check engine light and confirm the code with a scan tool. Clear it, then see if it comes back after a cold start.
Next, open the hood and give those glow plug wires and connectors for cylinder #1 a good look. I’ve caught more issues just by spotting corrosion or a loose terminal. Sometimes it helps to have a buddy wiggle the wires while you keep an eye on things.
With the engine off and cool, unplug the glow plug for cylinder #1. Grab your multimeter and check the resistance-most good plugs read between 0.5 and 2 ohms. If you see infinite resistance or a reading way out of that range, swap the plug.
If the glow plug looks fine, move on to the wiring. With the ignition on (engine still off), check for voltage at the connector when the system’s active. No voltage? Now you’re looking at a possible control module or wiring fault.
If power’s missing, trace the wire back to the glow plug control module. Test for continuity and inspect for burnt or damaged spots.
Still no luck? Only then do I consider the control module-or the PCM, but that’s a last resort in my book.

Be thorough-sometimes cleaning up a crusty connector is all it takes to cure the problem. Don’t rush through these steps, or you’ll risk missing the obvious.

Common Mistakes When Fixing dtc P0671

A classic mistake I see is folks jumping straight to swapping out the glow plug, never checking the wiring or connectors first. That’s a quick way to waste money. Another one? Skipping the multimeter check-just because a plug looks fine doesn’t mean it’s working. And don’t forget about the control module; I’ve had jobs where the wiring and plug were perfect, but the module was to blame. Guesswork doesn’t pay off here. Take it step by step and confirm each part before moving on.

How Serious Is the P0671 engine code

If you’re in a cold climate, leaving this code unresolved is asking for trouble. A dead glow plug can make starting your diesel a real headache-or downright impossible-when the temperature drops. Keep forcing hard starts, and you’ll wear out your starter and battery fast. Plus, unburned fuel can end up washing down cylinder walls or gumming up your exhaust. Ignore it, and you could be looking at much pricier repairs. Trust me-don’t put this fix on the back burner.

P0671 Repair and Troubleshooting

After you’ve tracked down the fault, here’s what usually gets things back in shape:

Swap out the faulty glow plug in cylinder #1
Repair or replace any bad wiring or crusty connectors
Change out the glow plug control module if it’s not sending juice
In rare situations, repair or reprogram the PCM if that’s the root cause

Once repairs are done, clear the code and give the system a cold start test. If obd p0671 is still showing up, retrace your steps-something’s still hiding. But in most cases, these fixes do the trick.

Conclusion

At the end of the day, P0671 tells you the glow plug circuit for cylinder #1 isn’t doing its job, which means hard starts are likely-especially in the cold. Always check the basics first: the wiring, connectors, and plug itself. Don’t jump straight to the expensive parts. Letting this slide can make for even bigger headaches down the road. Tackle it early, and most of the time, a new glow plug or a wiring fix will have you back on the road, headache-free.

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