Glow plugs

From TDIClub

Glow plugs are used to preheat the combustion chamber of a diesel engine.

The glow plug codes are a popular code to be thrown on our cars. Several versions of the DTC are:

• 16764 / P0380 code (Glow plug/heater circuit electrical fault (Q6))
• 17055 / P0671 code (Cylinder 1 Glow plug circuit (Q10))
• 17056 / P0672 code (Cylinder 2 Glow plug circuit (Q11))
• 17057 / P0673 code (Cylinder 3 Glow plug circuit (Q12))
• 17058 / P0674 code (Cylinder 4 Glow plug circuit (Q13))

The first code is for 2001 and older cars. They have a 2 wire glow plug harness, so they just throw a generic code for the glow plugs.

The other codes are examples of codes you might find on a 2002 or newer car. They have a 4 wire harness, so the ECM can specify the problem plug. However, the troubleshooting procedure is the same for all codes.

NOTE: I have tested a few 2002's recently for glow plug problems. I don't know if this is the case for all of them, but the ones I tested have the cylinder order backwards. For example, if the code specifies a bad plug at cylinder # 1, then cyl # 4 may be bad. The last car I tested had 2 bad plugs according to the codes. Cylinders 1 & 2 were indicated as being bad, but actually it was plugs 3 & 4 that were bad. So don't always trust the codes. Check for yourself with the multimeter before replacing any plugs.

Now why was a code thrown? The ECM measures the resistance accross the harness wires. If it finds an imbalance in the resistance between the wires, It thinks there is a bad plug since a bad plug will have high or infinate resistance. But the big problem is that there are other factors that can cause the resistance deviation other than a bad plug. A bad relay, corroded or oxidised harness, a bad ground or a blown fuse can all cause the code to appear. So I will go step by step through each of the possibilities so you can test and rule out or confirm the actual cause.

The first step I suggest is to check the plugs themselves. Afterall, they are the intended culprit that should light the MIL. What you need to do is remove the engine cover and locate the glow plug harness. You will need a 10mm socket and a digital multimeter for this step.

Here is the harness:

To remove it, just hook your finger under each terminal and pull. They snap on and off like spark plug boots. Once it is off, just rotate it out of the way so you can expose your plugs: