P0671 is a somewhat rare diagnostic trouble code. It is generic (meaning it has the same meaning for all vehicles manufactured after 1996). But it only applies to vehicles with diesel engines.
P0671 is usually caused by a faulty glow plug or wiring problem.
P0671 Definition: 1st Cylinder Spark Plug Circuit
This is the definition of P0671, broken down:
Cylinder 1 spark plug
Diesel engines do not use spark plugs. Instead, they use glow plugs.
The spark plugs heat the combustion chamber before starting the vehicle. Once started, the heat and compression are enough to keep the engine running (diesel engines have higher compression ratios than gasoline engines).
The PCM (powertrain control module) uses data from the engine and transmission temperature sensors to determine if the engine is cold enough to warrant using the heaters.
In the case of P0671, the code means that the circuit that supplies power to the glow plug is defective. That’s all. It does not specify whether it is open, shorted, low or high-voltage
With P0671, there will be noticeable symptoms, although a faulty glow plug is usually not enough to cause a modern diesel engine to fail.
- Check engine light with P0671
- Difficult start
- The feeling of a “misfire” until the engine warms up
- Does not start
- Low engine power until it warms up
- Diesel smell in the exhaust
If the engine starts, let it idle and warm up before driving. You want it to get hot enough for cylinder 1 to start lighting on its own. Even if the engine runs and drives fine once it has warmed up, it continues to pour unburned fuel into the exhaust until it does.
P0671 Diagnosis + Causes
Although P0671 refers to a circuit problem, it can also be caused by a bad glow plug.
1. Determine if the glow plug is bad
There are a few ways to tell if your glow plug is working properly. If they are easy to access, you can do the “swap test”
Remove the glow plug from cylinder 1. Then, replace it with another glow plug (whichever is easiest to access). Once this is done, use your scanner to erase the OBDII memory.
With the vehicle cold, start it. Wait for the check engine light to come on again
If the code is still P0671, you know there is a wiring problem, and you will need to troubleshoot the harness.
If the code changes to P067X (X being the cylinder with the defective glow plug), replace the glow plug with a new one, and that’s it.
You can also use a test light to determine if a heater is bad. Here’s how to do it.
2. Check the wiring harness
Now that you know you don’t have a bad glow plug, it’s time to check the wiring harness.
Check for damage (think cracked wiring, poor grounding, or a loose connection at the heater). The glow plug wiring harness has fusible links, so if the harness is hot on one side of the link but not the other, you will need to replace it.
If everything seems fine, you will need to get an OHM meter and check the resistance at the glow plug. You will have to look up the resistance for your exact engine and vehicle model year.
P0671 is usually fairly easy to locate. Good luck repairing your vehicle!